AbstractIn the northeastern Jajarm region, within the structural zone of the Eastern Alborz, a complete sequence of the Elika Formation (Early–Middle Triassic) and the Shemshak Group (Late Triassic–Middle Jurassic) is exposed. In this study, the upper part of the Elika Formation and the Upper Triassic Shahmirzad Formation of the Shemshak Group were investigated based on lithostratigraphic and petrographic analyses. The upper part of the Elika Formation, approximately 80 m thick, mainly consists of dolomite and dolomitic limestone with stromatolitic structures and lacks fossils, indicating deposition in a tidal environment. The Shemshak Group, with an approximate thickness of 2,050 m, comprises sandstones, silty sandstones, siltstones, shales, and fossiliferous limestones, and is subdivided into the Shahmirzad, Alasht, Shirin-Dasht, Fil-Zamin, and Densrit formations. The Shahmirzad Formation, about 285 m thick, predominantly consists of shale and sandstone and includes fossiliferous marker beds and diverse petrofacies such as litharenite and lithic arkose. The results indicate that the Shemshak Group exhibits significant lateral variations due to deposition in a fluvial–deltaic environment, and the observed litharenites reflect uplifted sequences associated with orogenic activity and foreland basin subsidence. The results of this study provide a detailed lithostratigraphic framework and fundamental data for identifying bauxite horizons and guiding future exploration.Keywords: Jajarm, Shemshak Group, Elika Formation, Shahmirzad Formation, Petrofacies IntroductionThe Shemshak Group in the Gol-e-Bini section, similar to other exposures of this group, is predominantly composed of siliciclastic deposits, including sandstone, siltstone, and silty sandstone. This succession was previously investigated by Fürsich et al (2009). According to these studies, the total thickness of the Shemshak Group was measured as 2,050 meters, comprising, from base to top, the Shahmirzad, Alasht, Shirindasht, Fil-e-Zamin, and Dansirit formations.In the Gol-e-Bini section, the lower boundary of the Shemshak Group unconformably overlies the dolomitic strata of the Elika Formation, whereas its upper boundary is disconformably overlain by marly limestones and marls of the Dalichai Formation. Both the lower and upper contacts are erosional disconformities associated with two major tectonic phases, namely the Early Cimmerian and Middle Cimmerian tectonic events, respectively.Variations in sedimentary successions among different depositional basins emphasize the importance of stratigraphic investigations, particularly lithostratigraphic subdivision, as one of the fundamental components of regional geological studies.The relatively homogeneous lithological characteristics and the scarcity of key beds have made the recognition of lateral facies variations within the Shemshak Group extremely difficult. Therefore, the present study focuses on a detailed stratigraphic investigation of the upper part of the Elika Formation and the basal formation of the Shemshak Group, including the Shahmirzad Formation. These formations were subdivided into lithological units, and where possible, key beds and marker horizons were identified and introduced. Materials & MethodsThe study of the Shemshak Group has been carried out in two stages: field and laboratory. Due to the lithological homogeneity of the succession and the absence of distinct key beds, tracing lateral facies variations within the Shemshak Group is inherently difficult. In this research, after a thorough review of previous studies and field surveys, suitable stratigraphic sections were selected for examination. Because mining activities and tectonic deformation have disrupted parts of the original sedimentary succession, no single complete stratigraphic section could be identified. Consequently, the succession was examined in three separate sections: Gol-e-Bini 4, Gol-e-Bini 6, and Zoo.During field investigations, the sedimentary succession was subdivided into lithological units based on bedding characteristics, sedimentary structures, fossil content, lithology, and both vertical and lateral facies variations.For further description and analysis, samples were collected from certain beds to prepare thin sections. In total, 100 sandstone samples were collected during fieldwork, of which 70 thin sections were prepared and examined using a polarizing microscope. Discussion of Results & ConclusionsThe upper part of the Elika Formation, with a thickness of about 80 meters, mainly consists of dolomites and dolomitics limestone containing stromatolitic structures and lacking any fossil content, indicating deposition in a tidal flat environment.The considerable thickness and lithological composition of the Shemshak Group suggest sediment accumulation within a shallow basin characterized by a high subsidence rate, most likely within a foreland basin setting. Progressive subsidence of the basin floor resulted in gradual landward transgression and rising relative sea level, eventually leading to the establishment of a carbonate platform upon which the sediments of the Dalichai Formation were deposited.Petrographic and Petrofacies analysis indicates that carbonate and chert rock fragments are abundant in the basal part of the Shemshak Group, whereas metamorphic rock fragments appear in its upper part. These variations in rock fragment composition are related to tectonic conditions and provenance. The carbonate lithic fragments were most likely derived from erosion of the Elika Formation, while the metamorphic fragments were sourced from the Gorgan Schists, which stratigraphically underlie the Elika Formation.The dominant lithology of the Shahmirzad Formation consists of shale interbedded with sandstone layers. A fossiliferous limestone bed in the upper part of the formation constitutes an important marker horizon within this siliciclastic succession. In addition, sandstone beds containing autochthonous calamites in the middle part of the formation represent another distinctive stratigraphic horizon.The predominant petrofacies in the Shahmirzad Formation include siltstone, litharenite with clay cement and matrix, litharenite with carbonate cement, feldspathic litharenite with carbonate cement, feldspathic litharenite with siliceous cement, lithic arkose with carbonate cement, and lithic arkose with clay cement.The Shahmirzad Formation representing the basal unit of the Shemshak Group that was deposited predominantly in a fluvial depositional environment characterized by channel sandstones, floodplain deposits, and swamp sediments. The occurrence of thin coal seams, root horizons, plant fossil remains and wood fragments, collectively indicate deposition under warm and humid climatic conditions. Evidence for a subsequent marine transgression is provided by carbonate beds in the upper part of the formation containing bivalve and echinoid fossils.
AbstractIn this study, the depositional and post-depositional history of carbonate rocks of the Asmari Formation in the Lorestan region was investigated. Three stratigraphic sections were analyzed: the northern Khorramabad section (92.3 m), the southeastern Khorramabad section (90 m), and the southwestern Khorramabad section (190 m). A total of 380 samples were collected for petrographic examination. The lower boundary of the Asmari Formation with the Shahbazan Formation is a conformable contact, while its upper boundary with the Gachsaran Formation represents a conformable unconformity. Thirteen microfacies were identified, including various dolomudstones, wackestones, packstones, grainstones, and rudstones containing different types of foraminifera, echinoderms, bryozoans, coralline algae, and anhydrite. These microfacies represent deposition on a carbonate ramp ranging from inner to mid and outer ramp settings. Several diagenetic processes were recognized, such as micritization, neomorphism, marine, meteoric, and burial cementation, dissolution, development of both fabric-selective and non-fabric-selective porosity, and replacement processes including pyritization, silicification, and dolomitization. Based on petrographic evidence, the paragenetic sequence of the Asmari Formation carbonates was interpreted to have formed in four diagenetic environments: marine, meteoric, burial, and uplift. These environments correspond to stages of early diagenesis (eogenesis), middle diagenesis (mesogenesis), and late diagenesis (telogenesis).Keywords: Asmari Formation, depositional environment, diagenesis, Zagros, Lorestan Introduction The Asmari Formation, with its dominant carbonate lithology, is rich in benthic foraminifera of Oligocene-Miocene age. Therefore, using this valuable fossil content and other skeletal components in this formation, the type of its depositional environment can be determined according to the models presented by Flügel (2010) and Wilson (1986). The Asmari Formation deposits can be considered as the last widespread marine transgression in the Zagros Basin (Amiri Bakhtiar and Noraiyanejad 2022). In terms of age, it begins in the Oligocene and continues up to the Burdigalian of the Early Miocene (Motiei 2000). Based on its fossils and age, the Asmari Formation can be divided into three parts: Lower, Middle, and Upper Asmari (Motiei 2000). In most outcrops in Lorestan, the Lower Asmari limestones are deposited directly over the carbonate rocks of the Shahbazan Formation, which are of Late Eocene age. In more northern areas, the Lower Asmari is absent, and it appears that the interval from the Late Eocene to the Oligocene was subaerial. Toward the southern regions, the Late Eocene–Oligocene limestones of the Lower Asmari grade into the deeper-water shales of the Pabdeh Formation (Paleocene–Oligocene). From Lorestan toward the Dezful Embayment, the Lower Asmari limestones were deposited along the margin of the deeper Pabdeh basin. The study area is situated within the Zagros Fold-Thrust Belt, specifically in the Lorestan sedimentary province. Three stratigraphic sections were selected in this region for detailed analysisThe study of the Asmari Formation in the Lorestan sedimentary basin is highly important, primarily due to its role as one of the most significant carbonate hydrocarbon reservoirs in the Zagros Basin. The primary objective of investigating this formation in this part of the basin is to identify facies variations, reconstruct depositional environments, and understand its diagenetic evolution, thereby enabling the prediction of reservoir quality and porosity distribution across different segments of the basin. A precise analysis of the petrophysical properties and sequence stratigraphy of the Asmari Formation in Lorestan provides a framework to improve reservoir modelling, evaluate the processes controlling hydrocarbon migration and accumulation in the area, and reduce exploration risks during the drilling of development and appraisal wells. Material & Methods In this research, following multiple field visits and the study of the 1:250,000 scale geological maps of Khorramabad and Pol-e Dokhtar, three suitable outcrop sections of the Asmari Formation were selected within the Zagros Fold-Thrust Belt (Lorestan sedimentary basin). Section 1: Located on the northern limb of the Khorramabad َnticline, near Sarab-e Chenar village (Bastam area). The geographical coordinates are 48° 8’ 58" E longitude and 33° 41’ 36" N latitude. Section 2: Located southeast of Khorramabad, along the road to Sepiddasht. The geographical coordinates are 48° 13’ 10" E longitude and 33° 36’ 40" N latitude. Section 3: Located approximately 85 km south of Khorramabad (Varah-Zard village) and 11 km north of Pol-e Dokhtar. The geographical coordinates are 47° 43’ 2" E longitude and 33° 13’ 26" N latitude. These sections were chosen to ensure they exhibit maximum thickness, significant lithological variations, easy accessibility, and minimal cover. A total of 380 rock samples were collected from these sections: 90 samples from the northern Khorramabad section, 90 samples from the southeastern Khorramabad section, and 200 samples from the southern Khorramabad section. Thin sections were prepared from all samples for petrographic analysis. These thin sections were prepared at Lorestan University and studied and photographed using an Olympus BH2 polarizing microscope equipped with a Nikon D70 camera. Carbonate rock nomenclature followed the method proposed by Dunham (1962) and the modified scheme from Embry and Klovan (1971). The interpretation of microfacies and depositional environments was conducted based on the works of Flügel (2010) and Wilson (1975). Discussion of Results & Conclusions In the three studied sections, 13 carbonate microfacies have been identified. These include: Anhydrite microfacies, fenestral dolomudstone, nodular dolomudstone, wackestone/packstone with extraclasts, mudstone to wackestone with bioturbation, wackestone/packstone with imperforate foraminifera, wackestone/packstone with imperforate and perforate foraminifera, wackestone/packstone with perforate foraminifera, bioclastic grainstone containing foraminifera, packstone to grainstone with echinoderms, bryozoans, and coralline algae, rudstone/floatstone containing foraminifera and coralline algae, floatstone/rudstone with coralline red algae, mudstone to wackestone with planktonic foraminifera. These microfacies were deposited within three distinct facies belts corresponding to a carbonate ramp: the tidal flat, lagoon, and open marine environments. Based on field evidence and the identified microfacies in the studied stratigraphic sections, the depositional environment of the Asmari Formation exhibits a continuous and gradual transition from the mudstone, wackestone, and packstone microfacies of the low-energy lagoonal environment associated with the inner ramp, to the grainstone facies of the high-energy lagoon at the beginning of the mid-ramp. This is followed by a gradual transition to packstone facies with elongated foraminifera and hyaline tests, and floatstone and rudstone microfacies of the distal mid-ramp, eventually leading to mudstone facies containing planktonic foraminifera characteristic of the open marine environment at the beginning of the outer ramp. The presence of evidence such as the abundance of non-porous benthic foraminifera with porcelains shells in a texture ranging from wackestone to packstone, red algae, weak to moderate sorting of allochems and supporting mud texture, the depositional environment of the Asmari Formation deposits in the stratigraphic sections is considered as a homoclinal carbonate ramp. Among the most important diagenetic processes in the studied sections are compaction, cementation, dissolution, porosity and replacement. Based on petrography evidence, the diagenetic sequence of the Asmari Formation occurred during three stages (eogenesis, mesogenesis, and telogenesis) and in four diagenetic environments (marine, burial, freshwater, and uplift).Early Diagenesis (Eogenesis): This stage includes some diagenetic processes, such as micritization and syntaxial rim cement, which are characteristic of early diagenesis in marine environments, have been identified in the studied sections, confirming the initial diagenesis stage.Freshwater Diagenesis: In the freshwater phreatic environment, intergranular pores are continuously filled with water, which can lead to the dissolution of metastable minerals like aragonite and high-magnesium calcite (Heydari and Wade 2014).Intermediate Diagenesis (Mesogenesis): During this stage, sediments are subjected to pressure and temperature resulting from burial at various depths, continuing until the threshold of diagenesis is reached. Processes occurring in this stage include physical and chemical compaction, blocky and drusy cements, dolomitization, and pyritization, all of which have been identified in the studied samples. Chemical compaction leads to the formation of features resulting from pressure dissolution, such as stylolites.Late Diagenesis (Telogenesis): During uplift, iron ions are introduced into the sediments by meteoric waters through fractures, forming hydrated iron oxides (oxidizing conditions), which gradually convert to hematite. Fractures and joints identified in the studied sections formed during this stage and are filled with freshwater cements.
Abstract This study investigates the lithostratigraphy and biostratigraphy of the Permian strata in the west of Sirjan City, within the southern metamorphic Sanandaj–Sirjan Zone (SSZ). The measured section is bounded by faults at its base and top. The studied strata unconformably overlie the metamorphosed Permian–Carboniferous sedimentary deposits and are overlain primarily by the dolomitized and recrystallized Permian–Triassic metamorphic rocks. Fourteen lithostratigraphic units were identified, their characteristics shaped by both original sedimentary basin fluctuations and post-diagenetic metamorphism. The microfossil assemblage is notably distinct from those of adjacent structural zones, comprising 11 fusulinid and 29 smaller foraminifera species. This unique fauna suggests that during the Permian, the SSZ constituted an independent basin situated on the northern margin of the Neo-Tethys Ocean and the southern margin of the Cimmerian Superterrane. Based on the identified smaller foraminifera and fusulinids, the age of the studied section is constrained to the Bolorian through Murgabian (Middle to Late Permian). Keywords: Permian, Sanandaj–Sirjan, lithostratigraphy, Biostratigraphy, smaller foraminifera Introduction The Sanandaj–Sirjan Zone (SSZ), extending 1500 km in length and 150–250 km in width, is one of the major structural zones of Iran (Aghanbati 2004). This zone contains sedimentary and igneous rocks ranging from Precambrian to Late Cretaceous, which have been intensely overprinted by multi‑stage regional metamorphism under various metamorphic facies (Nadimi and Konon 2012; Mehdipour Ghazi and Moazzen 2015; Hassanzadeh & Wernicke 2016; Gharibnejad et al. 2022). A prominent stratigraphic feature distinguishing this zone from Central Iran and the Zagros is the widespread outcrops of the Permian marine strata, particularly in the Sirjan area. The study of the Permian deposits in the SSZ is complicated because they record the breakup of the Cimmerian supercontinent from the northern margin of Gondwana at the end of the Paleozoic and the subsequent birth of the Neo-Tethys Ocean. Field evidence indicates that from the latest Carboniferous to the Early Permian, a marine basin formed in the southern part of the SSZ, and its extent and depth increased progressively throughout the Permian. The occurrence of shelf deposits and diverse marine fossils points to the formation and development of a marine environment that later evolved into the Neo-Tethys Ocean during the Mesozoic. Despite the necessity of detailed stratigraphic investigations to resolve palaeogeographic uncertainties, the intense folding, fault displacements and regional metamorphism in this belt have largely destroyed original fossil content, texture, and sedimentary structures, rendering conventional stratigraphic studies impossible in most areas. Nevertheless, these difficulties should not preclude stratigraphic efforts where feasible. Therefore, the present study focuses on the lithostratigraphy and biostratigraphy of the relatively undeformed and weakly metamorphosed Permian strata in the Sirjan area, located in the southern part of the SSZ. The aim is to determine the age of these deposits and to compare their faunal content (particularly small foraminifers and fusulinids) with other regions of Iran. Material & Methods The Permian deposit outcrop studied in this research is located in western Kerman Province, west of Sirjan city, within the SSZ (Fig. 1A). The outcrop lies west of the Sirjan salt marsh and northwest of the village of Kheyrabad, accessible via the Sirjan–Kheyrabad road and unpaved roads west of Kheyrabad (Fig. 1B). The measured section is on the eastern slope of Mount Hezarchil at 55°11′57.41″E and 29°31′6.03″N. The study area is in the southern part of the SSZ. According to the 1:100,000 Zardu geological map (Sabzehei 1994), rock exposures near the section consist mainly of metamorphosed limestone and shale units of Late Paleozoic (Carboniferous–Permian) to Early Mesozoic (Triassic) age (Fig. 1C). These rocks have undergone intense folding and multiple fault displacements and appear dark (light brown to black) on satellite imagery (Fig. 1D). Forty‑five samples were collected from the Permian outcrop. Due to faulted lower and upper boundaries, the outcrop limits were clearly distinguishable in the field (Fig. 2). During sampling, efforts were made to select samples with minimal recrystallization, stylolites, and other diagenetic or metamorphic effects. Sampling intervals were measured by tape, with denser and more regular sampling in fossiliferous, non‑metamorphosed layers. The true thickness of the section is 195 m. Most samples showed fusulinid remains, but in many cases, the fusulinid tests were destroyed, leaving only recrystallized or replaced molds. Therefore, three thin sections were prepared from each fossiliferous sample and one thin section from each non‑fossiliferous sample. Thin sections were studied using binocular and polarizing microscopes, and fossil content and petrographic features were photographed with a 10‑megapixel camera. Microfossil identification followed multiple references, cited in the biostratigraphy section. Finally, the identified fossil content was plotted on the measured stratigraphic column, and the range of each fossil along the section was delineated. The fossil assemblage from the studied section was compared with those from other structural zones of Iran to establish the relationship between the study area and other geological domains. Discussing of Results & Conclusion The lower boundary of the studied section rests upon strongly deformed metamorphic sedimentary units due to fault activity. These units include fault breccia, metaconglomerate, and metaquartzite with interbeds of schists and marbles. Intense shearing in the underlying unit and brecciation at the base of the section confirm a faulted lower boundary (Fig. 2). Fifteen lithological units are distinguishable above the basal fault contact as follows (Fig. 2): Unit 1 – Recrystallized and stylolitic limestone, 8 m thick (Fig. A3), with abundant calcite veins. Fusulinid remains occur as dissolved and calcite‑replaced molds (Fig. B3, C3), most of which are unidentifiable. Unit 2 –Fossiliferous limestone, 17 m thick (Fig. A3), with much better-preserved texture and fossil content than the underlying layers. Unit 3 –Sandstone (litharenite/quartzarenite), 6m thick, in sharp contact with the previous unit, showing cross‑bedding (Fig. A3). Unit 4 –Uniformly thick‑bedded fossiliferous limestone, 27 m thick (Fig. A3), where most identified species appear. Unit 5 –Thick‑bedded, unfossiliferous, recrystallized dolomitic limestone, 11 m thick (Fig. A3). Unit 6 –Litharenitic sandstone, 3.5 m thick (Fig. A3). Unit 7 –Highly fossiliferous limestone, 3.8 m thick (Fig. A3). Unit 8 –Massive recrystallized limestone, 6.5 m thick (Fig. A3, A4), with dissolved/replaced fusulinid traces. Unit 9 –Medium‑ to thick‑bedded fossiliferous limestone, 6 m thick (Fig. A4, B4) with lower fusulinid diversity compared to other microfossils. Unit 10 –Slightly metamorphosed shale, 3 m thick (Fig. B4). Unit 11 –Recrystallized calcareous sandstone, 5 m thick (Fig. B4, A5). Unit 12 –Intensely recrystallized and fractured massive limestone, 23 m thick (Fig. A5, B5). Unit 13 –Fossiliferous limestone succession, 54.7 m thick: Lower part medium‑ to thick‑bedded, middle part thick‑bedded to massive and the upper part medium‑ to thick‑bedded limestones (Fig. B5). Unit 14 –Recrystallized, stylolitic dolomitic limestone with calcite veins, 14.5 m thick (Fig. B5, A6, B6). Unit 15 –Medium‑ to thick‑bedded fossiliferous limestone, 6 m thick (Fig. B6), terminated upward by a fault contact. This unit is relatively rich in fossils, containing large fusulinids. Above the upper fault, thick‑bedded to massive dolomitic succession (Fig. B6) with intense fracturing and abundant calcite veins are present. Microscopic studies revealed two main foraminiferal groups: fusulinids and non‑fusulinids. Fusulinids assemblage includes 11 genera and 11 species as Afghanella sp., Cancellina ovalis, Codonofusiella cf. nana, Dunbarula sp., Eopolydiexodina persica, Grovesella sp., Mesoschubertella thompsoni, Misellina ovalis, Polydiexodina sp., Skinnerella sp. and Yangchienia sp. Non‑fusulinid foraminifers include: Climacammina cf. aljutovica, Climacammina elegans, Climacammina cf. procera, Climacammina sp., Climacammina cf. tudiola, Climacammina valvulinoides, Cribrogenerina gigas, Cribrogenerina major, Cribrogenerina sumatrana, Cribrostomum sp., Cryptoseptida sp., Deckerella sp., Deckerella composita, Deckerella geyeri, Deckerella cf. quadrata, Deckerella cf. tenuissima, Diplosphaerina inaequalis, Geinitzina sp., Langella conica, Langella cf. perforate, lunucammina sp., Nodosinelloides camerta, Pachyphloia sp., Padangia sp., Palaeotextularia sp., Palaeotextularia consobrina, Palaeotextularia bella and Palaeotextularia longiseptata. The fossil assemblage of the studied section was compared with those reported from the Alborz, east-central Iran, and Zagros basins. The assemblage of the SSZ is almost unique and does not fully resemble neighboring zones. The Yazd Block (east-central Iran) – No fusulinid‑bearing Permian strata; the Permian consists of dolomites of the Jamal Formation. Tabas Block (east-central Iran) – Fusulinids such as Armenina spp., Misellina spp., etc., show partial similarity up to the middle Kubergandian, but younger strata are unfossiliferous. Alborz (Ruteh Formation, Murgabian–Midian) – Fusulinids like Dunbarula mathieui, Yangchienia haydeni, and Neoschwagerina margaritae were reported but are absent in our section (likely because the Ruteh Formation is younger). Zagros (Dalan Formation) – Eopolydiexodina persica from Murgabian deposits matches the occurrence of the same species at the base of the Murgabian in our study. Thus, the studied assemblage shows the greatest similarity with east-central Iran (Tabas Block) and the Zagros, indicating a palaeogeographic affinity of the southern SSZ with these areas. During the Permian, the SSZ was part of the Cimmerian supercontinent (bordered by Paleo-Tethys to the north and Neo-Tethys to the south). The greater similarity with southern east-central Iran and the Zagros suggests that the studied section lay on the southern margin of the Cimmerian supercontinent, within the northern waters of the Neo-Tethys Ocean (Fig. 7). Based on fusulinid ranges and correlation with previous studies (Leven 2003; Leven & Gorgij 2008, 2011a, b; Fassihi et al. 2019, 2020, 2023; Hosseinipour 2024, among others), two biozones are recognized in the studied section (Figs. 8, 9): Eopolydiexodina persica Zone – Murgabian age: This zone begins at 131 m above the base of the measured section. The Kubergandian–Murgabian transition is marked by the first appearance of Eopolydiexodina persica, Afghanella sp., and Polydiexodina sp., and the last occurrence of Codonofusiella cf. nana and Cancellina ovalis. Immediately before and after this transition, Skinnerella sp. becomes more abundant. At this level, various Deckerella species (formerly common) disappear, while diversity and abundance of Cribrogenerina increase. Cancellina ovalis‑Codonofusiella nana Assemblage Zone – middle–upper Kubergandian age: This biozone is recorded from 77 m above the base of the measured section. Within this zone, various species of Deckerella and Palaeotextularia are present in most samples. At the base of the section, the Bolorian sediments exist, but due to a lithological change (fossiliferous limestone passing into sandstone) and the absence of fusulinids, the precise Bolorian–Kubergandian boundary cannot be determined. Therefore, this boundary is placed below the first sandstone unit, consistent with the last occurrence of Misellina ovali (Leven 2003; Wang et al. 2018).
Abstract The aim of this research is to investigation the lithofacies and sedimentary environment of the Kashkan Formation (middle Eocene) in the Lorestan sedimentary basin. In this regard, four stratigraphic sections were selected. The Kashkan Formation in the studied area has a variable thickness of 150 to 200 meters. It is mainly composed of conglomerate and sandstone strata, with lesser amounts of fine-grained lithofacies including mudstone, siltstone, and shale. Field studies on the four sections led to the identification of 12 lithofacies, which include five conglomeratic lithofacies (Gm, Gms, Gp, Gh, Gt), five sandstone lithofacies (Sp, St, Sm, Sh, Sr), and two mudstone lithofacies (Fm, Fl). Based on the vertical and lateral changes of facies, the depositional environment of these deposits was a braided river system with a gravelly and sandy bed in proximal areas. The clastic sequences of the Kashkan Formation overlie the marine carbonates of the Taleh Zang Formation, which indicates a sea regression and the progradation of braided rivers toward the basin. A decrease in accommodation space relative to sediment supply caused the progradation of braided river facies within the LST facies suite for the Kashkan Formation. Keywords: Sedimentary environment, Kashkan Formation, Zagros, Lorestan. Introduction Terrigenous rocks in the Zagros region of Iran have received relatively little attention. These rocks (e.g., the Amiran, Kashkan, Aghajari, and Bakhtiari formations) have mostly been studied only from the perspectives of lithostratigraphy or stratigraphic position (Motiei 1993). The Kashkan Formation is most extensively distributed in the Lorestan region, and its main facies include conglomerate, sandstone, and siltstone. This formation is situated between the carbonate Telezang Formation below and the Shahbazan Formation above. Where the Telezang Formation is absent, the Kashkan Formation rests directly on the Amiran Formation. The type section of the Kashkan Formation was introduced by James and Wynd (1965) in the Amiran Anticline near the city of Mamulan. Terrigenous formations are of great importance in the Zagros in terms of tectonics, sedimentary environment, climatic changes, and hydrocarbon generation and storage. The Kashkan Formation is essentially a terrigenous unit and is widely exposed in Lorestan. This formation is named after the Kashkan River that flows through this province. Among the few studies conducted on the Kashkan Formation, Yousefi Yeganeh et al. (2012) investigated the sedimentary environment and trace fossils of this formation within the Lorestan sedimentary basin, focusing on the northern, southeastern, and northwestern parts of Lorestan Province. The main goal of this research is to identify and describe the outcrops of the Kashkan Formation along the Khorramabad–Pol-e Zal Freeway (southern and southwestern Lorestan)—an area that has remained largely understudied. Through detailed analysis of lithological units and sedimentary environments in four selected sections, the depositional processes of this formation will be better understood. Finally, by integrating these findings with previous research, a comprehensive understanding of the depositional history of the Kashkan Formation in the Zagros sedimentary basin will be completed and enhanced. Materials & Methods The sections were selected to maximize thickness, facies diversity, and accessibility while minimizing cover. In this regard, four stratigraphic sections were chosen along the Khorramabad–Pol-e Zal Freeway. In these sections, field characteristics—including grain size, bedding, geometric form of beds, sedimentary structures, contact types between beds, and sediment stacking patterns—were carefully examined and recorded. The thickness of the formation was measured using a Jacob staff and compass. Gravels within conglomeratic units were studied using the ribbon method (Tucker 2003). This method involves dividing conglomeratic outcrops into sequential lateral or longitudinal ribbons; within each ribbon, the size, roundness, sorting, lithology of clasts, and their percentages are systematically recorded to document lateral and vertical depositional changes in a regular, comparable manner. The orientation of the foresets of current ripples was used to determine paleocurrent direction. Sandstone samples were collected based on facies variations using a point-sampling method from the middle part of the beds. Clastic facies were classified according to Miall's (2006) codes, and sandstone classification followed Folk (1980). By integrating data from microscopic and field studies, lithofacies and facies associations representative of sub-environments within a sedimentary system were identified and differentiated. Finally, the sedimentary model of the Kashkan Formation was interpreted. Discussion of Results & Conclusions Field studies on the examined sections led to the identification of 12 lithofacies. These include coarse-grained conglomeratic lithofacies (Gm, Gms, Gp, Gh, Gt), medium-grained sandstone lithofacies (Sp, St, Sm, Sh, Sr), and fine-grained mudstone lithofacies (Fm, Fl). The architectural elements identified in the four studied sections include channel elements (CH), overbank fines (OF), gravelly bar elements (GB), sandy bar elements (SB), and sediment gravity flow elements (SG). The SG (sediment gravity flow) element is introduced as the main and dominant element in proximal braided rivers with debris-laden flows, which is entirely consistent with the high percentage of conglomeratic facies in the studied deposits. This element primarily consists of massive conglomerates with very poor bedding, a lack of large-scale sedimentary structures, and poor sorting. The formation of the SG element is attributed to high-energy fluvial flows, particularly debris flows and hyperconcentrated flows. These flows typically develop under conditions of sudden discharge increases, bank instability, or intense bedload activity within braided river channels, leading to the rapid deposition of coarse-grained sediment. In terms of depositional setting, the SG element is interpreted mainly as a fill of main and subsidiary channels within a braided river system, representing peak flow energy phases. The abundance of this architectural element indicates the predominance of high-energy conditions, a relatively steep channel gradient, and a continuous supply of coarse-grained sediment to the sedimentary basin. Therefore, the SG element in the studied deposits is presented directly within the framework of a braided river model with an effective role played by debris flows. Sequence stratigraphy of the Kashkan Formation—as a prominent representative of high-energy fluvial depositional environments in the Zagros (Lorestan) basin—provides a powerful tool for reconstructing the basin's evolutionary history and understanding the influence of sea-level fluctuations on river dynamics. The dominant lithological composition of the Kashkan Formation includes thick conglomerates, coarse-grained sandstones, and mudstone, which directly indicates high-energy depositional conditions, such as those found in braided river systems. The significance of sequence stratigraphy for the Kashkan Formation extends beyond understanding sea-level changes; it holds the key to the tectonic history of the Zagros Basin by identifying sedimentary units related to erosion and deposition during periods of base-level fall. The formation of sedimentary sequences, particularly lowstand systems tracts (LSTs) that occur during periods of pronounced sea-level fall (i.e., reduction in river base level), is often associated with deep incision of riverbeds and the development of valleys and channels that are subsequently filled by conglomeratic and sandy sediments. These processes are closely linked to regional tectonic uplifts and changes in basin slope. Therefore, identifying and delineating LST units within the Kashkan Formation can serve as an indicator of tectonic activity and uplift events associated with the Zagros orogeny. Based on the vertical and lateral changes in facies, the depositional environment of these deposits was a proximal braided river system with a gravelly and sandy bed. Paleocurrent direction analysis for this formation shows a northeast-to-southwest trend, which is evidence of unidirectional (fluvial) flows within the Kashkan Formation. The shallow depth and low sinuosity of the channels, along with the presence of coarse-grained gravelly and sandy sediments, support the interpretation that the clastic facies of the Kashkan Formation accumulated in a proximal braided river environment.
AbstractThe Dalan Formation, a major carbonate-evaporite succession in the Persian Gulf Basin, is a significant gas-bearing reservoir in fields such as South Pars. This study integrates mud gas geochemistry and petrophysical evaluation to characterize hydrocarbon fluid types and assess reservoir quality in zones K3 and K4. Mud gas concentrations (C1–C8) were analyzed using Pixler diagrams and Haworth parameters (Wh, Bh, Ch), while petrophysical evaluation included gamma-ray, density, neutron, and resistivity logs to estimate effective porosity, water saturation, and hydrocarbon saturation. Results indicate that Zone K3 is a dry gas reservoir dominated by methane and ethane, with moderate to low porosity and hydrocarbon saturation, and limited economic potential. Zone K4 exhibits dual behavior, with a lower water-bearing interval of poor quality and an upper interval of moderate to good quality containing wet gas and light condensates. Integration of geochemical and petrophysical data provides accurate identification of hydrocarbon types, productive intervals, and fluid contacts, offering a robust scientific basis for reservoir development planning. This approach is particularly valuable in complex carbonate-evaporite reservoirs, reducing exploration risk and optimizing production strategies.Keywords: Dalan Formation, Mud gas analysis, Reservoir petrophysics, Haworth-Pixler gas ratios, South Pars IntroductionThe Dalan Formation, one of the most significant carbonate-evaporite sequences in the Persian Gulf Basin, plays a crucial role in the country’s gas supply. Despite extensive petrophysical studies, integrated analyses combining mud gas geochemistry and petrophysical data have received limited attention. Identifying hydrocarbon fluid types and evaluating reservoir quality, particularly in zones K3 and K4, is essential for optimizing exploration and production strategies. This study aims to address this gap by integrating real-time mud gas measurements with well log and petrophysical data from a well in the South Pars gas field. Materials & MethodsMud gas data, including hydrocarbon concentrations from methane (C1) to heavier hydrocarbons (up to C8), were recorded continuously during drilling using on-site mud logging systems equipped with gas chromatographs and flame ionization detectors. Quality control procedures were applied to remove background gas, drilling-induced artifacts, and unreliable measurements using the Gas QC index. Valid data were analyzed through Pixler diagrams and Haworth parameters (Wh, Bh, Ch) to identify hydrocarbon types and productive intervals. Petrophysical evaluation utilized gamma-ray, density, neutron, and resistivity logs, combined with lithological and mineralogical interpretation, to estimate effective porosity, water saturation, and hydrocarbon saturation. Probabilistic petrophysical models enabled correlation between fluid types and reservoir quality, providing a comprehensive understanding of the K3 and K4 intervals. Discussion of Results & ConclusionsZone K3 (3262–3379 m) is predominantly a dry gas reservoir, dominated by methane and ethane. Petrophysical analysis indicates moderate to low effective porosity (3–20%) and hydrocarbon saturation (40–60%), within the lower interval (3300–3382 m) displaying poor reservoir quality due to high anhydrite content and elevated water saturation. Mud gas ratios (C1/C2–C1/C5) and Haworth indices consistently indicate a dry gas system with limited heavier hydrocarbons.Zone K4 (3382–3547 m) exhibits dual behavior: the lower interval (3385–3450 m) shows low porosity, high water saturation, and poor reservoir quality, whereas the upper interval (3450–3500 m) demonstrates moderate to good quality, with effective porosity of 5–20% and water saturation of 40–70%. Mud gas geochemistry and Haworth analysis reveal the presence of heavier hydrocarbons and condensates (wet gas), corroborated by increased resistivity and cross-over effects in density–neutron logs. Pixler diagrams indicate that while the majority of K4 remains in the gas domain, certain depths approach the gas/oil boundary, confirming the existence of mixed fluids.The integration of mud gas geochemistry and petrophysical data highlights significant heterogeneity within the Dalan Formation. Zone K3 is a mature dry gas reservoir with limited economic potential, while the upper interval of K4 contains condensates that enhance production prospects.Conventional methods such as well logging and reservoir testing, although valuable, may be insufficient in complex carbonate-evaporite settings due to lithological heterogeneity and operational limitations. In contrast, mud gas analysis provides a rapid, cost-effective, and complementary approach to assess hydrocarbon types, identify productive zones, and reduce exploration risk. The combined use of Pixler and Haworth methods allows precise differentiation between dry gas, wet gas, and light condensates, enhancing the accuracy of reservoir characterization.Data were collected from a single well, limiting the ability to directly generalize results across the entire South Pars field. Instrumental errors, depth alignment uncertainties, and variations in drilling conditions may influence the accuracy of gas geochemistry measurements. Nevertheless, the study provides a reliable scientific basis for understanding fluid distribution in the upper Dalan Formation. Conclusions of this study follows:Zone K3 is a dry gas reservoir dominated by methane and ethane, with low to moderate reservoir quality and limited production potential.Zone K4 consists of a lower poor-quality, water-bearing interval and an upper interval with moderate to good quality containing wet gas and light condensates, contrary to prior assumptions of exclusively dry gas.Integration of mud gas geochemistry and probabilistic petrophysics enables accurate identification of hydrocarbon types, productive intervals, and fluid contacts.The upper K4 interval represents a valuable target for condensate production and should be prioritized in reservoir development planning.This integrated approach provides critical insights for the South Pars gas field and similar complex carbonate-evaporite reservoirs, offering a scientific basis for optimized well placement, production strategy, and risk mitigation.
Abstract The Asmari Formation, one of the most important carbonate reservoirs in the Zagros Basin, was studied in the Chahar Bishe Oil Field to identify its facies, depositional environments, and sequence stratigraphic framework. Examination of 316 meters of core and cutting samples of a well from Chahar Bishe Oil Field led to the recognition of fourteen carbonate facies that were deposited within four main facies belts: tidal flat, semi-restricted lagoon, reefal barrier, and open marine. The vertical succession of facies indicates a general shallowing-upward trend from open-marine to nearshore environments, suggesting deposition on a low-angle homoclinal carbonate ramp. Sequence stratigraphic analysis resulted in the identification of three third-order depositional sequences, each composed of a Transgressive Systems Tract (TST) and a Highstand Systems Tract (HST). Type-2 sequence boundaries and Maximum Flooding Surfaces (MFS) were clearly recognized, reflecting relative sea-level fluctuations during the Miocene. The results of this study define the Asmari Formation as a homoclinal carbonate ramp in which facies distribution was mainly controlled by sea-level oscillations and variations in depositional energy. Keywords: Asmari Formation, Carbonate facies, Depositional environment, Sequence stratigraphy, Chahar Bishe Oil Field Introduction The Asmari Formation represents one of the most significant carbonate reservoirs within the Zagros Basin and plays a fundamental role in hydrocarbon production across southwestern Iran. Deposited during the Late Oligocene to Early Miocene, this formation records the evolution of a shallow-marine carbonate platform that developed widely across the Dezful Embayment and adjacent sub-basins. Previous studies have consistently interpreted the Asmari depositional system as a homoclinal carbonate ramp, characterized by a gradual transition from tidal flat to open-marine environments. However, despite extensive investigations in major oilfields such as Ahvaz and Marun, relatively limited data are available from smaller fields such as the Chahar Bishe Oil Field. This lack of detailed facies and sequence stratigraphic frameworks introduces uncertainties in reservoir correlation and geological modeling at local scales. The present study aims to (1) identify carbonate facies and depositional environments, (2) reconstruct the depositional model, and (3) establish a sequence stratigraphic framework for the Asmari Formation in the Chahar Bishe Oil Field. The results provide new insights into local depositional controls and improve the understanding of reservoir heterogeneity within the Zagros Basin. Material & Methods This study is based on the analysis of 316 meters of core and cutting samples obtained from well No. 2 in the Chahar Bishe Oil Field. A total of 105 thin sections were prepared and examined using a polarizing microscope for microfacies analysis. Facies classification was conducted based on standard carbonate microfacies schemes, integrating textural and compositional attributes. Identified components include skeletal grains (e.g., echinoids, bryozoans, red algae, larger benthic foraminifera such as miliolids and nummulitids) and non-skeletal grains (e.g., peloids, ooids, intraclasts). Sequence stratigraphic interpretation was carried out using vertical facies stacking patterns and key stratigraphic surfaces, following established models. Key surfaces such as sequence boundaries (SB) and MFS were identified, and depositional sequences were subdivided into TST and HST. Discussion of Results & Conclusions Facies analysis and depositional environments: In this study, fourteen carbonate facies were identified and grouped into four main depositional belts as follows: Tidal flat (Facies A): Characterized by dolomudstone and mudstone with fenestral fabrics, bird’s-eye structures, and evaporitic minerals, indicating supratidal to intertidal conditions with restricted circulation. Semi-restricted lagoon (Facies B): Dominated by wackestone to packstone textures with miliolids, peloids, and limited faunal diversity, reflecting low-energy, restricted marine conditions. Reefal barrier (Facies C): Composed of coral framestones and floatstones, representing high-energy environments within the euphotic zone. Open marine (Facies D): Characterized by floatstone facies rich in red algae, bryozoans, and large hyaline foraminifera, indicating deposition in deeper, low-energy settings with normal marine salinity. The vertical facies succession exhibits a general shallowing-upward trend, transitioning from open marine to tidal flat environments, consistent with progradational stacking patterns. Depositional model: The absence of turbiditic deposits, lack of abrupt facies changes, and gradual lateral transitions indicate deposition on a low-gradient homoclinal carbonate ramp. The platform consists of inner, middle, and outer ramp settings, reflecting a continuum of depositional energy from low-energy open marine environments to high-energy shoal and tidal flat settings. Sequence stratigraphy: Three third-order depositional sequences (DS1–DS3) were identified within the studied interval.DS1: Initiated with open-marine facies and culminated in tidal flat deposits. The MFS is marked by the deepest marine facies. DS2: Defined by a Type-2 sequence boundary (SB2), with a transgressive trend from tidal flat to open marine followed by regression. DS3: Characterized by a transgressive phase from tidal flat to reefal environments, followed by a highstand dominated by shallow lagoonal and tidal deposits. Each sequence consists of TST and HST systems tracts, with clearly identifiable MFS surfaces. The stacking patterns reflect relative sea-level fluctuations during the Miocene. Facies distribution within the Asmari Formation is primarily controlled by relative sea-level changes and depositional energy gradients along the carbonate ramp. The development of restricted lagoonal facies and tidal flat deposits during highstand conditions suggests reduced accommodation space and increased progradation. In contrast, transgressive phases are marked by the expansion of open-marine and reefal facies, indicating increased accommodation and deeper depositional conditions. The dominance of homoclinal ramp geometry suggests minimal tectonic segmentation during deposition, although subtle variations in sequence thickness may reflect localized subsidence patterns. Comparison with regional studies indicates a strong correspondence between identified sequence boundaries and global sea-level curves, suggesting that eustatic controls played a major role, superimposed on regional tectonic influences. Fourteen carbonate facies were identified, reflecting a systematic variation in depositional energy and water depth. The vertical facies distribution indicates an overall shallowing-upward trend. The Asmari Formation in the Chahar Bishe Oil Field was deposited on a low-angle homoclinal carbonate ramp comprising four main depositional environments: tidal flat, lagoon, reef, and open marine. Sequence stratigraphic analysis reveals three third-order depositional sequences, each composed of TST and HST systems tracts, bounded by Type-2 sequence boundaries and marked by well-developed MFS. These findings highlight the dominant role of relative sea-level fluctuations in controlling facies architecture and depositional cycles, providing a robust framework for reservoir characterization and stratigraphic correlation in the Zagros Basin.
AbstractThe purpose of this study was to investigate the mineralogy, geochemistry, and provenance of clastic strata of the Aghajari Formation (Upper Miocene–Pliocene) in the north of Hoseynieh and Andimeshk. Petrographic studies and modal analyses of sandstones indicate that quartz, feldspar of igneous origin, and lithic fragments (igneous, metamorphic, and sedimentary) are the main constituents of the sandstones. It was also revealed that the sandstone intervals fall within the recycled or transitional recycled orogenic provenance field, which accumulated during the late Miocene–Pliocene under warm and semi-arid climates. The clay mineral content of these sediments is mainly chlorite and illite, of detrital origin. Th/Co versus La/Sc cross-plots indicated a siliceous source rock, and Ti/Zr versus La/Sc and La/Th versus Hf suggest andesitic source rocks for the Aghajari Formation. Furthermore, Th/Sc versus Zr/Sc showed a first-order sedimentation cycle. The orogenic events during Miocene–Pliocene (the Savian and Strian tectonic phases) in the Folded Zagros led to the erosion of a mixture of igneous–ophiolitic rocks from the Neotethyan oceanic crust and metamorphic rocks exhumed in northern Lorestan and Kermanshah regions, along with the sedimentary successions of the folded Zagros (the Amiran, Talehzang, Kashkan, Shahbazan and Asmari formations). These formations supplied the clastic sediments of the Aghajari Formation within the Hoseynieh and Andimeshk syncline.Keywords: Petrography, Geochemistry, Aghajari Formation, Provenance, Zagros Basin IntroductionSedimentary rocks are the main source of information about past conditions. Based on the chemical composition of sediments and siliciclastic sedimentary rocks, the provenance and other depositional processes, such as weathering, transportation, and diagenesis, can be evaluated (Dickinson and Suczek 1979; Sharafi et al. 2018; Asiedu et al. 2019; Salehi et al., 2018; Zamanian et al. 2019; Peng et al. 2020). Among the various controlling factors, the tectonic setting and lithology of the source area are the most important parameters governing the formation of siliciclastic deposits (Yan et al. 2012; Salehi et al., 2014; Khazaei et al. 2018). The tectonic setting of siliciclastic rocks can be interpreted using petrographic and geochemical data through discrimination diagrams, as these reflect sediment-generation processes, source-area morphology, and paleoclimatic conditions (Sabbagh et al. 2018; Pourdivanbeigi Moghaddam et al., 2020). The Aghajari Formation, the youngest unit of the Fars Group, was formally introduced by James and Wynd (1965) and consists of a thick succession of syn-orogenic red molasse deposits widely distributed throughout the Zagros region. Its age ranges from Middle Miocene to Pliocene and varies spatially across the basin (Motiee 2003). In this study, mineralogical and geochemical evidence are used to investigate the provenance of the Aghajari siliciclastic strata. Since sedimentary rocks represent the main remnants of eroded ancient crust (Condie et al. 2001; Basu 2003), provenance analysis provides key insights into the Miocene–Pliocene tectonic evolution of the Zagros Fold-Thrust Belt.Material & MethodsIn this study, three stratigraphic sections of the Aghajari Formation, including Paalam (600 m), Khoshab (550 m), and Piravali (520 m), were investigated in the Hosseinieh area of the Andimeshk region. Sampling in each section was carried out based on facies variations and, in some cases, systematically. A total of 180 rock samples (60 hand specimens from each section) were collected from different facies. To investigate the provenance of the Aghajari siliciclastic strata, 60 sandstone samples (mostly coarse-grained) were selected for thin-section preparation. From these, 17 thin sections (7 from the Aghajari Formation and 10 from the Lahbari Member) were selected for modal analysis. The percentages of cement and matrix in each sample were calculated relative to the total rock framework. To determine major and trace elements, identify lithological characteristics, and evaluate the tectonic setting, 20 mudstone samples were analyzed using ICP-MS at the Zarazma Laboratory (Tehran). These samples were selected to represent the entire stratigraphic succession under study. Additionally, 10 mudstone samples were analyzed by X-ray diffraction (XRD) at the Central Laboratory of Lorestan University to identify the mineralogical composition of fine-grained deposits. Modal analysis of sandstones was performed by counting 300–500 points per thin section following the Gazzi–Dickinson method (Dickinson 1970; Ingersoll et al. 1984), and sandstone classification was carried out according to Folk (1980). Petrographic and geochemical data, together with established discrimination diagrams and the Chemical Index of Alteration (CIA) (Nesbitt & Young, 1984), were used to infer the tectonic setting, palaeoclimate, and paleocurrent patterns. Discussion of Results & ConclusionsThe study area is located at the beginning of the Zagros folded zone. The Aghajari Formation is the main outcropping rock unit in the study area and consists mainly of sandstone, siltstone, claystone, mudstone, and shale. The investigated sandstones are mainly composed of sedimentary rock fragments, such as chert and carbonate fragments. The studied sandstones are generally poorly sorted, as seen in the thin sections. Petrographic studies revealed that sandstones of the Aghajari Formation are mainly lithic-arenite (sed-arenite), which is mostly composed of chertarenite and calclithite with calcite cement. The origin of the cement in these facies can be attributed to the dissolution of unstable carbonate grains, with an average composition of (Q38R60F2) in the upper part (Lehbari Member) and (Q33R66F1) in the lower part of the Aghajari Formation. The clay mineral content of these sediments is mainly chlorite and illite, of detrital origin.The grain size and particle geometry, as well as the degree of sorting of the studied sandstones, indicate that they are texturally immature in terms of textural maturity. Th/Co versus La/Sc cross-plots indicated a siliceous source rock, and Ti/Zr versus La/Sc and La/Th versus Hf suggest mixed acidic–basic and andesitic source signatures in the studied samples for the Aghajari Formation. Furthermore, Th/Sc versus Zr/Sc showed a first-order sedimentary cycle. The petrographic and geochemical studies of sandstones in the Aghajari Formation indicate that the investigated sediments have a transitional recycled origin, with recycling in an orogenic zone. Moreover, the results of the modal analysis indicate that the climatic conditions were semi-arid during the deposition of this formation. According to geochemical analyses, the investigated sandstones originated from intermediate or andesitic rocks. Furthermore, geochemical diagrams indicate that the studied sandstones formed in an oceanic arc-island field setting. The geochemical data in the A-CN-K triangular diagram indicate moderately weathered conditions in the source area of the Aghajari Formation.
AbstractIn this study, the stratigraphic distribution of foraminifera and the environmental evolution of the Permian–Triassic boundary (PTB) successions in the central Persian Gulf (upper Dalan and the basal part of the Kangan formations) were investigated through the integration of micropaleontological, microfacies, and isotopic analyses (δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr). The results reveal three local composite biozones within the upper Dalan and lower Kangan formations, along with the abundance of the index taxon Paradagmarita, which shows biogeographic affinity with approximately coeval strata in Turkey, the Caucasus, Saudi Arabia, and Oman. Microfacies analysis identified three facies belts including lagoonal, subtidal shoal, and tidal flat, indicating a general shallowing trend from lagoonal toward peritidal settings. A simultaneous decrease in δ¹³C and δ¹⁸O values was observed near the boundary, coinciding with the biotic crisis and the development of oxygen-depleted conditions. The increase in ⁸⁷Sr/⁸⁶Sr ratios across the boundary suggests enhanced influx of continental materials and intensified chemical weathering. The results also indicate a moderate increase in ooid size and significant changes in foraminiferal assemblages, reflecting a relatively shallow, warm, stressed, and oxygen-deficient environment. The novelty of this study lies in the combined application of three independent datasets, including sedimentological, geochemical, and paleontological, from multiple subsurface sections, enabling a more precise reconstruction of the environmental evolution and biotic events at the onset of the Triassic.Keywords: Foraminifera, Microfacies, Ooids, Carbon and oxygen isotopes, ⁸⁷Sr/⁸⁶Sr, Permian–Triassic boundary, Persian Gulf IntroductionThe Permian–Triassic transition marks the most profound biotic crisis in Earth’s history, with over 90% of marine species becoming extinct. This event significantly transformed carbonate platforms and their sedimentary environments. The Dalan and Kangan formations, representing the Late Permian to Early Triassic succession in the Zagros and Persian Gulf regions, record these changes in detail. Previous works have discussed the sedimentological and geochemical evolution of this interval (Abdolmaleki and Tavakoli 2016; Rafiei et al. 2016; Tavakoli et al. 2018; Haghighat et al. 2020). However, few studies have combined micropaleontological, isotopic, and microfacies evidence from multiple subsurface sections to elucidate the precise environmental evolution during this critical interval. The present study provides a comprehensive reconstruction of the Permian–Triassic boundary (PTB) based on integrated data from four wells in the central Persian Gulf. Materials & MethodsThe study is based on petrographic and geochemical analyses of 2,500 thin sections from the Dalan and Kangan formations in four wells (A, B, E, and F). Samples were taken at 30 cm intervals to ensure high stratigraphic resolution. Foraminiferal assemblages were identified under transmitted and polarized light microscopes. Microfacies were described and classified following Folk (1959), Dunham (1962), and Embry & Klovan (1971). Stable isotopes (δ¹³C and δ¹⁸O) were analyzed on micritic matrix samples to minimize diagenetic alteration. Samples with micritic fabrics (wackestones and fine packstones) were preferred due to their low permeability and better preservation of primary isotopic composition. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) were determined from wells B, E, and F. Petrographic and geochemical screening confirmed the absence of secondary dolomitization or recrystallization features in the selected samples, following the procedures described in Tavakoli (2015) and Abdolmaleki and Tavakoli (2016). Discussion of Results & ConclusionsForaminiferal Assemblages: Micropaleontological analysis revealed 51 species (41 genera) of benthic foraminifera dominated by Globivalvulina, Dagmarita, and Paradagmarita. Two major biozones were defined: the Glomomidiellopsis–Paradagmarita Assemblage Zone (Changhsingian) and the Microconchus–Ammodiscus kalhori Assemblage Zone (Griesbachian). These assemblages correlate well with equivalent zones in Saudi Arabia, Oman, Turkey, and the Caucasus (Haghighat et al. 2020). The disappearance of Late Permian taxa and the dominance of opportunistic forms (Ammodiscus kalhori, Microconchus phlyctaena) above the boundary indicate severe environmental stress and low-oxygen conditions.Microfacies Evolution: Nine microfacies types were recognized, ranging from ooid and bioclastic grainstones to thrombolitic boundstones and dolomitic mudstones. The upward transition from subtidal grainstones to peritidal thrombolitic facies indicates progressive shallowing. Ooid grainstones in the uppermost Dalan suggest deposition in high-energy shoal settings. In contrast, microbial boundstones in the lowermost Kangan represent early recovery of carbonate production under oxygen-depleted, restricted conditions. This pattern matches earlier models for the PTB carbonates in the Persian Gulf (Tavakoli et al. 2018; Davoodi et al. 2024).Isotopic Trends: The isotopic curves show a pronounced negative excursion in δ¹³C (from +4‰ to −1‰) and δ¹⁸O, synchronous with a rise in ⁸⁷Sr/⁸⁶Sr ratios across the PTB. The δ¹³C drop corresponds to global disturbances in the carbon cycle, potentially related to methane release, enhanced continental weathering, and volcanogenic CO₂ emissions (Tavakoli and Rahimpour-Bonab 2012). The gradual depletion of δ¹⁸O toward the boundary likely reflects rising seawater temperature and meteoric diagenesis under greenhouse conditions (Abdolmaleki and Tavakoli 2016; Naderi-Khujin et al. 2016). Elevated ⁸⁷Sr/⁸⁶Sr ratios across the boundary further support intensified continental weathering and influx of radiogenic strontium during the end-Permian climatic crisis.Integrated Interpretation: Integration of faunal, sedimentological, and isotopic data indicates a shift from a well-oxygenated, open-marine platform during the Late Permian to a restricted, shallow, and stressed lagoonal system in the Early Triassic. The decline in biodiversity, increase in microbial facies, and negative isotopic excursions reflect the combined effects of eustatic fall, climatic warming, and oceanic anoxia. These changes represent the regional expression of global end-Permian perturbations. The integrated multiproxy approach employed here refines previous reconstructions (Haghighat et al. 2020; Nazemi et al. 2021; Davoodi et al. 2024; Rezvannia et al. 2025; Shahkaram et al. 2025) and provides a more complete picture of the environmental transition in the central Persian Gulf.
AbstractBrachiopods, as one of the most important benthic fauna in the Late Ordovician, show a great abundance and diversity in many continents. In this study, a quantitative approach of multivariate analyses was conducted in order to study the palaeobiogeography of the brachiopods of Iran and their relationship with the brachiopods from other parts of the world, such as Baltica, Avalonia, Laurentia, South China, Kazakhstan, France, and Portugal, during the Late Ordovician (early Katian). Based on the results of the cluster analysis (CA), six main clusters and two sub-clusters were distinguished. The results obtained from the principal component analysis (PCA) method indicate the differentiation of seven main groups and show a high similarity with the main clusters obtained from the CA method. Based on the results of CA, the brachiopods of Bojnourd, Iran, are placed in a separate cluster together with the Avalonian brachiopods such as Shropshire, Powys (Wales), Anglesey (Wales), and Meath (Ireland). Based on the PCA scatter plot, the brachiopods of Bojnourd are placed in the same group with the Avalonian brachiopods. They are distinguished from the brachiopods of the Zagros and Anarak regions in central Iran, which form a separate group with the brachiopods of France, Portugal, and Morocco. The results of the PCA method indicate that the brachiopods from intracratonic Laurentian Basin such as New York, Manitoulin Island, Kentucky, and Indiana form a distinct group and are differentiated from brachiopods from continental margin such as the Appalachian Basin, Tyrone (Northern Ireland), and Girvan (Scotland).Keywords: Brachiopods, Late Ordovician, Palaeobiogeography, Multivariate analyses, Katian IntroductionDuring the Late Ordovician, much of Laurentia was covered by a shallow epicontinental sea that created carbonate platforms in the intracontinental basins and pericratonic shelves (Finnegan et al. 2012). This marine transgression event and the creation of carbonate structures likely indicate a greenhouse warming episode in the Late Ordovician. In addition, some sedimentological and geochemical data support the hypothesis of a cooling episode in the Late Ordovician, leading to the cold and glacial climate of the Hirnantian (Page et al. 2007; Trotter et al. 2008; Buggisch et al. 2010). Interpretations of the climate change during the Late Ordovician are controversial and, like the Boda event in the late Katian, have been interpreted as both a warming and a cooling episode (Fortey & Cocks 2005; Cherns & Wheeley 2007).As one of the most important groups of marine invertebrates of the Paleozoic, brachiopods have a high diversity and abundance in the Ordovician and are therefore of great importance in palaeobiogeographic studies. According to Webby (2000), major fossil groups such as brachiopods show three global diversity maximum during the “Great Ordovician Diversification Event,”. Each palaeocontinent had different brachiopod diversification trajectories during the Ordovician. Based on Harper & Rong (2001), rhynchonelliform brachiopods diversified during the Dapingian and Darriwilian. The Ordovician brachiopods of Baltica show four diversity maxima in the mid-Darriwilian, late Darriwilian, late Sandbian, and late Katian and are different from the brachiopod diversity curves of Avalonia and Gondwana. The brachiopods of Gondwana had one diversity maxima during the late Sandbian (Hints & Harper 2001; Harper & Mac Nicaill 2002; Harper 2006). In South China, brachiopods had three diversity maxima in the early Floian, late Darriwilian, and late Katian (Zhan & Harper 2006).During the late Darriwilian, the earliest rhynchonellid brachiopods appeared in shallow marine environments of palaeotropical regions, including Laurentia, Siberia, and Kazakhstan (Jin 1996). During the Sandbian and early Katian, the total number of rhynchonellide genera increased from five to fifteen. By the late Katian, rhynchonellide brachiopods became widespread in the epicontinental seas of Laurentia and some of the genera such as Hiscobeccus was endemic to Laurentia (Sohrabi & Jin 2013a).The controlling mechanism for the major changes in biogeographic patterns is not well understood. Because of the importance of the Katian brachiopods in palaeogeographic interpretations, a quantitative approach was conducted to investigate the Katian brachiopods of Iran and other regions of the world. Measuring the faunal similarity of the Iranian brachiopods with those from other regions of the world can provide a comprehensive interpretation of the palaeoclimatic and palaeogeographic control on the brachiopods’ evolution and their changing biogeographic patterns. Material & Methods In this study, the palaeobiogeography of the Late Ordovician brachiopods was investigated. The brachiopod data were compiled from various regions, including Baltica, Avalonia, Laurentia, Australia, Kazakhstan, France, South China, Portugal, Morocco, and Iran. Most of the brachiopod data in the present study are from the formations of the early Katian.The Laurentian brachiopods data were obtained from various regions of North America, including the Ottawa Valley, Lake Simcoe, northern Rocky Mountains, western Newfoundland, Manitoulin Island, New York, Kentucky, Indiana, Oklahoma, South Dakota, Nevada, California, Mississippi Valley, Champlain Valley, Hudson Valley, Appalachian Valley, Girvan (Scotland), and Tyrone (Northern Ireland). The brachiopods of Baltica are from the eastern Baltic (Estonia and Lithuania) of Keila and Oandu age (Rõõmusoks 2004; Hints 1998, 2010). The brachiopods of southern Norway are from the Oslo-Asker area, which are related to deep-water facies (Hansen 2008). The Late Ordovician brachiopods of Avalonia are from Waterford and Wexford (southeast Ireland), Meath (east Ireland), and Powys and Anglesey (Wales) (Cocks 2008). In Britain and Ireland, the late Sandbian–early Katian strata include Scotland, Shropshire, Wales, and Ireland. The brachiopods of Scotland are from the Caradoc aged formations in the Girvan area including the Craighead Limestone, Myoch Formation, Whitehouse Bay, and Albany Mudstone Formation. The brachiopods of the Shropshire region include the Caradoc aged formations such as the Acton Scott, Onny Shale, Cheney Longville, Spy Wood Grit, Horderley Sandstone, Whittery Shale, Hagley Shale, Whittery Volcanic, Hoar Edge Grit, Woolston, Smeathen Wood Beds, and Cheney Longville formations. The brachiopods of northwest Wales, Anglesey, include Sandbian–early Katian formations such as the Garn Formation, Llanbabo Formation, and Crewyn Formation. In northern Ireland, the brachiopods are from the Tyrone region including the Bardahessiagh Formation of the Burrellian age. The brachiopods of southern and southeastern Ireland are from Meath, Wexford, and Waterford and include the Burrellian age formations of the Duncannon Group, such as the Annestown Formation, Lower Tramore Volcanic Formation, Grange Hill Formation, Upper Tuffs and Shales of Grangegeeth Volcanic Series, Collon Formation, and Clashford House Formation.The early Katian brachiopods of Kazakhstan are related to the Chu–Ili, Ishim–Selety, and Boshchekul terrains and include the Anderken and Dolankara formations (Popov et al. 2002; Nikitin et al. 2006).In South China, the early Katian brachiopods include the Pagoda Formation (Zhan & Jin 2007; Bergstrӧm et al. 2009). The early Katian brachiopods of Portugal are related to the Cabeço do Peão and Ferradosa formations (Henry & Romano 1978; Cooper 1980; Romano 1980, 1982, 1991; Young 1985, 1988). In Morocco, the Late Ordovician brachiopods are from the Khabt-el-Hajar Formation (Fortey & Cocks 2005). Most of the Katian rhynchonelliform brachiopods of Gondwana are from the western Mediterranean regions such as France, Spain, and Portugal (Torsvik & Cocks 2011). In Iran, the Late Ordovician brachiopods include the Bojnourd region (Ghelli Formation), the Anarak region in Central Iran (Chah Gonbad Formation), and the Zagros region (Siahu Formation). Multivariate analyses: The data used in this study include a large number of genera in general, as well as a large number of endemic genera from different regions. In this study, multivariate analyses were conducted based on the early Katian brachiopods dataset to investigate the palaeobiogeography of the brachiopods of Iran and their relationships with the other brachiopods from Baltica, Avalonia, Laurentia, Australia, Kazakhstan, France, South China, Portugal, and Morocco.The dataset was generated based on binary data that includes 261 brachiopod genera of the early Katian from 30 geographical regions. In this dataset, the geographic regions were selected as locations and the brachiopod genera as variables (presence or absence). The dataset was subjected to multivariate analyses using PAST software (Hammer et al. 2001; Hammer & Harper 2006), which was developed for analyzing paleontological data.In order to distinguish the distribution patterns of the brachiopods in time and space, cluster analysis (CA) and principal component analysis (PCA) methods were employed. To perform cluster analysis (CA), a dendrogram algorithm was generated based on the paired group method by using the Raup-Crick similarity coefficient. The Raup-Crick similarity coefficient shows fully segregated clusters.The dataset was also subjected to PCA by using the variance-covariance algorithm in PAST software. The result was plotted in the PCA scatterplot, based on principal components 1 (X-axis) and 2 (Y-axis), which correspond to eigenvalues 1 and 2, respectively. Discussion of Results & Conclusions In this study, the results of multivariate analyses of the Late Ordovician (early Katian) brachiopods from 30 geographical regions including Laurentia, Baltica, Avalonia, South China, Kazakhstan, France, Portugal, Morocco, and Iran indicate several palaeogeographic patterns. Based on the results of CA and PCA, several clusters and groups were recognized.The results of CA indicate several distinct clusters. Based on the CA dendrogram, five main clusters (A–F) and four subclusters (A1 and A2) were identified.Cluster A consists of two subgroups, clusters A1 and A2. Cluster A1 contains the brachiopods of Lithuania-North Estonia in the East Baltic, which indicate relatively shallow and warm-water carbonate environments during the early Katian. Cluster A2 comprises the brachiopods from Shropshire, Meath (Ireland), Anglesey and Powys in Wales, and Bojnourd in Iran. The cluster of Bojnourd brachiopods with the brachiopods of Shropshire, Meath (Ireland), and other Welsh regions indicates the close affinity of the Bojnourd brachiopod fauna with those of Avalonia during the early Katian. Cluster B includes western Newfoundland, New York, Kentucky, Indiana, Manitoulin Island, Lake Simcoe, British Columbia, Appalachian Basin, Great Basin, Tyrone (Northern Ireland), and Girvan (Scotland). Western Newfoundland, New York, Kentucky, Indiana, Manitoulin Island, and Lake Simcoe were located on tropical carbonate platforms at mid- to high-latitudes. The Appalachian Basin (from Pennsylvania and Tennessee to Alabama), British Columbia (Advanced Formation), Girvan (Scotland), and Tyrone (Northern Ireland) represent the continental margin of Laurentia during the Late Ordovician. Cluster C includes brachiopods from southeastern Ireland (Wexford and Waterford), South China, and southern Norway (Oslo-Osker), and corresponds to cluster D in the PCA plot. Cluster D in the CA analysis represents the brachiopods from France and Portugal. Cluster E in this dendrogram includes the brachiopods from the Zagros and Anarak regions of Iran.Cluster F in the dendrogram includes the brachiopod faunas of Kazakhstan terranes such as Chu-Il, Boshchekul, and Ishim-Selety and shows low similarity with the brachiopod faunas from other regions. The brachiopod faunas of Morocco are separated from other brachiopod faunas in this dendrogram, which indicates their low similarities with the brachiopod fauna from other areas.In the PCA method, the brachiopod data were scattered in the PCA plot based on principal components 1 (X-axis) and 2 (Y-axis). According to the PCA scatterplot, seven groups A1, A2, B, C, D, E, and F were distinguished, which are similar to the clusters identified in the CA dendrogram.Group A1 consists of the brachiopod faunas from the eastern Baltic, Lithuania, and northern Estonia, and shows consistency with cluster A1 in the CA plot. Group A2 includes the brachiopod faunas from Shropshire, Powys (Wales), Anglesey (Wales), Meath (Ireland), and Bojnourd (Iran). It is interesting to note that the brachiopod faunas of the Bojnourd region are located among the Avalonian brachiopods within cluster A2 in the CA diagram.Group B includes the brachiopod faunas of epicontinental Laurentian such as Lake Simcoe, Manitoulin Island, New York, Kentucky, and Indiana and are located in proximity to the Avalonian regions. The position of the brachiopod faunas of Bojnourd close to the Avalonian and epicontinental Laurentian brachiopod faunas indicates the similarity of the brachiopods from this region of Kopeh-Dagh in northwestern Iran with those of Laurentia and Avalonia during Katian time.With the onset of the marine transgression over Laurentia during the early Katian, the brachiopod faunas of North America began to show a distinction between pericratonic and intracratonic settings. Scoto-Appalachian brachiopod fauna on the southeastern margin of Laurentia were more closely related to the brachiopod fauna of Avalonian and deep-water Baltica facies than to the intracratonic Laurentian fauna. In contrast, the intracratonic (epicontinental) Laurentian brachiopod fauna was more similar to the Lithuanian-northern Estonian brachiopod fauna than to the Scoto-Appalachian brachiopod fauna on the cratonic Laurentian margin.Group C, in the right portion of the PCA plot, shows brachiopods from the Appalachian, Girvan (Scotland), and Tyrone (Northern Ireland) regions. This group, which is clearly separated from the fauna of other regions especially Laurentia, is consistent with the concept of the Scoto-Appalachian fauna that introduced by Jaanusson (1979) and Whittington and Williams (1955). Group D in the PCA diagram corresponds to brachiopod faunas from the Oslo-Oskar area (Norway), South China, and southeastern Ireland (Wexford-Waterford), which corresponds to cluster C in the CA diagram.Group E includes brachiopods from the Zagros and Anarak regions of Iran, Morocco, France, and Portugal. In the CA diagram, the brachiopod faunas from the Zagros and Anarak regions are grouped in cluster E, and the brachiopod faunas from France and Portugal are grouped in cluster D, which indicates the high similarity of the brachiopods of these regions.The group F in the PCA plot corresponds to cluster F in the CA diagram and is related to the brachiopods of the Chu-Il, Boshchekul, and Ishim-Selety regions of Kazakhstan. According to the results of the CA and PCA, the brachiopods of the Kazakh regions show low similarity to the brachiopods of Laurentia, Baltica, and Avalonia, which could be due to the presence of endemic species in the Kazakh regions and very limited faunal connection with the brachiopods of the other areas during the Katian.The brachiopod faunas of epicontinental Laurentia from the Ottawa Valley, Lake Simcoe, Ontario, Manitoulin Island, western Newfoundland, New York, Hudson Valley, Champlain Valley, Kentucky, Indiana, Mississippi Valley, and Oklahoma show a higher similarity to the brachiopod faunas of mainly Avalonian origin than to the Scoto-Appalachian brachiopods of pericratonic regions.The early Katian Scoto-Appalachian brachiopods show a higher affinity to the brachiopods from the western margin of Laurentia such as the northern Rocky Mountains in British Columbia and Great Basin.The differentiation of Laurentian pericratonic and intracratonic brachiopod fauna during the early Katian has been interpreted as a palaeobiogeographical pattern (Sohrabi & Jin 2013).Based on the results of this study, the brachiopods of the Bojnourd region of Iran show more similarity with the Avalonian brachiopods rather than with the brachiopod faunas of the Zagros and Central Iran regions, as shown in cluster A2 in CA and Group 2 in the PCA diagram. The high degree of faunal similarity between the brachiopods of the Bojnourd region and the Avalonian brachiopods could be related to similar environmental conditions of these brachiopods during the early Katian.The close faunal affinity of the brachiopod faunas of the Bojnourd and Kopeh-Dagh regions with those of Avalonia was more likely attributable to the position of Bojnourd region of Iran at relatively lower latitudes which had different environmental conditions than those of the Zagros and Central Iran regions during the early Katian. Also, the low degree of faunal similarity between the Bojnourd brachiopods and the brachiopods of the same age in the Zagros and Central Iran regions could be interpreted as the beginning of brachiopod endemism in the Bojnourd region during the Late Ordovician (early Katian).During the early Katian, the Kopeh-Dagh region of Iran was more likely part of the microplates (terranes) adjacent to the supercontinent of Gondwana at similar latitudes to Avalonia and separated from the Zagros and Central Iran regions. The close similarity of the brachiopod faunas of Zagros and Central Iran with the brachiopods of France and Portugal, which were parts of high-latitude Gondwana, indicates their Gondwana palaeogeographical affinity during the Katian.By collecting more Late Ordovician brachiopods from different regions of Iran and compiling a comprehensive dataset, a better interpretation of the palaeobiogeographic pattern of the brachiopods can be obtained, which could result in a more accurate palaeogeographic positioning of Iran during the Late Ordovician.
.AbstractThe Permian–Triassic carbonates of the Kangan–Dalan formations in the central Persian Gulf represent one of the largest gas reservoirs in Iran and worldwide. Integrated petrophysical–core zonation, supported by multivariate cluster analysis, was applied to reduce reservoir heterogeneity and improve the understanding of reservoir properties. The results were compared with petrophysical logs, scanning electron microscopy, and pore-throat size distribution. Diagenetic processes were found to play a key role in reservoir quality. Fabric-destructive dolomitization in the upper K2 and lower K4 units generated micro-scale pathways connecting primary pores. Dissolution in the lower K2 and upper K4 units further enhanced pore connectivity, resulting in high permeability within these zones. In contrast, selective dolomitization and dissolution in K1 and K3 did not produce effective flow pathways. These processes led to the development of distinct reservoir zones, including high-porosity/low-permeability (Zone 2), high-permeability/low-porosity (Zone 3), and non-reservoir zones (Zones 4 and 5). Additionally, a thin interval with both high porosity and permeability (Zone 1) occurs as a transitional layer within the lower K4. Although diagenesis is the dominant control, primary depositional facies also influenced reservoir characteristics. All zones were successfully identified by the proposed algorithm.Keywords: Diagenesis, Petrophysical Zonation, Clustering, Permian–Triassic, Dolomitization IntroductionReservoir quality is one of the most critical parameters influencing the performance of hydrocarbon reservoirs, with pore-throat size distribution acting as a key controlling factor. This distribution is governed by both primary depositional attributes and secondary diagenetic processes (Tucker and Bathurst 1990; Cerepi et al. 2003; Baron et al. 2008). Diagenesis plays an especially significant role in carbonate reservoirs, where processes such as dolomitization, cementation, and dissolution can substantially alter porosity and permeability (Anselmetti and Eberli 1999). Studies have shown that reservoir zones with similar petrophysical behavior often reflect comparable diagenetic histories (Ehrenberg 2006).In many cases, core data are limited, highlighting the need for well-log analysis as an alternative tool. Several log responses are sensitive to diagenetic alterations, making them useful for reservoir characterization when integrated with petrographic observations. Among these, sonic logs and derived velocity-deviation curves have proven effective in distinguishing pore types and depositional–diagenetic trends, thereby enhancing geological and petrophysical interpretations. Multivariate cluster analysis is among the most powerful approaches for reservoir zonation. While it has been widely applied for electrofacies classification in both clastic and carbonate settings (Gill et al. 1993; Ye and Rabiller 2000), its large-scale application for defining reservoir zones remains limited. Combining well-log responses with petrographic and petrophysical parameters provides a more robust basis for identifying reservoir units.This study evaluates the application of multivariate cluster analysis for integrated petrophysical zonation in the Kangan and Dalan formations of the central Persian Gulf. The proposed workflow aims to establish a practical framework for accurate reservoir characterization by linking log responses, diagenetic features, and pore system evolution. Materials and MethodsA key well with 420 m of continuous core from the Kangan and Dalan formations in the central Persian Gulf was selected. Core plugs were taken every 30 cm, cut at both ends, and had thin sections prepared. One-third of each section was stained with Alizarin Red-S (Dickson 1966) to distinguish calcite from dolomite. Thin sections were studied under a polarizing microscope to record depositional facies and diagenetic features. Twenty representative samples were selected for scanning electron microscopy (SEM) and pore-throat size distribution was determined by mercury injection up to 60,000 psi.Core plugs were cleaned, dried, and analyzed for porosity (Boyle’s law) and permeability (Darcy’s law). Petrophysical data from neutron porosity, bulk density, photoelectric factor, and sonic logs were integrated with petrographic porosity estimates to assess the effects of dolomitization and dissolution. For data analysis, customized programming in MATLAB was applied instead of conventional software, providing flexibility in clustering, optimization, and algorithm testing. Hierarchical clustering was employed due to its ability to handle diverse datasets and dynamically determine the number of clusters (Xu and Tian 2015). To further constrain diagenetic effects, velocity-deviation logs were calculated by comparing measured and predicted compressional velocities. Positive deviations indicate cementation or compaction, while negative values reflect enhanced porosity from dissolution or fracturing.This integrated workflow allowed for precise petrophysical zonation by linking log responses, petrographic features, and diagenetic alterations. Discussion of Results & ConclusionsThe integrated zonation results reveal that intervals with similar porosity and permeability values can be effectively distinguished, reflecting the diverse impact of pore types on flow properties. Each identified zone shows a characteristic porosity–permeability distribution controlled by diagenetic processes. Zone 1 exhibits both high porosity and permeability, largely associated with touching-vug porosity (Lucia 1995) where dissolution, fracturing, and fabric-destructive dolomitization enhanced pore connectivity. This thin interval acts as a transitional layer between the upper dissolution-dominated K4 and the lower dolomitized K4. Zone 2 displays slightly lower porosity but still retains effective reservoir quality, corresponding mainly to the lower K2 and upper K4 where dissolution was the dominant diagenetic process. Zone 3 includes the largest number of samples, with moderate porosity but significantly enhanced permeability due to well-connected pore systems created by fabric-destructive dolomitization. In contrast, Zone 4, representing parts of K1 and K3, shows reduced reservoir quality as porosity is partly occluded by anhydrite cement, leading to low permeability despite moderate porosity values. Zone 5 is non-reservoir, characterized by very low porosity and permeability due to pervasive anhydrite cementation and loss of primary pores.Velocity-deviation analysis confirms these patterns. Negative deviations in Zones 1 and 3 indicate dissolution-related microporosity and microfractures, producing higher-than-expected permeability. Zone 2 shows values close to zero with a slight negative trend, reflecting micropores with limited connectivity. In contrast, positive deviations in Zone 4 clearly point to cementation and compaction, while Zone 5 shows weakly negative trends but insufficient pore connectivity to sustain flow. Thus, negative deviations are reliable indicators of dissolution-enhanced reservoirs, whereas positive values reflect cementation-dominated intervals.Mercury injection capillary pressure (MICP) analysis further supports these findings. Dolomitized samples with preserved primary porosity show relatively uniform pore-throat distributions and moderate permeability, while fabric-destructive dolomitization and dissolution create wider throat-size spectra and higher permeability (Zones 1–3). In contrast, limestone samples with moldic pores sealed by anhydrite exhibit poor pore connectivity and reduced flow potential (Zones 4–5). SEM observations confirm these relationships, showing dissolution-enhanced pore networks in productive zones versus anhydrite-filled throats in non-reservoir intervals.Overall, the Permian–Triassic carbonates of the Kangan and Dalan formations exhibit a complex diagenetic history that strongly influences reservoir quality. Five reservoir zones were defined by hierarchical clustering of well-log data (NPHI, RHOB, Sonic, PEF) calibrated against petrographic and petrophysical observations. Among them, Zones 1–3 represent the productive units, with Zone 3 being the most significant due to widespread fabric-destructive dolomitization and pore connectivity. Zones 4 and 5, affected by anhydrite cementation, represent poor-quality or non-reservoir intervals. This integrated approach demonstrates that multivariate clustering, when combined with petrography, SEM, and MICP data, provides a robust framework for characterizing carbonate heterogeneity. The methodology is flexible, does not require prior training datasets, and can be applied to other fields. Importantly, zones with similar diagenetic histories and pore characteristics are shown to share comparable flow properties, offering a reliable basis for linking porosity, permeability, and reservoir performance.
AbstractOnline evaluation of gas in drilling mud provides valuable information about reservoir horizons and the type of fluid during drilling. This study investigates the productive horizons of the Fahliyan Formation reservoir in the Yadegaran Field located in the Abadan Plain using geochemical evaluation of hydrocarbon gases in the drilling mud. The recording of mud gas data was conducted by a gas chromatograph with a flame ionization detector, and the hydrocarbon ratios(Pixler, Wetness, Balance, and Character) were calculated for two wells in the Fahliyan reservoir. The results of these ratios indicated that the Fahliyan Formation has reservoir quality, and the fluid is light oil. The relationship between the Wetness and Balance ratios divides the Fahliyan reservoir into two reservoir horizons. The lower horizon contains light oil, where the difference between these two ratios is slight, , while the greater difference in the upper horizon is due to low production capacity accompanied by residual oil.Keywords: Mud gas, Fahlyian Formation, Yadavaran Field, Productive Zone, Reservoir continuity IntroductionDuring drilling, valuable data is obtained that can be used to characterize the productive zones of hydrocarbon reservoirs. One of these data is the information on gas associated with drilling mud. By identifying the trends in gas composition and changes in their ratios, the petrographic changes and fluid content of the drilled formations can be examined (Farouk et al. 2014). The measurement of hydrocarbon gas amounts in the mud is performed by gas chromatography, which includes quality control of sampling at specified times and analysis of gas contents (Ferroni et al. 2012). The more accurate the identification of hydrocarbon gases and the broader the spectrum of hydrocarbons included, the higher the quality and clarity of formation evaluation during drilling, determination of reservoir fluid levels, and identification of the productive zone (Arief & Yang, 2020). The purpose of this research is to investigate the productive zones of the Fahliyan Formation in the Yadavaran Field located in the Abadan Plain, using geochemical evaluation of hydrocarbon gases in the drilling mud. Materials & MethodsThe record of mud gas information during drilling in two wells of the Yadavaran Field was carried out using a gas chromatograph equipped with a flame ionization detector. The gases are separated from the mud by gas trap motors and introduced into the gas chromatograph, where they are recorded based on the amount of gas and the time it takes to enter the detector. It is worth noting that the device is calibrated daily using standard samples. The quality of the recorded data is evaluated after the gas is analyzed by the device. For this purpose, the Gas Quality Ratio (GQR) index is used. This index is obtained from the ratio of the total gas amounts to the sum of hydrocarbon component amounts multiplied by their respective carbon atom numbers (Wiersberg & Erzinger 2007; Newton et al. 2014). A GQR value within the range of 0.8 to 1.2 indicates good data quality. The recorded results from the drilling mud in the two studied wells demonstrate the appropriate quality of these data. Discussion of Results & ConclusionUpon entering the Fahliyan Formation at a depth of 4050 meters, the concentration of hydrocarbon gases in the drilling mud, which was below 1000 ppm before this horizon, increases. This result is entierly consistent with the oil shows observed on the drilling cuttings. The ratio of methane to heavier gases such as ethane, propane, and butane can indicate gas, oil, and water intervals (Pixler 1969). A C1/C2 ratio between 2 and 15 indicates an oil zone, while a ratio between 15 and 65 indicates a gas zone. The higher this ratio, the richer the gas or the lower the hydrocarbon density. If the C1/C2 ratio is less than 2, it indicates residual oil, and if it is above 65, it signifies a non-productive zone (Pixler 1969). Interpretation of the Pixler C1/C2 ratio for the samples is between 2 and 15, indicating oil fluid in the Fahliyan reservoir, and the deviation of some data towards the gas suggests a higher API gravity of the oil. The Wetness ratio increases with increasing gas density; the Balance ratio is, in fact, a direct ratio between light and heavy hydrocarbons, used alongside the Wetness ratio for interpretation, and has an inverse relationship with it (Mode et al. 2014; Sahu 2018). Practically, a straightforwardrelationship between Wetness and Balance ratios is used to determine fluid type and fluid contact during drilling. If the Balance ratio is greater than the Wetness, it is predicted that gas exists in the layer, whereas if the Wetness ratio is greater than the Balance, oil is predicted in the layer. The closer the curves are to each other, the lighter the oil. The greater the distance between the curves, the heavier the oil or the presence of residual oil (Mode et al. 2014). Higher ratios of Wetness than Balance also confirm the oil fluid for the Fahliyan reservoir. The trend of these two ratios differs between the upper and lower horizons of the reservoir. These two ratios have little difference in the lower horizon, indicating a productive zone with light oil. In the upper horizon of the Fahliyan reservoir, the Wetness and Balance ratios differ more, which can be due to the presence of a heavier oil composition or a non-productive zone. Based on previous geological and petrophysical studies (Mohseni et al. 2016; Ramezani Akbari et al. 2017; Tavoosi Iraj et al. 2023), the reservoir quality of the upper horizon is low, and the hypothesis of low production capacity accompanied by residual oil seems more plausible. Moreover, the C1/C4+C5 ratio is used to determine the amounts of heavy hydrocarbons, and a high value of this ratio indicates low amounts of heavy hydrocarbons. The high value of this ratio indicates low amounts of heavy hydrocarbons in the Fahliyan reservoir. This ratio also divides the Fahliyan reservoir into two different reservoir horizons, with the lower horizon showing higher ratios.
AbstractThis study investigates the impact of the Paleocene–Eocene Thermal Maximum (PETM) event on the organic petrographic characteristics of the base Pabdeh Formation in the Tang-e-Hati section, located at the Kuh-e-Gurpi Anticline, SW Iran. To delineate the Paleocene–Eocene boundary, nanofossil analyses were employed. Additionally, the collected samples from the studied section were investigated using organic petrographic methods under reflected white light. The nanofossil results indicate that the Paleocene–Eocene boundary is situated approximately 26.5 meters from the base of the Pabdeh Formation, specifically between subzones NP9a and NP9b, marked by the presence of the key species: Discoaster araneus, Rhomboaster cuspis, and R. spineus. Furthermore, the organic petrographic results suggest that the studied marls were deposited under oxidizing conditions. Results from this study are consistent with the presence of a grey marl with relatively darker color, lower fossil concentration, and higher organic matter concentrations compared to the lower and upper parts. Results from this study conclusively suggest that deposition of the grey marl was associated with a short-term relative sea-level fall, which in turn led to an increased sedimentation rate in the basin and a greater influx of terrestrial organic matter.Keywords: Purple shale, Organic petrography, Paleocene–Eocene boundary, PETM incident, Tang-e-Hati IntroductionOrganic geochemistry, focusing on the study of organic compounds in sediments and their interactions with geological processes, plays a vital role in hydrocarbon exploration (Peters et al. 2005). One of the key analytical approaches in this field is organic petrography, which enables the identification of macerals, kerogen types, thermal maturity, and palaeo-depositional environments of potential source rocks (Hackley & Cardott 2016). The Paleocene–Eocene Thermal Maximum (PETM) was a short-lived but intense global warming event, associated with a 5–8 °C rise in global temperatures, ocean acidification, and major disruptions in the carbon cycle (Zachos et al. 2008).In southwestern Iran, the Pabdeh Formation, with its continuous marl and purple shale strata, provides an excellent opportunity to assess variations in organic matter preservation across the PETM (Motiei 1993). Although numerous studies have investigated the geochemical properties of the Pabdeh Formation (Alizadeh et al. 2012; Safaei-Farouji et al. 2022; Hosseiny et al. 2024), little is known about its organic petrographic characteristics, particularly across the Paleocene–Eocene boundary. This study aims to fill that gap by examining the organic matter variations and palaeo-depositional conditions during the PETM by means of organic petrography techniques. Material & Methods In this study, 24 samples were systematically collected from the base of the Pabdeh Formation at the Tang-e-Hati section (southern flank of the Kuh-e-Gurpi Anticline). Sampling intervals were generally less than 5 meters; however, in the vicinity of the Paleocene–Eocene boundary, the interval was reduced to approximately 1 meter or less to allow for a more detailed investigation of PETM-related changes. The collected samples, consisting of both consolidated and unconsolidated materials, were transferred to the laboratory for calcareous nannofossil and organic petrographic analyses. Calcareous nannofossils were prepared using the standard smear slide technique (Bown & Young, 1998), and examined under a polarized Olympus BX60 microscope at 1250× magnification. Species identification was carried out based on established references (Perch-Nielsen 1985; Agnini et al. 2014) and biozonation and boundary placement followed the schemes of Martini (1971), Romein (1971), and Aubry (1998). For organic petrographic studies, polished pellets were prepared from small fragments (approximately 1.5 × 1.5 cm) of the collected samples. These fragments were embedded in a 2:1 mixture of epoxy resin and hardener using standard protocols. After 24 hours, the samples were removed from the molds and polished according to the standard procedures (Bustin et al. 1985; Taylor et al. 1998). Petrographic observations were performed using a Zeiss Axioplan II microscope at 100× magnification under oil immersion. Discussion of Results & ConclusionsIn this study, the Paleocene–Eocene boundary was identified at approximately 26.5 meters above the base of the Pabdeh Formation, based on the recognition of nannofossil subzones NP9a and NP9b. Organic petrography revealed three distinct sections differing in organic matter content and color: i) a lower purple shale with low organic matter contents which was precipitated under oxidizing conditions, ii) a middle grey marl with higher organic content linked to the PETM event and increased water acidity, and iii) an upper purple shale with decreased organic content and a return to oxidizing conditions. Changes in color and organic matter content correspond to fluctuations in the relative sea level, sedimentation rate, and pH of the water. During deposition of the lower purple shale, higher sea levels and lower sedimentation rates favored good fossil preservation. During the PETM interval, sea level dropped, sedimentation rate increased, and water acidity increased, resulting in reduced fossil preservation and increased terrestrial organic matter input. After the PETM, sea level rose again, terrestrial organic matter input decreased, and fossil preservation improved. These results highlight the interplay of sea-level changes, sedimentation rates, and water pH in controlling the organic petrographic characteristics of the base Pabdeh Formation during the PETM, providing valuable insights for reconstructing palaeo-depositional environments in this part of the Zagros Basin.
This study investigates the influence of lithological characteristics on the evolution of water quality in the Zuzan Plain, Northeast Iran, a region characterized by limited water resources. A total of 23 water samples were collected from various sources across the area. Physical parameters, including acidity (between 7.2 and 8.7), dissolved solids (1180–7415 mg/l), electrical conductivity (ranging from 2100 to 12,210 microsiemens/cm), and salinity, were measured on-site using an Extech model multimeter. Water hardness (100–1960 mg/lCaCO3) was analyzed at the laboratory. Statistics software was used to analyze the correlations between elements and their relationship with the region’s lithology. Water facies were identified using RockWorks 16 software. The Piper and Stiff diagrams revealed that most samples exhibited sodic facies with chloride type, except for two samples, which displayed sodic facies with sulfate type. Furthermore, the Gibbs diagram was employed to distinguish the dominant hydrogeochemical processes, indicating that both evaporation and rock-water interactions strongly influence the groundwater composition. To better understand spatial controls, a groundwater flow direction map was prepared, clarifiying the distribution of water facies in relation to regional hydrogeology. Cluster analysis and principal component analysis of the water samples revealed two main components influencing the source variations. The presence of acidic igneous units, limestone, sandstone, conglomerate, siltstone, and shale in the region suggests their significant role in controlling the chemical composition of the water. Limestone, due to its higher solubility, is identified as the primary regulator of water quality in the Zuzan Plain, while igneous and sedimentary rocks exert secondary influences.
The discovery of a new specimen of Portunus withersi (Glaessner 1933) from the Lower Miocene Qom Formation in the Vartun section, north of Isfahan (Central Iran), contributes to our knowledge of this portunid species, to which most fossil portunids found in the Miocene of Iran have been attributed. New images of the holotype are also presented herein. The Miocene decapod fauna of Iran, exhibiting a clear Indo-Pacific affinity, reflects an emerging loss of faunal homogeneity between the decapod communities on either side of the Tethys Realm. This differentiation is related to the progressive closure of the Tethys Seaway, which interrupted the connection between the proto-Mediterranean and Paratethys seas and the Indian Ocean. This stands in contrast to the relative faunal homogeneity observed during the Paleogene, as evidenced by the presence—on both sides of the Tethys Realm—of genera such as Zanthopsis M’Coy 1849, Palaeocarpilius A. Milne-Edwards 1862, or Lophoranina Fabiani 1910, among others, and even species such as Retrocypoda almelai Vía Boada 1959.
This study focused on the sedimentological attributes of the dunes in the Najaf Governorate, southern Iraq. Twelve spot samples were collected from two areas, the first was approximately 3 km from Madhlum Village, and the other was west of Al-Manathera District. Several analyses were performed, including grain-size analysis, XRD, petrography using a polarized microscope, magnifying lenses to distinguish light minerals, and SEM-EDS survey. These sediments are compound of silt, sand, and mud fractions, with sand particles predominating in the dune fields. Muddy sand was categorized according to this pattern. According to XRD, quartz is the predominant mineral at the studied locations, followed by feldspars, calcite, and gypsum. Petrographic analysis reveals that monocrystalline quartz is more abundant than polycrystalline quartz. The various types of rock fragments include metamorphosed, igneous, carbonate, mudstone, and chert fragments. Alkaline feldspar is more than plagioclase. The mineral content was classified as litharenite to feldspathic litharenite. The provenance of the sand dunes indicates recycled orogeny. The paleoclimate in the source area ranged from semi-humid to sem-iarid.
The facies analysis of the Eocene rocks found in South Samawa has led to the identification of two significant Eocene formations: the Rus and Dammam. The Dammam Formation holds substantial geological importance due to its extensive coverage across much of southern Iraq, serving as a primary source for the region's aquifers. Rocks consisting of porous limestones and dolomitic limestones. The microfacies analysis revealed five distinct textures including floatstone, rudstone, wackestone, packstone, and mudstone. These textures are classified into nine sub-microfacies. The Rus Formation with mixed carbonate-evaporitic units has deposited in an inner ramp setting from the lagoon to the sabkha sub environments. In contrast, the Dammam Formation has been deposited in various settings, including a peritidal, lagoon, shoal, restricted marine platform, and open marine inner platform. Six distinct depositional sequences are identified within the formation, each reflecting varying depositional environments. Sequence A, dating to the Lower Eocene, with lagoon microfacies indicating the Maximum Flooding Surface. Sequence B, also from the Lower Eocene, is marked by alternating limestone types and an MFS defined by shallow open marine microfacies. Sequence C, representative of the Middle Eocene, reveals a transition from shoal to peritidal facies. Sequence D, spanns the end of the Middle Eocene to the early Late Eocene. In contrast, Sequence E, representing the Upper Eocene, exhibits transitions from lagoonal to peritidal microfacies. Finally, Sequence F, dating to the Lower Euphrates. Overall, the findings highlight the complex interplay of sea-level changes and tectonic processes in shaping the sedimentary architecture of the Dammam Formation.
AbstractDam reservoirs serve as suitable places to settle sedimentary materials carried by rivers. Pollutants absorbed into these sediments may lead to water contamination, increased aquatic animal mortality, and loss of biodiversity. Potentially toxic elements (PTEs) are classified as highly hazardous pollutants due to their long persistence in the environment. Due to their destructive effects on humans and the environment, it is important to examine sensitive and special areas for contamination with them. Therefore, spatial zoning of these pollutants in surface sediments of aquatic ecosystems is very effective in identifying the entry routes and determining their origin, as well as in identifying sedimentation conditions and determining sensitive zones for pollution reduction management. The sampling of surface sediments with the aim of zoning the sediments of the Zayandehroud Dam reservoir in terms of the level of contamination with PTEs based on geochemical indicators was carried out from 20 stations in this lake in December 2022. Then, the physical and chemical variables of the sediments were measured, including pH, EC, Eh, percentage of moisture, bulk density, dry density, grain size, percentage of organic matter, percentage of calcium carbonate, and concentrations of 26 elements. The results showed overall increasing changes in pH, bulk density, dry density, percentage of organic matter, percentage of calcium carbonate, and sand-sized particles from the dam crest to the lake entrance, while the percentage of moisture and Eh showed an overall decreasing trend. Only some elements, including silver, arsenic, chromium, copper, nickel, and lead, exceeded the US sediment quality guidelines (ERL and ERM) at some stations, among the 26 elements measured in the 20 surface sediment samples. Which was found towards the dam crest for arsenic and lead, chromium except for a few points, nickel and copper were found throughout the study area, and silver was found towards the lake entrance. Also, the Enrichment Factor (EF) of lead and copper was lowest at the lake entrance (EF<2) and increased towards the dam crest, and there was also very high enrichment (EF= 20-40). The EF of silver showed the highest enrichment (EF> 40) at one station at the lake entrance. The Geoaccumulation Index also showed non-polluted (Igeo<1) to slightly polluted (Igeo= 1-2) for cadmium and antimony in the entire study area, and slightly polluted (Igeo= 1-2) for silver towards the lake entrance and for lead near the dam crest. The PLI index based on total elements showed pollution (PLI>1) only near the dam crest.Keywords: Zonation, Surface sediments, Elements, Geochemical indices, Zayandehroud Dam Introduction Potentially toxic elements (PTEs) are classified as highly hazardous pollutants due to their long persistence in the environment. Due to their destructive effects on humans and the environment, it is important to examine sensitive and special areas for contamination with them. Therefore, spatial zoning of these pollutants in surface sediments of reservoir dames is very effective in identifying the entry routes and determining their origin, as well as in identifying sedimentation conditions and determining sensitive zones for pollution reduction management (Kabata et al. 2007). Due to the increasing population growth and corresponding increasing drinking water need, along with the development of agricultural and industrial sectors in the Gavkhooni Basin in arid to semi-arid region of the central plateau of Iran, the optimal utilization of the Zayandehroud Dam is crucial. For this purpose, determining the amount and spatial distribution pattern of PTEs in the surface sediments of the Zayandehroud Dam reservoir using geochemical indices was determined as the main goal of this study. Material & MethodsThe sampling of surface sediments of the Zayandehroud Dam reservoir was carried out using Ekman grab sampler from 20 stations in December 2022. Sampling of surface sediments was carried out along the dam lake and in nine rows on the left and right sides of the old course of the Zayandehroud River. Sediment samples were placed in one-kilogram plastic containers with lids, coded, and transported to the laboratory. Then, the physical and chemical variables of the sediments were measured, including pH, EC, Eh, percentage of moisture, bulk density, dry density, grain size, percentage of organic matter, percentage of calcium carbonate, and concentrations of 26 elements.In order to determine the contamination levels of various elements, a comparison was made with the US sediment quality guidelines (ERL and ERM; Long et al. 1995). Moreover, in order to determine the level of sediment pollution, the EF was used as a suitable indicator to indicate the degree of pollution (Li et al. 2021), and the geoaccumulation index (Igeo) was used to measure the intensity of pollution (Muller 1969), and the pollution load index (PLI) was used (Weissmannová & Pavlovský 2017).The normality of the data distribution and the presentation of a summary of the statistical information of the physical and chemical variables of the sediments and the calculated indices were performed in SPSS software, version 18. Due to the non-normality of most of the variables, the Spearman correlation test was used to determine the relationship between the different variables. Correlation coefficients of 0.7 to 0.85 and greater than 0.85 were considered strong and very strong correlation coefficients, respectively (Dawson et al. 2007). Also, in order to prepare a zoning map of the surface sediments of the Zayandehroud Dam reservoir in terms of the Pollution Load Index (PLI), ArcMap software and the interpolation command using the inverse distance weighting (IDW) method were used. Discussion of Results & Conclusions The results showed overall increasing changes in pH, bulk density, dry density, percentage of organic matter, percentage of calcium carbonate, and sand-sized particles from the dam crest to the lake entrance, while the percentage of moisture and Eh showed an overall decreasing trend. The results of the sediment size gradation generally showed similarities with the spatial pattern reported in previous studies conducted on surface sediments of the Zayandehroud Dam reservoir (DaryaTarsim Consulting Engineers 2011).In general, changes in element concentrations in surface sediments can be divided into two groups. Calcium and thorium elements are in the first group and other elements are in the second group. Only some elements, including silver, arsenic, chromium, copper, nickel, and lead, exceeded the US sediment quality guidelines (ERL and ERM) at some stations, among the 26 elements measured in the 20 surface sediment samples. Which was found towards the dam crest for arsenic and lead, chromium except for a few points, nickel and copper were found throughout the study area, and silver was found towards the lake entrance. Also, the EF of lead and copper was lowest at the lake entrance (EF<2) and increased towards the dam crest, and there was also very high enrichment (EF= 20–40). The Enrichment Factor of silver showed the highest enrichment (EF> 40) at one station at the lake entrance. The Geoaccumulation Index also showed non-polluted (Igeo<1) to slightly polluted (Igeo= 1-2) for cadmium and antimony in the entire study area, and slightly polluted (Igeo= 1–2) for silver towards the lake entrance and for lead near the dam crest. The PLI index based on total elements showed pollution (PLI>1) only near the dam crest.Based on the significant positive correlation coefficients between most elements of the surface sediments of the Zayandehroud Dam reservoir, it can be stated that the majority of the studied elements are of natural origin. The enrichment of elements such as chromium, nickel, and copper observed at most stations is probably of natural origin and is consistent with the upstream geology of the Zayandehroud Dam Basin. The increased concentration and enrichment of elements such as silver and lead observed in some stations show the effect of human activities.
Abstract Understanding reservoir connectivity is critical for reducing exploration risk and optimizing field development. This study uses surface geochemistry techniques to investigate the relationship between the Khami reservoir horizon in the Chilingar and Garangan oilfields, located in the Southern Dezful Embayment. A total of 154 surface soil samples were collected from the field closures and the inter-field area. These samples were analyzed using acid extraction and fluorescence analysis to evaluate hydrocarbon migration, identify geochemical anomalies, and assess potential reservoir connections. Hydrocarbon gas ratios (C1/C2, C2/C3, and C1/(C2+C3) vs. C2/(C3+C4)) revealed that surface hydrocarbons are primarily oil-derived. The composition and ratios of samples from the inter-field area closely matched those from the closures, suggesting a shared source. Fluorescence analysis provided R1 ratios (three-ring to two-ring aromatics) and inferred an average API gravity of 36˚, indicating high-quality, light oil consistent with field samples. Geochemical anomaly maps revealed similar hydrocarbon compositions and quality between the two fields, supporting a likely connection through a saddle structure. These findings demonstrate the effectiveness of surface geochemistry in assessing reservoir connectivity and reducing exploration risks.Keywords: Surface geochemistry, Reservoir connectivity, Chilingar and Garangan oilfields, Acid extraction, Fluorescence analysis IntroductionDiscovering new hydrocarbon resources and enhancing the production efficiency of existing fields remain critical challenges for global oil and gas producers. Until the 1940s, oil drilling was primarily based on visible surface hydrocarbon seeps (Link 1952; McGregor 1993). Hydrocarbon seeps refer to the surface or near-surface accumulation of light or high molecular weight hydrocarbons, often indicating the presence of subsurface reservoirs. The global demand for energy in the 20th century pushed oil exploration from empirical methods toward more scientific approaches. Surface geochemistry, introduced in the 1930s, became a significant tool for oil exploration, examining direct and indirect hydrocarbon seepage linked to deep reservoirs (Tedesco 2017). In Iran, surface geochemistry was initiated in 2014 by the Energy Researchers Ariana (ERA) company. After decades of oil production in the Zagros Basin, better reservoir management is crucial to maximizing efficiency. The Chilingar and Garangan oilfields, located in the southern Dezful Embayment, are closely situated and separated by a saddle structure. Reservoir studies show connectivity between the lower and upper Khami reservoirs in these oilfields. Understanding this connection is vital for planning field development and identifying weakly connected reservoir zones. This study uses the soil gas method, a direct surface geochemistry technique, to analyze reservoir connectivity and hydrocarbon-rich zones. The findings aim to guide efficient production management, reduce drilling risks, and advance field development strategies at lower exploration costs.Materials & MethodsThis study utilized direct geochemical techniques, including acid extraction and fluorescence analysis, to detect subsurface hydrocarbon reservoirs. Surface prospecting relies on vertical hydrocarbon migration, which creates detectable seeps and geochemical anomalies. Sampling followed pre-existing geophysical lines, with 154 soil samples collected across 35 lines. Acid extraction analysis measured light hydrocarbons (C₁–C₅) released via a hydrochloric acid reaction, while fluorescence analysis quantified polycyclic aromatic hydrocarbons (PAHs) using high-performance liquid chromatography (HPLC) and UV excitation. Both methods identified surface anomalies linked to potential subsurface accumulations, underscoring their utility in hydrocarbon exploration (Schumacher 1996). Discussion of Results & ConclusionsThis study focused on the Chilingar and Garangan oilfields, located in the southern Dezful Embayment, using surface geochemistry to investigate hydrocarbon seepage and assess reservoir connectivity between the two fields. Hydrocarbons typically migrate vertically due to high subsurface pressure, with stratigraphic layers, faults, and fractures facilitating this movement. In oil and gas reserve areas, light hydrocarbons in sediments indicate a potential reservoir, with higher concentrations suggesting economically viable accumulations. If anomalies between the two fields match those over the anticline, it supports the hypothesis of a hydrocarbon connection. To determine hydrocarbon content, soil samples were analyzed using acid extraction (AE) and fluorescence methods. Acid extraction showed that methane-to-ethane (C1/C2) and ethane-to-propane (C2/C3) ratios point to oil-associated hydrocarbons (Pixler 1969). The area between the two fields displayed similar hydrocarbon concentrations to those above the fields. Standard ratio charts, such as C1/(C2+C3) vs. C2/(C3+C4), confirmed an oil origin for most hydrocarbons, with anomalies indicating high concentrations between the fields. Fluorescence analysis revealed that inferred API gravity and aromatic hydrocarbon ratios (R1) suggest high-quality hydrocarbons, including oil and gas condensates. The changes in the interpolated parameters reveal the presence of major faults in the region and the influence of the north-south trending fault in the western part of the Garangan oilfield. Areas surrounding the old wells in the Chilingar field exhibit a lower value of the inferred API index in the interpolated map, due to reservoir pressure depletion caused by prolonged production. The results of both methods strongly indicate the presence of liquid hydrocarbons between the fields, supporting the likelihood of a reservoir connection through a saddle structure. These findings are crucial for efficient exploration, reducing risks, and guiding field development.
AbstractLower Carboniferous deposits of Kalmard area identified with informal Gachal formation which in different outcrops have different lithologic features. This formation is consists of four members A, B, C and D that predominantly consists of carbonate and evaporite rocks. In the Gachal section, the B, C, and D members are outcropped. Member B consist of 200 m massive to thick-bedded dolostone, C member 150 m white massive gypsum, and D member from 50 m carbonate deposits with intercalation of evaporite interlayers. The lower boundary is not exposed but the upper boundary with Khan Group is unconformable. In this paper, Member D, in the Godar-e-Gachal is investigated. Based on lithologic and microscopic studies, 20 carbonate microfacies are identified which belong to the tidal flat, open to the semi-restricted lagoon, bar/shoals and open marine sub-environments. Vertical changes of microfacies and depth change curve represent that the supratidal, intertidal and lagoonal microfacies are thicker than open marine microfacies. Member D of the Gachal formation were deposited in the homoclinal ramp that was situated in the southern margin of paleo-Tethys Ocean in the Visean–Serpukhovian? in the hot and dry climatic conditions that is comparable with the conditions of modern Persian Gulf homoclinal ramp. The Member D deposits of Gachal formation consist of three depositional sequences that differentiated with sequence boundary type 2. Relationships between Member D and C in the lower part and with Khan Group in the upper part are identified with sequence boundary type 1. The above-mentioned third depositional sequences shows the late Visean age that correlateable with the Kaskaskia IV supersequence. The boundary between Gachal formation and Khan Group is correlateable with falling of sea-level in the global scale in the late Visean–Serpukhovian? stages which represent a clear disconformity and erosional surface between Gachal formation and Khan stratigraphic Group.Keywords: Gachal formation; Kalmard area; Lower Carboniferous; Sequence stratigraphy IntroductionCentral Iran is one of the basic tectonostratigraphic and complicated units in the geology of Iran that is located in the center of Iran and has a triangle shape. The Kalmard Block is a little part of Central Iran that has a northeastern trend and is located between the Kalmard Fault in the east and Naeini Fault in the west. The Lower Carboniferous rocks in the Kalmard area mainly consist of carbonate rocks and a unit C of Gachal formation that is composed of approximately 170 m gypsum and anhydrite that the name of Gachal formation derived from this unit (Aghanabati 1977). Unit D of the Gachal formation is composed of carbonate rocks such as limestone, dolomitic limestone with intercalations of gypsum and red paleosoils and collapse breccias that the complete section of this unit is seen in the Godar-e-Gachal section studied in this paper. With respect to complete Unit D in the southern part of Kalmard area and a necessary of the study of sequence stratigraphy of Tournaisian–Visean strata in this part of Central Iran, this stratigraphic section has been selected. Materials & MethodsFor identification of microfacies characters and analysis, environmental conditions, and sequence stratigraphy of unit D of Gachal formation, the Godar-e-Gachal section has been selected that has 50.4 m thickness. With respect to thickness and lateral facies change, carbonate-evaporate sedimentary cycles and key stratal sequence stratigraphy surfaces. Thirty-two rock samples have been collected and 100 microscopic thin sections have been prepared. Classification and studies of carbonate rocks were conducted based on Dunham (1962) and reconstruction of the depositional environment was based on Walther’s law of correlation of facies (Walther, 1984 in Middleton 1973). Also, vertical and lateral facies changes and comparison with recent sedimentary environments were examined by using the standard carbonate platform models (Wilson 1975; Flügel 1982; Carozzi 1989; Burchette & Wright 1992). Discussion of Results & ConclusionsThe identified depositional sequences of Member D of the Gachal formation have a lot of similarities with global depositional sequences in the Early Carboniferous (Sloss 1988). By studying Member D of Gachal formation, the following results have been obtained:The thickness of unit D changes from north to south (22 to 98 m). In the Godar-e-Gachal section the thickness of Member D is 50.4 m. With respect to carbonate rocks and intercalation of gypsum beds, it seems that this member was deposited in the south of Kalmard mixed carbonate –siliciclastic platform, under hot and dry climatic conditions. Based on microscopic and field studies, microfacies types are deposited in the tidal flat, lagoon, bar and open marine sub-environments in the Klamard homoclinal platform in the lower Carboniferous was located in the passive margin of the southern part of the Gondwanaland. Member D of the Gachal formation consists of three depositional sequences that the first depositional sequence has sequence boundary type 2 in the lower part with gypsum of Member C, and the third depositional sequence has a sequence boundary type 1 in the upper part with the Khan formation which, indeed, is a disconformity and an erosional surface. Glacial events belong to the Milankovitch orbital forcing and tectonic processes, and are main factors for the formation of sedimentary cycles and depositional sequences of Member D of Gachal formation.
AbstractLand subsidence represents a severe environmental hazard, causing significant infrastructure damage and threatening cultural heritage sites. The Isfahan–Borkhar region of central Iran, with its dry climate and diverse topographical conditions, has been highly susceptible to this phenomenon. Using remote sensing techniques, particularly radar interferometry (InSAR), this study investigates subsidence rates over the 2019–2023 period. Advanced machine learning methods, namely Decision Tree (DT), Random Forest (RF), and Extreme Gradient Boosting (XGBoost) are employed to develop a susceptibility map divided into five probability classes: very high, high, medium, low, and very low. The analysis incorporates 145 Sentinel-1 radar satellite images and factors such as elevation, groundwater levels, rock composition, vegetation cover and fault proximity. Among these, RF emerges as the most effective algorithm, achieving a classification accuracy of 95.63%, while XGBoost proved inefficient for certain critical subsidence zones. Results reveal that subsidence risk is concentrated in the central and eastern parts of the region due to excessive groundwater extraction and geological vulnerabilities. Conversely, the western and northwestern areas exhibit lower risk due to stable geological formations and controlled groundwater usage. These findings aim to inform regional planning and subsidence mitigation strategies.Keywords: Land subsidence, Radar interferometry (InSAR), Machine learning, Isfahan–Borkhar, Sentinel-1, Environmental hazards. IntroductionLand subsidence, a significant and growing environmental concern, occurs as a result of both natural geological processes and human activities. It is defined as the gradual or sudden sinking of the Earth's surface due to subsurface changes. Globally, subsidence poses a serious threat to infrastructure, ecosystems, water resources, and historical landmarks, making it a critical issue for urban planners, environmental scientists, and policymakers. The drivers of subsidence vary across regions, but in arid and semi-arid regions like central Iran, it is predominantly caused by excessive groundwater extraction.In recent years, Iran has experienced severe land subsidence, primarily fueled by agricultural overreliance on groundwater resources. Studies estimate that approximately 3.5% of Iran's land area has been affected by subsidence, with some regions showing annual sinking rates of over 10 centimeters. Such rates are alarmingly high and have direct implications for urban development, agriculture, and even national heritage conservation. Isfahan–Borkhar, the focus of this study, stands as a prime example of this hazard due to its unique combination of climatic, geological, and hydrological vulnerabilities.The rapid urbanization and agricultural intensification in Isfahan–Borkhar have exacerbated the strain on groundwater reserves, leading to widespread subsidence and its associated risks. Furthermore, the region's geological context including its proximity to the Qom–Zefreh Fault and the presence of loose alluvial deposits makes it particularly prone to land sinking. While individual studies have explored the impact of factors like groundwater depletion and soil characteristics, comprehensive research that simultaneously examines multiple influencing variables remains limited.The advancement of remote sensing technologies, particularly radar interferometry (InSAR), has provided researchers with powerful tools for subsidence monitoring. InSAR techniques allow for high-precision measurements of surface deformation across large areas, making them invaluable in understanding and modeling subsidence patterns. When combined with machine learning approaches, these technologies offer even greater potential for developing predictive models and susceptibility maps, enabling proactive risk management. This study aims to bridge the existing research gap by employing a multi-factor approach using machine learning algorithms including Decision Tree (DT), Random Forest (RF), and Extreme Gradient Boosting (XGBoost) to map and analyze the subsidence susceptibility of the Isfahan–Borkhar region. By doing so, it seeks to inform mitigation strategies and contribute to sustainable land and water resource management in one of Iran's most vulnerable areas. Materials & MethodsStudy area: The Isfahan–Borkhar Plain covers an area of approximately 3,743 km² in central Iran, extending across key cities, including Isfahan, Shahin Shahr, and Najafabad. Elevations range from 1,500 to 1,730 meters above sea level, while its dry climate and varied topographical features amplify the region's vulnerability to subsidence. Proximity to critical geological structures, such as the Qom–Zefreh Fault, further complicates the scenario.Data sources: The study employed 145 ascending Sentinel-1 radar satellite images processed via the SBAS (Small Baseline Subset) technique to derive displacement data between 2019 and 2023. Ancillary datasets included groundwater, geological features, digital elevation models (DEM) and its derivatives, such as slope and aspect, vegetation and land cover maps. All input datasets (as input factors for machine learning algorithms) were co-registered to match the resolution of the InSAR-derived maps (100 meters).Machine learning algorithms: Three machine learning algorithms including decision tree (DT), random forest (RF) and extreme gradient boosting (XGBoost) were tested. In addition, factors were ranked based on their importance using RF Discussion of Results & ConclusionsSubsidence measurements: InSAR analysis revealed subsidence rates ranging up to 116.8 mm annually and cumulative displacement reaching 506.29 mm over the five-year period. These values align closely with reports from local monitoring agencies and geological surveys.Algorithm performance: Among the algorithms tested, RF demonstrated the highest classification accuracy, achieving 95.63%. It successfully mapped five susceptibility classes, including the high-risk "very high subsidence" category. DT produced moderately reliable results, with an accuracy of 90.58%, while XGBoost was the least effective, achieving just 75.42% accuracy and failing to predict high-risk subsidence zones accurately.Susceptibility mapping: The RF algorithm highlighted central and eastern parts of Isfahan–Borkhar as the most vulnerable to subsidence, driven primarily by excessive groundwater withdrawal and geological factors. The presence of loose, porous sedimentary layers in these areas exacerbates the phenomenon, particularly near the Qom–Zefreh Fault. Conversely, western and northwestern sections showed reduced risk due to stable geological structures composed of shales and limestones with lower porosity.Field observations confirmed the presence of visible subsidence indicators, including cracks on building walls and soil fractures. These findings emphasize the urgent need for targeted mitigation strategies, such as controlled groundwater extraction and urban planning.Factor ranking: Elevation emerged as the most critical subsidence predictor, followed by groundwater levels and rock composition. Notably, sedimentary deposits in eastern regions exhibit higher porosity and susceptibility, while western areas resist subsidence due to their geological stability. Vegetation and land cover contributed minimally, reinforcing the predominance of hydrological and geological parameters.Conclusion: The findings of this study underscore the critical need for a holistic and data-driven approach to understanding and managing land subsidence in regions like Isfahan–Borkhar. By integrating InSAR with advanced machine learning algorithms, this research provides valuable insights into the spatial distribution and key drivers of subsidence susceptibility. It highlights that subsidence in Isfahan–Borkhar is not solely a consequence of groundwater depletion but also strongly influenced by elevation, geological structures, and proximity to fault lines, such as the Qom–Zefreh Fault.In summary, the integration of advanced remote sensing and machine learning techniques represents a promising pathway for addressing the challenges posed by land subsidence. By shedding light on the factors driving subsidence and providing actionable insights, this study not only contributes to the academic discourse but also supports informed decision-making to safeguard lives, infrastructure, and cultural heritage in one of Iran's most affected regions.
