Sediments of the Holocene of the southern Arabian Gulf (United Arab Emirates) exhibit a wide range of sedimentary facies which include: (a) pelecypod sands mixed with lime and argillaceous mud offshore; (b) pelecypod-rich grain-dominated sediments in the deeper tidal channels between the barrier island lagoons and deeper portions of the Khor al Bazam; (c) coral reefs and coralgal sediments of coastal margins to the west; (d) oolite shoals that accumulate in front of barrier islands to the east or ebb tidal tidal-deltas and tidal-channels; (e) grapestone-rich sediments that occur on exposed coastal terraces of the western Khor al Bazam and on the leeward side of the reefs and oolite shoals in eastern Abu Dhabi; (f) pelleted lime muds that accumulate within the protected lagoons of eastern Abu Dhabi; (g) cyanobacterial mats and mangrove swamps lining the inner shores of the protected lagoons of Abu Dhabi and the east Khor al Bazam; and (h) supratidal salt flats, (sabkhas) where evaporite minerals precipitate at the surface or within the sediments along the inner shoreline. The settings of the southern coast of the southern Arabian Gulf (Abu Dhabi coastline) can be used as a comparative model for understanding ancient carbonate/evaporite depositional and diagenetic processes. Similar facies associations of shallow-water carbonates and evaporites occur in the subsurface of the Arabian Gulf and include the Permo-Triassic Khuff Formation andthe Upper Jurassic Arab and Hith (Anhydrite) formations. examples from North and South America, Australia, and Europe include the Ordovician Red River Formation of the Williston Basin, the Ordovician Bauman Fjord Formation of Ellesmere Island, the Devonian of Western Canada and Western Australia, the Pennsylvanian of the Paradox Basin, the Permian of West Texas and the Permian Zechstein of the Netherlands, Northern Germany, Denmark, Poland, the UK, and the North Sea area, as well as Jurassic sedimentary rocks of the Gulf of Mexico and parts of the Lower Cretaceous of Southeastern Texas.

Sabkhas are key landforms along the southern coast of the Arabian Gulf and represent modern analogues for depositional and diagenetic processes controlling properties and quality of ancient hydrocarbon-bearing carbonates. While previous investigations of coastal sabkhas in Qatar have mainly focused on dolomitization processes, presented here is one of the first studies reconstructing facies changes and coastal formation in great detail. In the sabkha of Al-Kharayej (Gulf of Salwa), fifteen different facies types were distinguished based on twelve sediment cores, two trenches, as well as grain-size distribution, X-ray powder diffraction, thin section and microfossil analyses. Age estimates were based on seventy-eight C-14-AMS and optically stimulated luminescence data. The sabkha parasequence comprises pre-transgressive dune sands, a thin, transgressive layer of reworked dune material, a mid-energy open-coast to open-lagoon facies, a low-energy lagoon facies, saline lake facies (salina: swallow-tail gypsum and gypsum mush) and the supratidal sabkha characterized by diagenetic overprinting (buckled gypsum crusts and halite crust). Transgressive marine flooding created open-coast to open-lagoon sedimentation after ca 7000 cal yr bp, followed by initial spit formation at the northern sabkha end at the beginning of the relative sea-level highstand (6000 cal yr bp). This main outer spit prograded southward and a more narrow, low-energy spit, diverted landward, closing a small lagoon in the northern sabkha 4500 to 4000 cal yr bp. The falling relative sea-level and longshore drift intensified the southward extension and widening of the main spit, and the main lagoon became more shallow. At 2000 to 1500 cal yr bp, the outer spit had almost closed the main lagoon, leading to salina and, finally, sabkha conditions. It is shown how specific local conditions (coastline orientation; wind, wave, tidal energy, longshore drift; depositional relief; sediment sources) created a spit-controlled sabkha that is genetically distinct from the classical model of shore-perpendicular accumulation of coarser sediment during high tides or storms.

The Ca Voi Xanh (CVX) gas field is located offshore Vietnam along the eastern margin of the southern Song Hong Basin. Reservoir rocks are carbonates of Middle Miocene (Langhian and Serravallian) age which developed on an isolated platform (length approximately 100 km, and width approximately 15 km) on top of the Triton Horst structural high. Shallow‐water corals and large and small benthic foraminifera are the main faunal constituents of the Langhian carbonates, whereas overlying Serravallian carbonates, the principal reservoir at CVX, are dominated by deeper‐water coralline red algae (rhodoliths) and large benthic foraminifera (LBF). The Serravallian carbonates consist of rhodolith‐LBF grainstones and packstones to mud‐lean packstones. Langhian carbonates consist of coral‐LBF grainstones‐packstone/rudstones.This paper documents the workflow used to develop an integrated sequence‐stratigraphic and reservoir rock‐type framework for the Ca Voi Xanh reservoir, and the impact of this work on reservoir quality prediction and modelling. Sequence stratigraphic interpretations of the Serravallian carbonates are based on data from three wells including detailed sedimentological core descriptions tied to well‐log character. The carbonate succession consists of three third‐order depositional sequences (Ser1, Ser2 and Ser3). Two well‐developed exposure surfaces can be identified: the base‐Serravallian sequence boundary (Ser1 SB), and the base‐Tortonian sequence boundary (Tor1 SB). The Serravallian shows an overall shallowing‐upward trend from carbonates with more horizontally‐oriented coralline red algae (encrusted and bored pavements/hardgrounds) in the lower part of the section, to carbonates with large, roundish irregular rhodoliths towards the upper part. Petrographic thin section, stable isotope (oxygen and carbon), and fluid inclusion analyses confirm a freshwater (vadose and phreatic) diagenetic overprint of the carbonates below the exposure surfaces (sequence boundaries).Partial dolomitization of the Serravallian carbonates was observed at Wells‐2 and ‐3 in the CaVoi Xanh field. Dolomite formation could be related to hydrodynamic fluid interactions associated with the development of a freshwater lense during the top‐Serravallian sea‐level fall (Tor1 SB), favoring dolomitization by sea water during the subsequent sea‐level rise. Alternatively, dolomitization could be related to CO2 and hydrocarbon charging, causing secondary leaching of the Mg‐rich coralline red algae and large benthic foraminifera, leading to an enrichment of the water with Mg‐ions and thus favoring downward dolomitization.A reservoir rock type (RRT) scheme was developed for the Langhian and Serravallian carbonates at the Ca Voi Xanh field based on a combination of depositional environment interpretations together with diagenetic characteristics and reservoir parameters (porosity and permeability).The Langhian is characterized by two RRTs depending on the degree of cementation (RRT‐3L) and dissolution (RRT‐5L). Serravallian RRTs are separated into dominantly packstone (RRT‐1) and dominantly grainstone textures (RRTs‐2, ‐3, ‐5 and ‐6). RRTs with grainstone texture show varying degrees of cementation (RRTs‐2 and ‐3) and dissolution (RRTs‐5 and ‐6). Early diagenetic dolomitization (RRT‐4) preferentially affects packstones (RRT‐1) but also grainstones (RRT‐2, ‐3 and‐5).The geologic (static) model consists of both matrix and non‐matrix components. Sequence stratigraphic concepts in combination with seismically‐derived palaeo‐reconstructions were used to guide the vertical and lateral distribution of reservoir rock types, which comprise the matrix component of the geological model. Different karst geometries and features consistent with known hydrogeologic processes were identified in the seismic discontinuity (variance) cube, and were used to interpret karst regions with different degrees (and/or types) of karst/fractures. These karst regions were used to populate the non‐matrix component of the geologic model along the Ser1 and Tor1 sequence boundaries.

Many have postulated that a specific microbial metabolism or the presence of microbes or/and their extracellular polymeric substances (EPS) can lead to the formation of dolomite. Although now there is the consensus that dolomite can be formed in the presence of microorganisms, the exact nature of the involvement of microbes in the dolomite nucleation remains a matter of debate. The focus is now in understanding how microbial mats determine the mineralogy of dolomite. Here we report the effect of the EPS extracted from phototrophic microbial mat isolated from a sabkha in Qatar dominated by cyanobacteria (Lyngbya aestuarii) in the formation of dolomite precursors at 25 degrees C and 40 degrees C. Both the temperature and the presence of EPS impact the size and morphology of minerals, promoting spherulitic and dumbbell growth in sulfate free solutions. The formation of proto-dolomite was enhanced by the abundance of carboxylated molecules in EPS which controlled the polymorphism of carbonates. Our study emphasizes the dual importance of organic matter and temperature in dolomite formation and their impact on mineral morphology and chemical composition in sabkhas.

The Khor Al-Adaid sabkha in Qatar is among the rare extreme environments on Earth where it is possible to study the formation of dolomitea carbonate mineral whose origin remains unclear and has been hypothetically linked to microbial activity. By combining geochemical measurements with microbiological analysis, we have investigated the microbial mats colonizing the intertidal areas of sabhka. The main aim of this study was to identify communities and conditions that are favorable for dolomite formation. We inspected and sampled two locations. The first site was colonized by microbial mats that graded vertically from photo-oxic to anoxic conditions and were dominated by cyanobacteria. The second site, with higher salinity, had mats with an uppermost photo-oxic layer dominated by filamentous anoxygenic photosynthetic bacteria (FAPB), which potentially act as a protective layer against salinity for cyanobacterial species within the deeper layers. Porewater in the uppermost layers of the both investigated microbial mats was supersaturated with respect to dolomite. Corresponding to the variation of the microbial community's vertical structure, a difference in crystallinity and morphology of dolomitic phases was observed: dumbbell-shaped proto-dolomite in the mats dominated by cyanobacteria and rhombohedral ordered-dolomite in the mat dominated by FAPB.

Barrier islands are important landforms in many coastal systems around the globe. Studies of modern barrier island systems are mostly limited to those of siliciclastic realms, where the islands are recognized as mobile features that form on transgressive coastlines and migrate landward as sea‐level rises. Barrier islands of the ‘Great Pearl Bank’ along the United Arab Emirates coast are the best‐known carbonate examples. These Holocene islands, however, are interpreted to be anchored by older deposits and immobile. The mid‐Holocene to late‐Holocene depositional system at Al Ruwais, northern Qatar, provides an example of a mobile carbonate barrier island system, perhaps more similar to siliciclastic equivalents. Sedimentological and petrographic analyses, as well as 14C‐dating of shells and biogenic remains from vibracored sediments and surface deposits, show that after 7000 years ago a barrier system with a narrow back‐barrier lagoon formed along what is now an exposed coastal zone, while, contemporaneously, a laterally‐extensive coral reef was forming immediately offshore. After 1400 years ago the barrier system was forced to step ca 3 km seaward in response to a sea‐level fall of less than 2 m, where it re‐established itself directly on the mid‐Holocene reef. Since that time, the barrier has retreated landward as much as 1000 m to its current position, exposing previously‐deposited back‐barrier lagoonal sediment at the open‐coast shoreline. In modern neritic warm‐water carbonate settings mobile barrier island systems are rare. Their construction and migration may be inhibited by reef formation, early cementation, and the relative inefficiency of sourcing beach sediments from open carbonate shelves. Carbonate barrier island systems likely formed more commonly during geological periods when ramps and unrimmed shelves predominated and in calcite seas, when meteoric cementation was minimized as a result of initial calcitic allochem mineralogy. As with their siliciclastic analogues, however, recognition of the influence of these transient landforms in the rock record is challenging.

Dedolomitization converts a dolomite into a calcite, and conventionally is interpreted to form in an eogenetic or telogenetic diagenetic environment where meteoric water dissolves evaporites to supply excess calcium. Hydrologic connectivity between a dolostone and the surface water may then lead to a dedolomitized interval, often times being a diagnostic indicator for an erosional unconformity, subaerial exposure, and/or karst. In contrast to the classic models, we present a case study for pervasive burial dedolomitization, unrelated to meteoric fluids. Dedolomitization of the Zechstein-2-Carbonate (Ca2) gas reservoir in NW Germany is strongly altering reservoir quality on a regional scale. The Ca2 shows a textbook correlation between reservoir quality and mineralogy. Petrographic analyses show that approximately 80% of all observed calcite reveal a dedolomite microtexture, causing a reduction of average matrix porosity by 5 to 10% compared to the dolomite. Dedolomite follows initial layering and abundantly forms massive concretions, which are surrounded by compaction-related curvature of the depositional layering in the dolomite host rock. Such an early burial timing is supported by a slight burial-related shift of similar to 3 parts per thousand delta O-18 and similar to 1 parts per thousand delta C-13 towards lighter values in the dedolomites compared to dolomite. An exceptionally large amount of calcium-rich fluids must have been mobilized to account for > 50% of the Ca2 rock volume to be dedolomitized. However, core fabrics related to meteoric diagenesis, such as karst fabrics or typical fresh water stable isotope signatures have been observed neither in the Ca2 nor in over- and underlying anhydrite beds. Instead, excess calcium likely comes from gypsum-to-anhydrite conversion and pressure solution within over- and underlying anhydrites. An influx of strontium-rich waters from the anhydrite beds is indicated by up to 5 times higher strontium contents measured in dedolomite fabrics compared to their dolomite host rock. Homogenization temperatures between 51 and 56 degrees C measured in some early fluid inclusions in dedolomite textures further exclude a pristine meteoric water input and suggest fluid entrapment in a burial depth range of 900 to 1400 m. Comparing these results with literature data furthermore indicates that mesogenetic dedolomite is either volumetrically underestimated in other carbonate-evaporite settings or formed under unique diagenetic conditions across the Southern Permian Basin during Zechstein times.

Sawda Nathil is one of a series of inland depressions (inland sabkhas and salinas) that extend nearly continuously along the Southern Qatar border with Saudi Arabia. Six to eight thousand years ago, these depressions were marine embayments that separated the peninsula of Qatar from the mainland. These embayments in-filled rapidly with marine sediments and dune sands, blown southeastward across Qatar. Over time, they have become progressively more evaporitic. Inland depressions like Sawda Nathil host a set of unique environments within Qatar. Most depressions are close to or below sea level, bringing water near the surface in the driest parts of Qatar. Evaporation to salt saturation creates thick gypsum and halite crusts (sabkhas), as well as shallow hypersaline lakes/ponds (salinas) with spectacular domal microbial gypsum stromatolites. The present ground surface is a mosaic of relict marine facies, deflated dune sands, and inland sabkhas and salinas. Sediment of four short push cores was photographed, described, and sampled for petrographic thin-section and X-ray diffraction (XRD) analyses. Radiocarbon (AMS) as well as optically stimulated luminescence (OSL) age-dating were carried out on three samples. Scanning electron microscopy (SEM), carried out on samples from a gypsum stromatolite, was used to resolve the microbial-influence on gypsum precipitation. Radiocarbon dating of marine shells provides ages of approximately 6600 un-calibrated 14 C years before present (year BP), coinciding with a well-documented sea-level highstand, approximately 2–4 m higher than present. During that time, Qatar was mostly an island, only connected in the south to the Arabian Peninsula by narrow land bridges (isthmuses). SEM examinations of gypsum stromatolites show gypsum crystals developing in close spatial association with microbial biofilms (filamentous structures). Whether this is purely a passive microbial-influenced gypsum mineralization process or an example where microorganisms actively control the gypsum crystal morphology to obtain ecological advantages, remains to be evaluated.

Carbonate rocks can be classified in terms of those properties relating to the pore system of lithified sediments, so-called petrophysical rock types', or depositional rock types' which are categorized based on characteristics directly reflecting their original depositional environment. Whereas petrophysical rock types are typically used to identify and distribute rock bodies within a reservoir with similar flow characteristics, depositional rock types ignore pore types and capture sedimentary structures, lithology and fossils. Both classification systems are extensively used to describe reservoir rocks, but the degree of plurality between them remains poorly understood and is the motivation for this study. To examine the degree of congruency between the two classification schemes, a field assessment was conducted for a 175km(2) area situated offshore Al Ruwais, northern Qatar, encompassing depositional environments spanning supratidal, intertidal, shallow subtidal and open marine conditions. A total of 350 surficial sediment samples were collected along 24 shore-normal transects. Each sample was assigned a petrophysical rock type' class based on analysis of sedimentary texture (grain size and sorting). Depositional rock type' classes, by contrast, were defined with reference to faunal content and, in turn, classes of mineralogy were delimited by weighting this content against the mineralogy of each faunal category. Of course, the samples studied correspond to unconsolidated sediments and not to indurated rocks. However, considering only primary porosity and permeability preservation, it is reasonable to assume that the classified sediments would become petrophysical rock types and depositional rock types when consolidated, following their primary grain size, sorting and grain type distribution. Therefore, the term rock type' is retained here for ease of terminology but, for clarity, these are sediment samples. The discrete samples were interpolated into continuous surfaces describing the distribution of depositional rock types, petrophysical rock types and mineralogy, and spatial correspondence between those surfaces was statistically evaluated. In order to link these parameters with environment of deposition, their correlation with water depth (as audited from airborne light detection and ranging) and ecological habitat (mapped from DigitalGlobe satellite imagery) was also assessed. The data reveal that spatial distributions of sedimentary faunal, petrographic and mineralogical properties do not show exactly congruent patterns. Other meaningful trends do exist, however. For example, the occurrence of certain depositional rock types is indicative of particular petrophysical rock types, and vice versa. Further, connections between petrophysical rock types and mineralogy are emphasized and offer insight as to how the evolution of matrix porosity might be predicted via diagenetic models tuned to specific sediment textures. Useful relationships are also identified between the occurrence of petrophysical rock types and depositional rock types, and both ecological habitat and water depth. The potential of such dualities is two-fold. Firstly, they can be applied to more realistically distribute petrophysical rock types and depositional rock types by environment of deposition in reservoir models and, secondly, the use of modern carbonate systems as subsurface analogues might be enhanced.

The Dohat Faishakh sabkha in Qatar is one of the rare modern environments where it is possible to study the formation of dolomite, a mineral whose origin has been long debated. In previous studies, dolomite formation in this area was considered to be the result of a penecontemporaneous replacement of aragonite, occurring in the presence of Mg-rich evaporated pore-waters. However, a re-investigation of the sabkha revealed that dolomite is not forming exclusively under the evaporitic conditions that characterize the supratidal zone, but also in microbial mats that colonize the lower intertidal zone, indicating that evaporated pore-waters are not a strict requirement for the mineralization process. Moreover, in the supratidal zone, portions of the sediment that are rich in dolomite are also relatively richer in organic material, which derives from partially degraded microbial mats buried in the sediments. Extracellular polymeric substances (EPS) that constitute microbial mats are recognized as an important component for the formation of Mg-rich carbonates. The presence of living and decaying microbial mats comprising EPS, rather than a replacement process, may be the key factor for dolomite formation in the Dohat Faishakh sabkha.

The sabkhas (i.e., salt flats) of Qatar are among the rare places on Earth where carbonate and sulfate minerals similar to those constituting economically important hydrocarbon reservoirs are still forming today, under the arid conditions that characterize the coastline of the country. Since the 1960’s, the sabkhas of Qatar have been studied with great interest as a modern analogue for ancient sedimentary sequences (e.g., Wells, 1962; Illing & Taylor, 1995; Alsharhan & Kendall, 2003). The results of these studies provided important insights for formulating stratigraphic models of subsurface hydrocarbon reservoirs. Notable examples of gas and oil reservoirs that formed in arid, evaporitic environments include the Permo-Triassic Khuff (which is estimated to contain about 15–20% of the world's gas reserves and is of fundamental importance for the economy of Qatar), the Jurassic Arab formations, and the Triassic Kurra Chine, all of the Middle East, and the Permian Zechstein of Northern Europe. Although extremely valuable, most of these early studies were based on purely physical and chemical approaches, which may have not fully captured the complexity of the mineralization processes occurring in the sabkha environment. Indeed, research conducted in more recent years has shown that microorganisms play an important and, as yet, poorly understood role for the mineralization processes occurring in these evaporitic environments (Bontognali et al., 2010; Bontognali et al., 2012; Bontognali et al., 2014; Brauchli et al., 2015; Paulo & Dittrich, 2013; Strohmenger et al., 2011).Here we present the results of a field campaign conducted in the Khor Al-Adaid sabkha, which is located in the southeast of Qatar, in a large tidal embayment composed of two shallow inland lagoons. The main goal of the field campaign was to identify regions of the intertidal zone that are particularly rich in microbial mats, and that represent ideal sites at which to study microbe-mineral interactions. Three sites of interest have been defined.Site 1 is characterized by the presence of microbial mats that develop in a restricted pond where abundant precipitation of gypsum takes place. This site is an ideal place to look for geochemical or mineralogical signatures of microbes in the gypsum crystals. This, in turn, may allow for the definition of new proxies for identification of microbially-mediated gypsum in ancient sedimentary sequences. Because gypsum and anhydrite are common seals of hydrocarbon reservoirs, a broad understanding of the mechanism of their formation is of unquestioned interest in the field of hydrocarbon exploration.Site 2 is characterized by the presence of thick (more than 5 cm) microbial mats. Spherical authigenic carbonate minerals are visibly forming in association with the extracellular polymeric substances constituting these mats. X-ray diffraction analyses revealed that dolomite is among the carbonate minerals forming at this site. Thus, the mats will be studied with the goal of providing new insights helpful in solving the long-standing enigma surrounding the origin of sedimentary dolomite. Dolomite is a common mineral in ancient sedimentary sequences (including many hydrocarbon reservoirs) but historically it has been very difficult to form in laboratory experiments that simulate Earth's surface conditions. For this reason, the mechanism of its formation remains highly debated. It has been proposed that microorganisms play a key role for overcoming the kinetic barriers that prevent dolomite formation at low temperature (Vasconcelos et al., 1995). Because this “microbial hypothesis” is not unanimously accepted, the mats present at site 2 represent an ideal material to study for better understanding and further demonstration of the existence of this biomineralization process.Site 3 is characterized by the presence of well-layered, domical microbial mats. Gas is produced in abundance within the mats, which likely influences pore-water chemistry and might, in turn, influence the rate and the type of mineral precipitation.Work is currently in progress to characterize the microbial diversity of the three sites through “next generation sequencing” methods, as well as characterization of the mineralogy and the isotopic composition of the carbonate and gypsum forming within the microbial mats. The ultimate goal is the better understanding of what role microbes play in the formation of ancient evaporitic sequences.ReferencesAlsharhan, A. S., and Kendall, C. G. S. C., 2003, Holocene coastal carbonates and evaporites of the southern Arabian Gulf and their ancient analogues: Earth-Science Reviews, v. 61, no. 3–4, p. 191–243.Brauchli, M., McKenzie, J.A., Strohmenger, C.J., Sadooni, F., Vasconcelos, C., Bontognali, T.R.R., 2015. The importance of microbial mats for dolomite formation in the Dohat Faishakh sabkha, Qatar. Carbonates and Evaporates, p. 1–7.Bontognali, T. R. R., Vasconcelos, C., Warthmann, R. J., Bernasconi, S. M., Dupraz, C., Strohmenger, C. J., and McKenzie, J. A., 2010, Dolomite formation within microbial mats in the coastal sabkha of Abu Dhabi (United Arab Emirates): Sedimentology, v. 57, no. 3, p. 824–844.Bontognali, T. R. R., Vasconcelos, C., Warthmann, R.J., Lundberg, R., McKenzie, J.A., 2012. Dolomite-mediating bacterium isolated from the sabkha of Abu Dhabi (UAE). Terra Nova 24, 248–254.Bontognali, T. R. R., McKenzie, J. A., Warthmann, R. J., and Vasconcelos, C., 2013, Microbially influenced formation of Mg-calcite and Ca-dolomite in the presence of exopolymeric substances produced by sulfate-reducing bacteria: Terra Nova, p. 1–6.Illing, L. V., and Taylor, J. C. M., 1993, Penecontemporaneous dolomitization in Sabkha Faishakh, Qatar; evidence from changes in the chemistry of the interstitial brines: Journal of Sedimentray Research, v. 63, no. 6, p. 1042–1048.Paulo, C., and Dittrich, M., 2013, 2D Raman spectroscopy study of dolomite and cyanobacterial extracellular polymeric substances from Khor Al-Adaid sabkha (Qatar): Journal of Raman Spectroscopy, v. 44, no. 11, p. 1563–1569.Strohmenger, C. J., Shebl, H., Al-Mansoori, A., Al-Mehsin, K., Al-Jeelani, O., Al-Hoseni, I., Al-Shamry, A., and Al-Baker, S., 2011, Facies stacking patterns in a modern arid environment: a casa study of the Abu Dhabi sabkha in the vicinity of Al-Qanatir Island, United Arab Emirates, in Kendall, C. G. S. C., and Alsharhan, A. S., eds., Quaternary carbonate and evaporite sedimentary facies and their ancient analogues: A Tribute to Douglas James Shearman, Volume 43: Chichester, West Sussex, UK, Wiley-Blackwell, p. 149–182.Vasconcelos, C., McKenzie, J. A., Bernasconi, S., Grujic, D., and Tiens, A. J., 1995, Microbial mediation as a possible mechanism for natural dolomite formation at low temperatures: Nature, v. 377, no. 6546, p. 220–222.Wells, A. J., 1962, Recent Dolomite in the Persian Gulf: Nature, v. 194, no. 4825, p. 274–275.

Maximizar la recuperacion de los campos de petroleo y gas depende de modelos geologicos que describan de manera realista la complejidad, la composicion y las propiedades de las unidades reservorio. Los sistemas costeros de clima arido actuales, como la linea de costa de Qatar, proporcionan analogos para los procesos de deposito y diageneticos que controlan la calidad como reservorio de los antiguos yacimientos. Muchos grandes yacimientos de Qatar y Oriente Medio se formaron bajo condiciones que son marcadamente similares a las que conforman las lineas de costas qataries de hoy en dia. Los principales controles de los patrones de sedimentacion costera son: 1) orientacion de la linea de costa, 2) energia eolica, de las olas y mareal, 3) clima, 4) nivel relativo del mar, 5) relieve deposicional y 6) fuentes de los sedimentos. Los fuertes vientos del NW que prevalecen (vientos Shamal) dirigen los patrones de circulacion marina somera, creando cuatro perfiles de deposito muy diferentes: a barlovento, a sotavento, oblicuo y protegido. Las lineas de costa a barlovento estan marcadas por el desarrollo de arrecifes y laminas litorales y arenas de playa. El perfil costero a sotavento esta dominado por un aporte de sedimentos eolicos, ya que las dunas de arena son transportadas por el viento dentro del mar. A lo largo de las lineas de costa a barlovento y oblicuas, los suelos endurecidos (roca de playa) estabilizan los patrones de circulacion, creando areas de retroplaya lodosas de alfombras microbianas y manglares. Las lineas de costa protegidas se caracterizan por arenas de paneroplidos de grano mas fino y playas de bajo relieve. El tamano de grano, la composicion y las dimensiones de las arenas costeras varian debido a la energia de las olas. Los depositos costeros estan igualmente afectados por las oscilaciones de alta frecuencia del nivel del mar. Hace aproximadamente 6000 anos, el nivel del mar era unos 2 a 4 metros mas alto de lo que es actualmente y la linea de costa qatari llegaba hasta 10 km tierra adentro. La mayoria de los depositos costeros y sabkhas son relictos de este antiguo nivel alto del mar. Las caidas puntuales del nivel del mar hasta el nivel actual dieron lugar a la formacion de sistemas de flechas de playa con avance hacia el mar. Los patrones de sedimentacion y su sobreimpresion diagenetica se estudiaron en detalle en la sabkha costera de Mesaieed, que representa un sistema costero oblicuo en relacion con la direccion predominante del viento. La detallada cartografia de campo, los analisis de dataciones por radiocarbono y la integracion de datos de sondeos geotecnicos, asi como de datos de numerosas calicatas someras, permitieron reconstruir el espesor del Holoceno, la datacion y la reconstruccion espacial del patron de progradacion de las flechas de playa en relacion con el nivel variable del mar, y la cartografia de la cantidad y la distribucion de la porosidad que destruye el yeso. La complejidad y la heterogeneidad espacial observadas en los sistemas costeros modernos son aspectos importantes a tener en cuenta para condicionar los modelos geologicos tridimensionales. Los sistemas de deposito modernos a lo largo de la linea de costa de Qatar, como el estudiado en la sabkha de Mesaieed, son particularmente utiles como analogos para condicionar conjuntos de datos del subsuelo en los modelos geologicos (estaticos) y de yacimientos (dinamicos).

Maximizing recovery in oil and gas fields relies on geological models that realistically portray the spatial complexity, composition, and properties of reservoir units. Present day arid climate coastal systems, like the coastline of Qatar provide analogues for depositional and diagenetic processes that control reservoir quality in ancient reservoirs. Many major reservoirs in Qatar and the Middle East formed under conditions that are remarkably similar to those shaping the Qatari coastlines of today. Major controls on coastal sedimentation patterns are: 1) coastline orientation, 2) wind, wave and tidal energy, 3) climate, 4) relative sea level, 5) depositional relief, and 6) sediment sources. Strong NW prevailing winds (Shamal winds) drive shallow marine circulation patterns, creating four very distinct depositional profiles: windward, leeward, oblique, and protected. Windward coastlines are marked by reef development and intertidal sheet and beach sands. The leeward coastal profile is dominated by an eolian sediment supply, as sand dunes are blown into the sea. Along windward and oblique coastlines, hardgrounds (beachrock) stabilize circulation patterns, creating mud-prone back beach areas of micobial mats and mangroves. Protected coastlines are characterized by finer-grained peneroplid sands and low-relief beaches. Grain size, composition, and dimensions of coastal sands vary due to wave energy. Coastal deposits are equally affected by high-frequency oscillations in sea level. Approximately 6000 years ago, sea level was about 2 to 4 metres higher than it is currently and the Qatari coastline was up to 10 km inland. Most coastal deposits and sabkhas are relics of this ancient highstand in sea level. Punctuated sea-level drops to present-day level have led to the formation of seaward-stepping beach spit systems. Sedimentation patterns and their diagenetic overprint were studied in detail at the coastal sabkha of Mesaieed, which represents an oblique coastal system relatively to the predominant wind direction. Detailed field mapping, radiocarbon age dating analyses, and the integration of geotechnical borehole data, as well as data from numerous shallow pits allowed reconstructing the thickness of the Holocene, the dating and spatial reconstruction of the progradational pattern of the beach spits relative to the varying sea level, and the mapping of the amount and distribution of porosity destroying gypsum. The observed spatial complexity and heterogeneity of modern coastal systems are important aspects to be considered for conditioning three-dimensional geological models. Modern depositional systems along the Qatar coastline, like the one studied at the Mesaieed sabkha, are particularly useful as analogs for conditioning subsurface data sets in geologic (static) and reservoir (dynamic) models.

Many hydrocarbon reservoirs - in Qatar and worldwide - are constituted of dolomite. For this reason, the origin of this Ca-Mg carbonate mineral has been extensively studied by generations of geologists, with the goal of exploiting gas and oil from rock reservoirs in the most efficient way. However, despite more than two centuries of research, several fundamental questions regarding the origin of sedimentary dolomite remain without a convincing answer. Recent research conducted in the field of geobiology suggests that dolomite formation may be the result of a microbial process, that is, organic molecules that in natural environments are produced by microorganisms seem to play a key role for dolomite nucleation at low temperatures in many geological settings. However, this innovative hypothesis is far from being unanimously accepted by the scientific community, and many details on the exact mechanism through which microorganisms mediate dolomite formation are still to be fully understood. The aim of this contribution is to summarize the most recent scientific studies that support the microbial model for dolomite formation, providing examples from culture experiments conducted in the laboratory using artificial growth solutions and from modern dolomite forming environments, such as the hypersaline lagoons located in the State of Rio de Janeiro (Brazil), the sabkhas of Abu Dhabi (UAE), and the sabkhas of Dohat Faishakh and Khor Al-Adaid (Qatar). Furthermore, we will elaborate on why we consider the coastal sabkhas of Qatar to be among the most ideal places on Earth where it is possible to study microbe-mineral interactions in evaporitic environments. In fact, thanks to the distinctive geology that characterizes this region, it is possible to obtain samples documenting the progressive transformation of the living microbial mats that mediate dolomite formation and other authigenic minerals into a fully lithified sediment, which is analogous to dolomite formations constituting economically important gas/oil reservoir rocks. This approach will provide key insights to test whether dolomite present in ancient evaporitic sequences can be interpreted as a fully biological product associated with early diagenesis or whether most of the dolomite forms during later stage metamorphic/replacement events that are controlled by purely abiotic processes. Finally, considering not only the scientific importance but also the aesthetic beauty of the Qatari evaporitic environments, we will discuss the idea and the challenges of transforming selected areas of the modern sabkhas into geoparks - protected natural reserves that would be of interest for the local Qatari population, as well as for tourists visiting Qatar.

Abstract Maximizing recovery in oil and gas fields relies on geological models that realistically portray the spatial complexity, composition, and properties of reservoir units. Present day arid climate coastal systems, like the coastline of Qatar provide analogues for depositional and diagenetic processes that control reservoir quality in ancient reservoirs. Many major reservoirs in Qatar and the Middle East formed under conditions that are remarkably similar to those shaping the Qatari coastlines of today. Major controls on coastal sedimentation patterns are:coastline orientation,wind, wave and tidal energy,climate,relative sea level,depositional relief, andsediment sources. Strong NW prevailing winds (Shamal winds) drive shallow marine circulation patterns, creating four very distinct depositional profiles: windward, leeward, oblique, and protected. Windward coastlines are marked by reef development and intertidal sheet and beach sands. The leeward coast profile is dominated by an eolian sediment supply, as sand dunes are blown into the sea. Along windward and oblique coastlines, shoreface hardgrounds stabilize circulation patterns, creating mud-prone areas of stromatolites and mangroves. Protected coastlines are characterized by finer-grained peneroplid sands and low-relief beaches. Grain size, composition, and dimensions of coastal sands vary due to wave energy. Coastal deposits are equally affected by high-frequency oscillations in sea level. Approximately 6,000 years ago, sea level was about 2 to 4 meters higher than it is currently and the Qatari coastline was up to 10km inland. Most coastal deposits and sabkhas are relicts of this ancient highstand in sea level. Punctuated sea-level drops to present day level have led to the formation of seaward-stepping beach spit systems. Sedimentation patterns and their diagenetic overprint were studied in detail at the coastal sabkha of Mesaieed, which represents an oblique coastal system relatively to the predominant wind direction. Detailed field mapping, radiocarbon age dating analyses, and the integration of geotechnical borehole data, as well as data from numerous shallow pits allowed reconstructing the thickness of the Holocene, the dating and spatial reconstruction of the progradational pattern of the beach spits relative to the varying sea level, and the mapping of the amount and distribution of porosity destroying gypsum. The observed spatial complexity and heterogeneity of modern coastal systems are important aspects to be considered for conditioning three-dimensional geological models. Modern depositional systems along the Qatar coastline, like the one studied at the Mesaieed sabkha, are particularly useful as analogs for conditioning subsurface data sets in geologic (static) and reservoir (dynamic) models. Introduction The peninsula of Qatar is located approximately 25 degrees north of the equator and measures roughly 190km in north-south and 90km in east-west direction. Strong, seasonal northwesterly winds locally called Shamal winds, drive marine circulation patterns. Other factors that control sedimentation patterns in coastal areas include: relative sea level, climate, depositional relief, and sediment sources (Jameson et al., 2009; Jameson et al., 2010). Together, these factors combine to produce four distinct coastal environments: 1) windward coastline: northern coastline (Al-Ruwayis area), oblique coastline: northeastern to eastern coastline (Al-Thakhira and Mesaieed areas), leeward coastline: southeastern coastline (Khor Al-Adaid area), and protected coastline: western coastline (Bir Zekreet and Al Zareq areas). These coastal areas together with the inland sabkhas of Dukhan (east of the Dukhan anticline) and Sawda Nathil are the focus of our research (Fig. 1).

Abstract Modern sabkhas are recognized as analogues to ancient evaporitic reservoirs and as Earth analogues to Martian paleo-environments. Sabkhas are normal marine coastal sediments modified by groundwater precipitation of evaporites and carbonates. Previous work on Holocene sabkhas has focused largely on dolomitisation in carbonate-evaporite systems. Little attention has been given to understanding the origins of evaporites in mixed clastic-carbonate systems and their influence on reservoir quality. Extensive and detailed geomorphological and sedimentological characterization of depositional environments in Qatar provides a framework within which to understand processes controlling the origins of evaporites, their spatial distribution and likely evolution through time. Mesaieed sabkha is a 4–6 km wide coastal plain which consists of an onlap wedge of Holocene sediments some 3–6 m thick reaching a maximum of 15 m, which onlaps onto Eocene bedrock. Within the sabkha, gypsum is the most abundant diagenetic mineral, reaching 20–50% of the sediment volume over several square kilometres, with minor calcite, dolomite, anhydrite and halite. Gypsum cementation is pervasive above and below the water table in the proximal sabkha, in sediments dated c.6,000 years before present (yr BP), whilst in the central part (c. 4,000 yr BP) gypsum is restricted to surface crusts and water table cements, and is largely absent in the distal (coastal) sabkha (= 2,000 yr BP). Preliminary analysis of hydrological and geochemical data suggests evaporative pumping of groundwater from the underlying aquifer is an important source of solutes in the upper part of the sabkha, whilst seawater recharges the lower sabkha via the porous and permeable Eocene carbonates. Evaporation close to the water table results in fluids reaching gypsum saturation, and active precipitation of gypsum is evidenced by depletion of calcium and sulphate in the shallow brines. This is most marked in the middle part of the sabkha where salinity is highest. These increased density fluids reflux downwards from the Holocene, to mix within the Eocene aquifer, where reaction with the Eocene carbonates results in relative enrichment of calcium. Introduction Many ancient sedimentary systems, particularly those deposited at low latitude, include units deposited in non-evaporitic settings but within which evaporitic minerals occlude significant volumes of porosity such as the Permian Zechstein Formation, the Permo-Triassic Khuff Formation, and the Jurassic Arab Formation. Much of the pore-filling or nodular anhydrite that is common at a wide range of burial depths may be secondary, precipitated from pore fluids rich in Ca2+ and SO42- (Kendall and Walters, 1978). Anhydrite is the product of dehydration of a gypsum precursor, which is the most abundant primary CaSO4 mineral (Warren, 2006). However, the sources of solutes forming these evaporites, flow pathways of fluids transporting these solutes and the stability of resulting evaporite precipitates has received little attention. Studies of ancient rock seldom attempt to distinguish between CaSO4 that was precipitated as gypsum prior to dehydration and CaSO4 precipitated at depth as primary anhydrite. Hence by using a modern arid environment to develop a better understanding of the distribution of early diagenetic CaSO4, and thus contribute to reconciling potential sources of secondary anhydrite formed during burial.

Abstract The evolution of coastal plains, their inhabitation patterns, present day shape, and surface hydrology of Qatar are related to changes in relative sea-level. Several factors, acting on different time scales, have contributed to sea-level changes. These include tectonism, glacio-eustasy and possibly isostatic rebound. The peninsula shape is the surface expression of the Qatar Arch, one of the largest structural features of the Arabian Plate. It plunges northward into the Zagros foredeep. Tertiary age, compressional flexure of the foredeep and plate tilting associated with Red Sea rifting are likely tectonic forces. Previous studies indicate the Arabian Gulf was a fluvial plain during the last glacial maximum 18,000 years before present (yr BP). The Gulf began flooding 14,000 yr BP in response to ice melting. The period between 14,000–9,000 yr BP is marked by a rapid rise (2m/100yr). Age dating of coastal deposits indicates that sea-level was about 2–4 meters higher than present between 8,000-3,000 years BP. Most coastal deposits are relicts of this sea-level highstand. During this period coral reefs formed a discontinuous fringe around the windward and oblique coastlines. A sea-level drop approximately 2,000 yr BP may account for the demise of the fringing reefs. Similar beaches are found elsewhere along the Gulf. The occurrence of Pliocene age fluvial gravel deposits of the Hofuf Formation on hill tops 30 to 90 meters above sea-level are interpreted as related to long term tectonic uplift, associated with the evolution of the Zagros foredeep and structural tilting of the Arabian Plate. Pleistocene shoreline deposits may be part of the same structural flexural event or reflect the marine isotope stage 5e. Data from Pliocene to present suggest that the sea-level history of Qatar reflects relatively high-frequency changes in seal-level driven by eustasy superimposed on a long term pattern of tectonic uplift. Introduction Relative changes in sea-level are a primary control on the sedimentological evolution of coastal plains, climate and human habitation. Coastal plains along the southern margin of the Arabian Gulf have been of particular interest to the oil and gas industry as an analogue to ancient evaporitic carbonate ramps in the subsurface (Jameson and Puls, 2009, Tucker and Wright, 1990). The influence of plate tectonics on Holocene sedimentation patterns in Qatar and elsewhere along the southern Gulf has not been well documented. The presence of beaches stranded well above present sea-level along the southern margin of the Gulf has been well known for over 40 years (Kassler,1973). However, little attempt has been made to establish the origins for these events or examine the impact of relative sea-level changes on depositional patterns. The 11,700 year span of the Holocene is simply too short to capture longer range tectonic processes. This paper derives tectonic data from surface geology from the Miocene-Pleistocene Hofuf Fm. (Al-Saad et al, 2002) and Pleistocene outcrops along the present day coast. Holocene sedimentation patterns are compared to both the long term tectonic trends and well-established, post glacial eustatic sea-level changes in order to document the relative influences tectonics and eustasy on sedimentation.

Arid climate carbonate reservoirs like the Permian Zechstein, the Permo-Triassic Khuff, the Triassic Kurra Chine, and the Jurassic Arab formations show early diagenetic dolomite clearly linked to facies successions. They are dominated by intertidal, microbial-laminated carbonates and often encased by salinaand sabkha-type evaporite layers. SEM analysis of samples from the Permian Zechstein Formation reveals filaments around dolomite crystals interpreted as fossilized extracellular polymeric substances (EPS) of ancient biofilms. Similar, fine-crystalline dolomite displaying authigenic spheroids of subhedral to euhedral dolomite rhombohedra, embedded in an organic matrix of EPS have been found in Recent and sub-Recent microbial mats and lagoonal deposits of the Abu Dhabi and Qatar coastal sabkha environments. The main factor controlling the occurrence of significant amounts of dolomite (up to 50%) within these sediments is interpreted to be the presence of an organic matrix and not, as proposed in previous studies, a replacement process. In contrast to arid carbonate settings (e.g. the Arabian Gulf), where relatively thick polygonal microbial mats are dominating the intertidal environment, only relatively thin leathery and crinkly-laminated microbial mats are found in the supratidal environment under humid conditions (e.g. the Bahamas). Penecontemporaneous dolomite is relatively abundant in arid carbonate coastal environments, often associated with evaporites and thick, aerially extensive microbial mats. Humid carbonate coastal environments show less penecontemporaneous dolomite, and microbial mats are less common. Dolomite is more common in carbonate reservoirs of the Arabian Peninsula, deposited under arid climatic conditions (e.g. Khuff and Arab formations), compared to those deposited under humid climatic conditions (e.g. Thamama Group). Microbial-mediated dolomite is therefore interpreted to play an important role in the dolomitization process observed in arid climate carbonate reservoirs. Dolomite is also found to form in small semi-closed pores or micro-niches (e.g. shell borings and micro-borings) within grain-dominated carbonate sediments. These micro-niches retain connate water and become anoxic due to microbial activity, thereby mediating the precipitation of dolomite. Microbial dolomite is therefore not exclusively restricted to sabkha settings, but can be found in any depositional environment. Microbial-Mediated Dolomite from Coastal Sabkha Environments of Abu Dhabi and Qatar: Analogues to Subsurface Arid Climate Dolomitized Reservoir Rocks Christian J. Strohmenger1, Fadhil Sadooni2, Tomaso R. R. Bontognali3, Judith A. McKenzie3 and Crisogono Vasconcelos3 1 ExxonMobil Research Qatar, Doha, Qatar 2 Environmental Studies Center, Doha, Qatar 3 ETH Zürich, Geologisches Institut, Zürich, Switzerland