The Upper Triassic in the Tarim Basin represents one of the most significant hydrocarbon source strata within the basin's Mesozoic-Cenozoic petroleum system. However, oil and gas reservoirs within the Upper Triassic are still rarely discovered, and whether deep-water sedimentation developed during this period remains controversial. In this study, a deep-water identification framework integrating core structures, well-log responses, and seismic facies was established based on extensive drilling, logging, and seismic data. Using wavelet transform analysis, a Triassic lake-level fluctuation curve was reconstructed; in particular, the Huangshanjie Formation interval was highlighted and examined in greater detail, and the relationship between lake-level variations and deep-water sedimentary systems was evaluated. The results indicate that three large-scale lake transgressions and two large-scale lake regressions occurred during the Triassic. From the Ehuobulak Formation (T1e) through the Karamay Formation (T2k) to the Huangshanjie Formation (T3h), the basin underwent a transgressive-regressive-transgressive evolution, corresponding to a rise-fall-rise in lake level. During the Huangshanjie Formation, accommodation space reached its maximum, and the lake basin expanded to form a laterally extensive semi-deep- to deep-lacustrine environment, providing the necessary accommodation and slope-break conditions for the development of deep-water turbidite systems. In the northern part of the basin, multiple sets of deep-water gravity-flow deposits are developed within the Huangshanjie Formation, including turbidite sand bodies, turbidite channels, and sublacustrine fans. These sand bodies have a sand content of approximately 15%-20%, forming effective reservoir intervals. At the same time, thick dark mudstones (240-300 m) of semi-deep- to deep-lacustrine facies are developed, with total organic carbon (TOC) ranges from 0.31 to 5.0 wt%. These mudstones act not only as high-quality source rocks but also as regional cap rocks. Comprehensive analysis suggests that the deep-water sedimentary system of the Huangshanjie Formation was jointly controlled by lake-level fluctuations and sediment supply. In the northern part of the basin, this system exhibits the geological conditions necessary to form a self-generating and self-storing lithologic reservoir, representing a favourable target for deep-water exploration in the Upper Triassic.

ABSTRACT The Triassic in the Tarim Basin is an important target for continental oil and gas exploration. However, the basin‐scale spatiotemporal configuration of sedimentary systems and the distribution of sand bodies remain poorly constrained due to the combined effects of multi‐stage tectonic activity, variable provenance supply, and complex sediment transport pathways, thereby hindering the prediction and exploration of high‐quality reservoirs. Seismic, logging, core, and heavy‐mineral data were integrated to investigate the provenance of Triassic deposits. Through seismic facies tracing, sedimentary facies identification, and provenance analysis, the types and distribution patterns of sedimentary systems under different provenance settings were systematically constrained. The results indicate that four major provenance systems developed during the Triassic in the Tarim Basin. The western provenance was mainly derived from the Awati tectonic belt and the Keping fault uplift, forming proximal alluvial fan and fan‐delta deposits. The southern provenance was controlled by the uplifted northern margin of the Kunlun Mountains and was represented by a distal fluvial delta system. The northern provenance was jointly influenced by the Kuruktag Mountains and the Tabei Uplift, forming a mixed depositional system comprising alluvial fans, fan deltas, and braided river deltas. The eastern provenance was mainly supplied by the Qilian Mountains and the Altyn Tagh tectonic belt, resulting in a distal braided river delta system. These results highlight the strong control of regionally differentiated provenance systems on Triassic sedimentary filling and provide a geological basis for basin‐wide reservoir prediction and exploration.

Reef-shoal complexes were extensively developed during the Middle and Late Ordovician in the Tarim Basin. Based on core observations from 18 drilling wells, petrographic analysis of more than 500 large thin sections, and correlation of outcrop sections, this study investigates the reefal deposits of the Middle–Upper Ordovician. The results indicate that the formation of these reefs is primarily associated with reef-building organisms, including receptaculites-sponges, corals, stromatoporoids, and calcimicrobes. Reef-shoal complexes consist mainly of framework reefs, lime-mud mounds and grainstone shoal or banks, with “small” reef and “big” shoal, as well as shallowing upward sequences. Laterally, these complexes are discontinuously distributed in sheets along the carbonate platform margin of the No. I fault zone in the northern margin of the Tazhong area; vertically, they exhibit multi-storey superimposed stacking, demonstrating a distinctive depositional architecture and forming the most important reservoir facies belt in the region. Furthermore, this study reveals that meter-scale sea-level cycles resulting in episodic meteoric freshwater dissolution, epikarstification during the late depositional stage of the Lianglitage Formation in the Tazhong area, as well as faulting and fracturing, collectively played a critical role in controlling the formation of favorable porous-vuggy and fractured layered reservoirs within the reef-shoal bodies. These insights have provided crucial guidance for the discovery of the largest Ordovician reef-shoal condensate gas field in China within the Tarim Basin.

The development and distribution of depositional and dispersal systems and their controlling factors have long been key topics in basin analysis. The Early Silurian Kalpintag Formation in the Tarim Basin, a nearly 1000 m thick marine siliciclastic succession with important hydrocarbon-bearing intervals, is characterised by a complex sequence architecture and diverse depositional systems. Based on the integrated analysis of cores, outcrops, well logs, and seismic data, the sequence architecture, sediment provenance, dispersal patterns and their key controls are systematically documented. Five major depositional systems are identified, including tide-influenced delta, estuary, tidal flat, braided-river delta, and shallow marine. Their distribution can be classified into three facies tracts: eastern braided-river delta, northern tide-influenced delta and estuary, and southern tidal flat and estuary. Strong uplift of the Altyn Mountains along the eastern basin margin produced steep slopes and abundant sediment supply, leading to extensive development of the braided-river delta system. Sediment input from the Tabei Uplift on a relatively gentle paleoslope controlled the tide-influenced delta and estuarine deposits in the north. In the south, the limited sediment supply from the Tanan Uplift and low-relief paleogeomorphology led to widespread tidal flat deposits. Two depositional sequences are identified in the Kalpintag Formation, each comprising transgressive (TST) and highstand (HST) systems tracts. The sequence architecture and shifts in sedimentary environments were clearly controlled by relative sea-level changes and correlated with the global eustatic curve.

Shale gas geochemical characteristics are critical for genetic identification and enrichment mechanism analysis. Current research focuses predominantly on marine shale gas, whereas studies on transitional shale gas (particularly the Permian Longtan Formation in South China) remain relatively limited, which constrains our understanding of the geochemical characteristics and genesis of transitional shale gases. The geochemical characteristics of transitional shale gas from the upper Permian Longtan Formation in western Hubei Province indicate that the organic matter is predominantly sapropelic, with some humic organic matter. The gas composition of the Longtan Formation consists primarily of CH4, with a low content of C2H6. The delta C-13(CH4) values range from -25.40 parts per thousand to -21.70 parts per thousand, the delta C-13(C2H6) values range from -32.00 parts per thousand to -27.02 parts per thousand, and the delta H-2(CH4) values range from -124.01 parts per thousand to -119.46 parts per thousand. These findings imply that shale gas is composed mainly of oil-type gas of thermal origin, with a potentially minor presence of mixed gas. Isotope analysis of the shale gas reveals that it has undergone reversal (delta C-13(CH4) > delta C-13(C2H6)), which is attributed to the cracking of liquid hydrocarbons under overmature conditions. The CO2 content ranges from 0.21 % to 2.33 % and the delta C-13(CO2) values range from -21.80 parts per thousand to -19.00 parts per thousand, suggesting that the CO2 in the study area is of organic thermal origin. Additionally, a geochemical evolution pattern suggests that the gas composition of different sedimentary phases is controlled by the type of kerogen and the degree of thermal evolution. The multistage cracking of organic matter thermal evolution products caused the dynamic changes in carbon isotopes.

ABSTRACT The Early Cambrian was a crucial transition for Earth's environment and life evolution. The Early Cambrian Yuertusi Formation black shale in the Tarim Basin, a typical organic‐rich black rock series, records regional sedimentary and paleo‐oceanographic information. Integrating outcrop, logging, core, seismic and litho‐geochemical data, this study documents the Yuertusi Formation's sedimentary features and organic‐rich deposit controls. Comprising lower black shale and upper carbonate, it divides into three sequences (Sq1–3). Three sedimentary facies (outer ramp, mid ramp, and inner ramp) formed in a ramp setting. Geochemical analysis shows the outer ramp had higher paleoproductivity. Oxygen isotopes and siliceous rock lithofacies suggest hydrothermal activities were in the outer ramp during Sq1. Cross‐plots and shale phosphorus indicate upwellings were active in mid‐outer ramps during Sq1–3, stronger in the mid ramp. Redox proxies show outer ramp Sq1 organic‐rich shales were anoxic, mid ramp Sq2–3 organic‐lean carbonates were oxic. Terrigenous influx (Al, Ti) didn’t correlate with TOC. In summary, the most organic‐rich deposits (TOC > 5%) in the outer ramp along the northern basin margin were controlled by hydrothermal‐induced high productivity and anoxic preservation, with siliceous, phosphate rocks and abundant trace elements.

The Cretaceous Kapushaliang Group of the northern Tarim Basin contains more than 800 m delta-lacustrine deposits, which are essential for oil and gas exploration. Substantial research has been carried out previously on the deltaic system in the Kapushaliang Group, but the paleo-environment and its influence on the delta evolution are poorly understood. Herein, we determine deltaic system distribution within sequence stratigraphy as well as investigate how the paleo-environment controls delta evolution. Sequence boundaries are identified through unconformities in the seismic profiles and abrupt changes in the well-logging curves, revealing five 3rd-order sequences (Sq1–Sq4 in the Yageliemu and Shushanhe Formations and Sq5 in the Baxigai Formation) are identified. Facies associations and delta distributions are determined using well-logging profiles, cores, outcrops, relative sand content, and seismic attributes. The paleo-environment is documented by mudstone sample testing in the Kuchehe outcrop. The gradual aridification of the climate and decreasing lake levels during the early stage of the Kapushaliang Group led to delta aggradation in the southern part. In addition, the uplift of the South Tianshan Mountains resulted in increased accommodation and exhibited a retrogradation pattern in the northern part. During the early stage of the Kapushaliang Group, increased lake level caused delta retrogradation before maximum transgression, and showed progradation after maximum transgression in the southern part. Increased accommodation and stable sediments resulted in delta retrogradation in the northern part. At the late stages of the Kapushaliang Group, the accommodation gradually decreased in both the southern and northern parts. The dry climate accelerated the water evaporation, and increased sediment supply led to delta progradation in the southern part and aggregation in the northern part. These results show that the deltaic system from Altyn Mountain has a weak tectonic background and is more sensitive to climate and lake level changes than the delta from Tianshan Mountain in the northern part within a 3rd-order sequence stratigraphy.

The Lower–Middle Jurassic of the Kuqa Depression consists of terrestrial clastic deposits containing coal seams and thick lacustrine mudstones, and is of great significance for oil and gas exploration. Based on the comprehensive analysis of core, well-logging, outcrop, and seismic data, the sequence stratigraphy, depositional systems, and the controlling factors of the basin filling in the depression are systematically documented. Four primary depositional systems, including braided river delta, meandering river delta, lacustrine, and swamp deposits, are identified within the Ahe, Yangxia, and Kezilenuer Formations of the Lower–Middle Jurassic. The basin fills can be classified into two second-order and nine third-order sequences (SQ1–SQ9) confined by regional or local unconformities and their correlative conformities. This study shows that the sedimentary evolution has undergone the following three stages: Stage I (SQ1–SQ2) primarily developed braided river, braided river delta, and shallow lacustrine deposits; Stage II (SQ3–SQ5) primarily developed meandering river, meandering river delta, and extensive deep and semi-deep lacustrine deposits; Stage III (SQ6–SQ9) primarily developed swamp (SQ6–SQ7), meandering river delta, and shore–shallow lacustrine deposits (SQ8–SQ9). The uplift of the Tianshan Orogenic Belt in the Early Jurassic (Stage I) may have facilitated the development of braided fluvial–deltaic deposits. The subsequential expansion of the sedimentary area and the weakened sediment supply can be attributed to the planation of the source area and widespread basin subsidence, with the transition of the depositional environments from braided river delta deposits to meandering river delta and swamp deposits. The regional expansion or rise of the lake during Stage II was likely triggered by the hot and humid climate conditions, possibly associated with the Early Jurassic Toarcian Oceanic Anoxic Event. The thick swamp deposits formed during Stage III may be controlled by the interplay of rational accommodation, warm and humid climatic conditions, and limited sediment supply. Milankovitch cycles identified in Stage III further reveal that coal accumulation was primarily modulated by long-period eccentricity forcing.

The Wusonggeer Formation of the Cambrian Series 2 is one of the important carbonate oil/gas bearing intervals in the Tarim Basin. It recorded significant paleoclimatic conditions and history of sea-level variation and has long been a popular research issue in the region. Based on drilling data and thin sections from core wells, 18 microfacies (Mf1-Mf18) and 11 microfacies associations (MA1-MA11) of the Formation are identified, and the main depositional facies include mixed tidal flat, evaporated platform, restricted platform, semi-restricted platform, platform margin, and slope-shelf deposits. According to unconformity contacts such as toplap and onlap, lithofacies transition and karst interfaces, the Wusonggeer Formation comprises a composite sequence (WCS1) and three shallowing-upward third-order sequences (WSQ1-WSQ3). The highstand systems tracts (HST) dominate each sequence and consist mainly of evaporated lagoon deposits (MA3), reef-shoal complexes and shoal deposits (MA8), while the transgressive systems tracts (TST) are predominately composed of intershoal deposits (MA9) and restricted tidal flat deposits (MA7). The depositional architecture from WSQ1 to WSQ3 evolved from restricted to evaporated carbonate platforms, revealing three depositional stages of carbonate platform in gradual arid and warm climatic background, including semi-restricted rimless platform in a relatively humid climate (WSQ1), restricted weak-rimmed platform in a semi-arid climate (WSQ2), and evaporated weakrimmed platform in an arid climate (WSQ3). The salinity changes of the seawater seem to be consistent with the relative sea-level change indicated by the stacking pattern of facies associations and this may suggest that the depositional cycles of the platform were mainly constrained by the synergistic processed of climate and sea level change. WSQ2 marks a key turning period from a relatively humid climatic condition to a dry and hot climate is suggested to be the result of the global climate change during the late Cambrian Series 2.

The paleokarst systems of the Ordovician carbonate rocks in the Tarim Basin, northwestern China, comprise economically significant oil and gas reservoirs and display complex cave architectures. Based on comprehensive analysis of seismic, well log, core, and outcrop data, the cave architecture and controlling processes of the Ordovician paleokarst systems in the western margin and central uplift belt of the basin are documented. Cave fills of the paleokarst systems are composed mainly of collapse breccias, crackle or mosaic breccias, chaotic breccias, terrestrial sediment fills, and calcareous muddy deposits. Primary architectural elements of the paleokarst systems include surface collapse caves or pits, fractured roofs and walls, sinkholes and associated small-scale caves, cave-level or fault-cave complexes, fractured layers with fractured pores or cavities, and densely spaced small cavity layers. The paleokarst cave structures are characterized by the development of multiple phreatic cave-level or fault-cave complexes and were constrained mainly by the interplay of changing phreatic zones due to multiple stage uplifts or relative sea-level falls and