The Maokou Formation of Middle Permian has huge resource potential and is an important target for natural gas exploration in Sichuan Basin. In recent years, significant exploration breakthroughs have been made in the dolomite of the 2nd member of Maokou Formation in central Sichuan Basin, and the gas production of several wells has exceeded one million cubic meters, which reveals the huge exploration prospect in this field. However, the genesis of dolomite is still unclear, which restricts the prediction of dolomite distribution. Focusing on the core exploration wells in Hechuan area, a detailed description of the petrological characteristics based on core and thin sections is carried out, and representative samples of dolomite and limestone are selected for carbon oxygen isotope, strontium isotope, rare earth element, and UPb dating analysis. Taking into account the geological background, it was clarified that: (1) Dolomite is mainly developed in the middle-upper part of the 2nd member of Maokou Formation, with a thickness of 1-25 m, and its original rock is grainy limestone. (2) The dolomitization fluid is mainly seawater, and dolomitization occurred in the quasicontemporaneous period-early burial period. (3) The shoal developed in a relatively paleogeomorphologic high part of the 2nd member of Maokou Formation was susceptible to syngenetic karstification, and a large fracture-cavern system developed in the phreatic zone. Fracture-cavern system were filled with bioclastic particles, marl and Mg2+ rich seawater, and dolomitization occurred during the shallow burial process. Based on the new research results of dolomite genesis, it is clear that the paleogeomorphologic high part is the favorable area of dolomite of the 2nd member of Maokou Formation, which provides a basis for the prediction of dolomite reservoir distribution in the study area and effectively guides the exploration deployment. Differences in the characteristics and distribution patterns of dolomite reservoirs of the Upper Cambrian Xiaqiulitage Formation in the western Tarim Basin. Based on a systematic analysis of rock thin section, carbon and oxygen isotope compositions, and U-Pb dating, the conclusions are drawn as follows: (1) The Xiaqiulitage Formation, with a total thickness of 350 m, is divided into six members, and is composed of grain dolomite, thrombolite dolomite, stromatolite dolomite, and laminated microbialite dolomite. Seven lithofacies association and two third-order sequences are identified in the Xiaqiulitage Formation, reflecting the overall transition of tidal flat subfacies to inner platform shoal subfacies from bottom to top. (2) The reservoir spaces are dominated by matrix dissolution pores, vugs (dissolution cavities), and intergranular fractures within breccias. The columnar stromatolitic dolomite and thrombolitic dolomite exhibit the best physical properties, followed by grain dolomite, with the overall characteristics of moderate-to-high porosity and moderate-to-low permeability. A comprehensive evaluation indicates that the reservoir properties are optimal in Member 1, Member 2, and Member 6, while Member 5 ranks slightly lower. (3) The dolomite was formed during the early diagenetic stage, and reservoir development is primarily controlled by the combined effects of sedimentary microfacies, unconformity surfaces, and high-frequency sequences. The reservoirs can be classified into two types: unconformity-karst dolomite reservoirs and inner mound-shoal dolomite reservoirs. This research provides critical support for evaluating favorable exploration zones in the Cambrian dolomite plays of the western Tabei area, and offers reliable evidence for hydrocarbon reservoir assessment, particularly in the Xiongying region.

Diagenesis of reservoirs affects pore development, distribution and reservoir quality. Relevant studies on the Middle Triassic Leikoupo Formation in central Western Sichuan Basin are still blank. In order to reveal the diagenetic evolution characteristics of dolomite reservoirs in the study area and their influence on reservoir quality, and to provide a theoretical basis for the exploration of carbonate oil and gas, the study on the dolomite diagenesis of Leikou Formation in western Sichuan Basin is systematically analyzed on the basis of data analysis such as core and thin section observations, dolomite order degree analysis, fluid inclusions and carbon-oxygen stable isotope tests. The results show that: (1) The Leikoupo Formation in central Western Sichuan mainly develops two different types of dolomite: micritic dolomite and micritic algal clast dolomite. The low degree of ordering and low formation temperature of the dolomite crystals indicate that they were formed by penecontemporaneous dolomitization. The reservoir spaces mainly consist of dissolved pores developed along algal frameworks, intergranular pores, and tectonic breccia interstices. (2) The dolomite in the study area has mainly undergone diagenetic processes such as fracturing, dolomitization, dedolomitization, dissolution, micritization, cementation, and surface-induced demineralization. Among them, structural fracture and dissolution play an improving role in the physical properties of the reservoir. Deep dissolution is the fundamental factor for the development of deep secondary pores. (3) The correlation between the development characteristics of dissolution pores and structural breccia and structural fractures is confirmed: acidic fluids were injected along the fracture space into the remaining pore development areas such as sandy shoal and the framework of the algal layer to form secondary dissolution pores in the late Indosinian stage. Vertical fissures and late structural breccia were formed in the early stage of the Himalayan Movement, and late dissolution and calcite vein filling occurred. Horizontal fractures formed in the late Himalayan period, further improving the physical properties of the reservoir. The research has for the first time clarified the diagenetic sequence and pore evolution model of the dolomite reservoir of the Leikoupo Formation in central Western Sichuan Basin, and proposes a three-stage reservoir control mechanism of "structural fracture-fluid dissolution-fracture modification", providing new geological basis for the exploration of the Leikoupo Formation

This study addresses the core issues of unclear sequence architecture and sedimentary evolution patterns of the bioclastic limestone member of the Carboniferous Bachu Formation in eastern Tazhong area, Tarim Basin. Based on a wealth of data, including core, thin section, logging, and geochemical data, we conduct an in-depth analysis of the petrological characteristics of the bioclastic limestone member, construct the sequence stratigraphic framework for this member, and explore its control on sedimentation and reservoir formation. The research findings demonstrate that: (1) The bioclastic limestone member in the eastern Tazhong area represents a mixed siliciclastic-carbonate sedimentation within a marine-terrestrial transitional setting, mainly composed of micritic to peloidal limestone/dolomite, calcarenite/doloarenite to calcirudite/doloyunrudite, mixed rocks, and transitional lithologies, with a relatively high content of terrigenous clastics. The distribution of lithologies exhibits distinct vertical segmentation and lateral zonation patterns. (2) Based on variations in lithology and sedimentary facies, the bioclastic limestone member, along with the underlying Donghe sandstone member, the lower mudstone member, and the overlying middle mudstone member, forms a complete threeorder sequence. The bioclastic limestone member itself represents a complete transgressive-regressive sequence, with the middle submember recording the relatively deepest marine flooding conditions during deposition. (3) The eustatic cycles exert a decisive influence on the evolution of sedimentary microfacies and diagenetic processes. The lower and upper submembers, deposited in shallow waters, are dominated by supratidal dolomicrite (dolomudstone) facies within evaporitic tidal flats. During relative sea-level rise in the middle submember, high-energy grain shoal complexes developed within intertidal settings, where superimposed high-frequency exposure events drove meteoric dissolution and penecontemporaneous dolomitization, thereby generating high-quality reservoirs with superior storage capacity. These dolomitized grain shoal and dolomicrite flats with pinprick vugs together constitute the most favorable reservoir facies of the bioclastic limestone member in the eastern Tazhong area, Tarim Basin, and represent the primary targets for future exploration and development

Taking the middle and upper sections of the Pinghu Formation on the Pinghu slope in Xihu Sag as an example, by synthesizing the research achievements of coastal sedimentary systems at home and abroad, this study innovatively integrates paleogeomorphology, sedimentary characteristics of tidal flat areas, and coastal sedimentary models to explore their impacts on tidal dynamics and sedimentary system distribution, providing key basis for subsequent large-scale lithologic trap oil and gas exploration and development. Comprehensive application of drilling and logging data, seismic data, and sedimentary process numerical simulation techniques is conducted to simulate the spatio-temporal evolution of the sedimentary system. Through qualitative description and quantitative measurement, the distributions of unique sedimentary units such as tidal channels, tidal gullies, tidal sand ridges, and tidal sand sheets are clarified, the controlling effects of coastal topography and sea-level changes on tidal sedimentary sand bodies are revealed, and three sedimentary models, namely barrier coast, barrier-free underwater low-relief coast, and barrier-free gentle slope coast, are constructed to improve the theoretical framework of coastal sedimentary systems. Further comparative analysis between numerical simulation and actual data shows that barrier coast sand bodies are sheet-like distributed during low sea-level periods, while underwater low-relief coast sand bodies are ribbon-like distributed during high sea-level periods. For the first time, large-scale tidal sand ridges in barrier and underwater low-relief sedimentary environments and restricted tidal channel sand bodies are identified as key targets for oil and gas exploration

In recent years, natural gas exploration breakthrough has been made in the Ordovician Majiagou Formation of Ordos Basin, but the formation and evolution of natural gas have not been thoroughly studied. Taking Daniudi gas field as an example, the typical characteristics of oil-cracked gas in Majiagou Formation are determined through the identification of bitumen in the core and thin section, the occurrence of hydrocarbon inclusions and Raman spectroscopy testing, and the analysis of geochemical data of natural gas. The formation conditions of oil-cracked gas are comprehensively analyzed, and the formation and evolution process of oil-cracked gas in Majiagou Formation are analyzed through the simulation of burial history and thermal history. The results show that: (1) Bitumen filling with diverse occurrences is found in the fractured porous reservoir of Majiagou Formation, and it is a dry bitumen with high degree of thermal evolution. Three phase hydrocarbon inclusions of oil, gas and bitumen are captured in calcite veins of Majiagou Formation, which confirms the existence of oil cracking gas process. (2) Based on the crossplot of geochemical index such as ln(C1/C2) and ln(C2/C3) of natural gas, it shows that the internal natural gas of Majiagou Formation is mainly oil-cracked gas. (3) During the deposition period of 3rd member of Majiagou Formation, Daniudi and its surrounding areas were situated at the margin of a saline depression, where thick source rocks of argillaceous dolomite and dolomitic mudstone developed with interbedded evaporates, creating favorable conditions for thermochemical sulfate reduction (TSR). (4) In the Early Jurassic, source rocks produced a large amount of oil. Under the effect of the relatively high paleotemperature in the Early Cretaceous, high-temperature oil cracking occurred, and TSR reaction occurred with significantly increased H2S content in the natural gas of O1m55 -O1m56 of Majiagou Formation in Daniudi gas field. (5) In Daniudi and surrounding areas, the source rocks of Majiagou Formation became mature earlier in the south and later in the north, and natural gas mainly migrated and accumulated from south to north along the strike-slip faults. This study has certain significance for the internal gas exploration of Majiagou Formation in Ordos Basin.

The tight gas resource potential of the Lower Jurassic Ahe Formation in Dibei area of the eastern Kuqa Depression is enormous, making it a promising region for increasing oil and gas reserves and production. For a long time, the coupling relationship between sweet spot model and oil and gas enrichment of tight sandstone reservoir in Ahe Formation is unclear, which restricts the efficient exploration and development of tight oil and gas reservoirs.Based on microscopic reservoir characterization, geological modeling, fault-fracture characterization and reservoir analysis, this paper investigates the sweet spot characteristics and hydrocarbon enrichment patterns of tight sandstone reservoirs in the Ahe Formation, and evaluates their resource potential. The study reveals that the Ahe Formation reservoirs exhibit an alternating distribution of tight layers and low-porosity / high-permeability zones laterally. Reservoir properties are significantly enhanced by fault-fracture modification, developing sweet spot areas at four hierarchical scales. The fracture-pore systems controlled by class I-II faults extend east-west direction, characterized by large scale and favorable porosity-permeability properties. The fracture-pore systems controlled by class Ⅲ-Ⅳ faults/ fractures are small in scale and pinch out within tight sandstones. The first hydrocarbon charging event in the Ahe Formation reservoirs occurred between 18 and 12 Ma, with porosity ranging from 15% to 18% during this phase. The primary charging fluid was crude oil, which accumulated in structural highs to form conventional oil reservoirs. However, these reservoirs were subsequently severely disrupted, leading to complete dissipation of the accumulated hydrocarbons. The second hydrocarbon charging phase commenced since 5 Ma, during which the reservoir underwent rapid densification, with porosity reduced to 6%-8%. Natural gas efficiently migrated along faults and fractures, accumulating preferentially within sweet spot zones of the reservoir. Class Ⅲ and Ⅳ faults/fractures zones establish effective connectivity between sandstone units within the Ahe Formation, forming optimal configurations with adjacent tight reservoirs and overlying mudstones. These structural features constitute critical controls on both trap effectiveness and hydrocarbon accumulation. The favorable area for oil and gas enrichment in tight sandstone of the Ahe Formation can reach 106 km2, mainly concentrated in the central and southern platform areas of the Dibei Slope. The lithological trap resources of natural gas are 1 699 × 108 m3 and petroleum are 778 × 104 t

Three marine shale formations of the Middle-Upper Permian—the Gufeng Formation (P2g), the 3rd member of the Wujiaping Formation (P3w3) and the 1st member of the Dalong Formation (P3d1), are well developed in the northeastern Sichuan Basin, representing promising new targets for enhancing shale gas reserves and production. Based on extensive core testing data, this study analyzes their geochemical characteristics, reservoir features and proposes corresponding development strategies to provide theoretical support for shale gas exploration in China. The key findings are as follows: (1) All three shale formations exhibit high organic matter abundance, with average total organic carbon (TOC) contents of 9.82% (P2g), 6.60% (P3w3) and 6.01%(P3d1), respectively. The organic matter is classified as type II1, and the maturity(Ro) exceeds 2.0%, indicating significant hydrocarbon generation potential. The Gufeng Formation is dominated by siliceous shale and calcareous shale facies, whereas P3w3 and P3d1 primarily consist of siliceous shale and mixed shale facies. The brittleness index of all three formations exceeds 70%. Organic pores are the dominant pore type, with mesopores serving as the primary pore category. (2) The P3w3 exhibits well-developed laminated fractures, highest pore connectivity index (average value of 5.17), high porosity and high gas content; the P2g has moderate pore connectivity index (average value of 2.57), high porosity and gas content; the P3d1 shows poor laminated fracture development, the lowest pore connectivity index(average value of 1.69), the lowest porosity value and relatively high gas content. (3) Compared to the Longmaxi Formation in southern Sichuan Basin, these three shale formations are characterized by high TOC content, high brittleness index, high gas content, thin thickness and deep burial depth. Targeted development technologies are thus required. Favorable areas for shale gas enrichment of these three formations are primarily distributed in the southeastern segment of the Kaijiang-Liangping Trough and the Chengkou-Fengjie-Lichuan-Shizhu area of the Chengkou- E'xi Trough.