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The reef-shoal gas reservoirs of the Changxing Formation are the major targets of natural gas exploration and development in the eastern Sichuan Basin. However, few studies have been seen on the development of reefs and their differences under the sequence framework of the entire eastern Sichuan Basin. Based on the pertinent core and seismic data for the Changxing Formation, principles of reef development were systematically studied, showing that: (1) The Changxing Formation is divided into three third-order sequences (i.e., Sq1–Sq3), based on the temporal and spatial evolution of reefs development. (2) Due to different platform margin structures, the reefs along the western platform margin mainly accreted vertically, whereas the eastern counterparts demonstrate obvious lateral migration. (3) Platform-margin reefs belt mostly developed in the Sq2-Sq3 highstand systems tract (HST) of the Kaijiang–Liangping troughs. Slope reefs developed in the Sq3 HST period, and intra-platform reefs developed in the Sq1-Sq3 period. (4) The Sq1 period was a transgression background, the sedimentary evolution from southwest to northeast towards the terrestrial clastic platform evolved into carbonate platform deposition. In this period, the reef was thin in thickness and small in scale, and the shoal facies were not developed. Because of the sea level rise, the carbonate platform range expanded, and reefs developed in Sq2 HST. The sq3 period was a carbonate platform sedimentary, and reefs’ development characteristics were similar to the Sq2 period. However, the platform margin reefs scale decreased and the distribution area increased; the intra-platform reefs scale and distribution were larger than the Sq2 period. This research results indicated exploration direction, which was conducive to breakthroughs of reef exploration in the eastern Sichuan Basin. [ABSTRACT FROM AUTHOR]

In the Hongxing area of the eastern Sichuan Basin, the Permian Wujiaping Formation developed a set of black, silicon-rich, and carbon-rich shale reservoirs, which are currently pivotal for enhancing shale gas reserves and production in the region. These shale reservoirs show strong similarities with the lower sections (layers ① to ③) of the gas-bearing shale in the Longmaxi Formation of the Jiaoshiba area in terms of sedimentary environment, shale quality, and gas content. Both formations are characterized by high carbon, silicon, and gas contents. However, in later stages of development, the shale gas reservoirs of the Wujiaping Formation were significantly less productive compared to those in the Longmaxi Formation in the Jiaoshiba area. To address this, the study analyzes the differences in reservoir quality and gas content between the two shale sets and their primary controlling factors. The results reveal that the shale reservoir in the Wujiaping Formation is characterized by a low silica mineral content and a high carbonate mineral content. Although it has a high organic carbon content, the kerogen types and organic matter are relatively poorer. The pore structure is marked by less developed organic matter pores and lamellar fractures, with smaller pore sizes. Overall, its reservoir physical properties and gas content are slightly inferior to those of the Longmaxi Formation shale in the Jiaoshiba area. Comparative analysis suggests that differences in sedimentary backgrounds and processes lead to primary quality differences in mineral and lithological composition, lamellation, and kerogen types between the two shale reservoirs. These differences form the material basis for variations in physical properties and gas content. Furthermore, differences in subsequent structural preservation conditions and other macro-geological factors lead to significant disparities between the two reservoirs in terms of pore types, sizes, porosity, and gas content. In addition, the study explores the impact of these distinctive characte-ristics on the development effectiveness of the two reservoirs and proposes corresponding engineering and technological strategies tailored to the differences in the development of these reservoirs.

To determine the diagenesis and evolution of the Cambrian DEPS (dolomite-evaporite paragenetic system) in the eastern Sichuan basin, the isotopic geochemistry characteristics of C, O, and fluid inclusions were studied. Four diagenetic systems were determined in the Cambrian DEPS based upon the hydrological system. These include the pore brine during the penecontemporaneous stage, sealed brine during the early diagenetic stage, compaction hot brine during the middle and late diagenetic stage, and mixed hydrothermal fluids involving deep thermal water during the later tectonic uplift stage. The character and source of fluids of various diagenetic stages and systems have a certain familiarity and inheritance features of development and evolution. However, fluids of various diagenetic stages and systems have different effects on the reservoirs. The diagenetic systems and manner of diagenesis are related to reservoir development closely. These include burial dolomitization and dissolution of sealed brine of the early diagenetic stage, and TSR (thermochemical sulfate reduction) and recrystallization of the mixed hydrothermal fluids involving deep thermal water during the tectonic uplift stage. Analyzing the basin structure indicates that a series of faults caused by the Cambrian detachment layer in the Eastern Sichuan Basin are the dominant factor controlling the spatiotemporal coupling relationship between the diagenetic systems and the reservoir in DEPS. The shoal subfacies control the regional distribution of the reservoir. During the early diagenetic stage, buried dolomitization helped with continued reservoir development, while TSR and recrystallization in the tectonic diagenetic stage improved reservoir quality by enhancing porosity and permeability. The Cambrian DEPS strata and the underlying Qiongzhusi Formation mud shale form a high-quality source-reservoir-caprock assemblage in the eastern Sichuan Basin. [ABSTRACT FROM AUTHOR]

The major controlling factors of organic matter and its enrichment model of the black shale from the Wufeng-Longmaxi Formation were explored by investigating the vertical variation characteristics, as well as major element and trace element abundances in the Wuxi Bailu section. The results show that the sedimentary tectonic setting of the Wufeng-Longmaxi Formation in the north-east margin of the upper Yangtze platform is located on the active continental margin, which is a passive continental margin and continental island arc. The parent rock in the source area is mainly felsic volcanic rocks mixed with small amounts of sedimentary recycling materials. Due to increased plate activity and a drop in sea levels, terrigenous pyroclastic input increased. The palaeoclimate was semi-humid, and a robust dysoxic-reduction environment and a high level of palaeoproductivity, causing the formation of the organic-rich shale in the Wufeng Formation. At the base of the Longmaxi Formation, the sedimentary water body was affected by global transgression, showing a strong anoxic-reductive environment, and the paleoclimate was a warm and humid condition. The palaeoproductivity level was high, resulting in the formation of organic shale. Due to the sea level drop at the top of the Longmaxi Formation, the sedimentary water was in an oxic-reduced environment, but the input of terrigenous pyroclastic matter increased. Because the paleoclimate was warm and humid and the palaeoproductivity level was high, organic-rich shale was formed. The findings demonstrate that terrigenous clastic input circumstances, palaeoproductivity conditions, and paleoredox conditions had the greatest influence on the enrichment of organic matter in the Wufeng-Longmaxi Formation. Thus, organic matter enrichment was controlled by multiple paleoenvironmental factors. [ABSTRACT FROM AUTHOR]

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In this study, taking the Jurassic Lianggaoshan Formation (J1l) tight sandstones in the eastern Sichuan Basin as an example, the types and well-logging responses of main sedimentological and diagenetic facies in the lacustrine delta-front are investigated based on summarizing the sedimentary characteristics and reservoir properties. Subsequently, further validation and application are conducted in the study area through machine learning. Research results show that the J1l lacustrine delta-front in the eastern Sichuan Basin mainly develops subaqueous distributary channels and mouth bar sand bodies, exhibiting typical densification reservoirs, with porosity and permeability distributed between 0.48% and 11.24% (av. 3.87%) and 0.0003-0.653 x 10-3 µm² (av. 0.026 x 10-3 µm²), respectively. Strong compaction and strong cementation are the primary factors leading to densification, whereas chlorite coatings and weak dissolution play constructive roles in preserving some primary pores, creating a small amount of dissolution pores, and enhancing permeability. In terms of manifestation, the pore-throat content with a radius greater than 0.006 µm governs the reservoir quality. Furthermore, five types of diagenetic facies are identified in the J1l subaqueous distributary channels and mouth bars: strong compaction facies (Type I), strong cementation facies (Type II), chlorite-coating and intergranular pore facies (Type III), weak dissolution and intragranular pore facies (Type IV), and medium compaction and cementation facies (Type V). Overall, the thick and coarse-grained subaqueous distributary channels can be considered as the preferred exploration targets for tight oil and gas, with type III and type IV diagenetic facies being the most favorable reservoirs, characterized by well-logging responses of high AC and low GR, DEN, and RT. Based on the fine division of sedimentological and diagenetic facies, establishing well-logging interpretation models and then employing machine learning to achieve sweet spot reservoir prediction can provide valuable insights for tight oil and gas exploration in regions lacking core data. [ABSTRACT FROM AUTHOR]

The eastern Sichuan Basin is rich in Cambrian oil and gas resources, but the drilling exploration of Gaotai Formation has not made great progress in recent years. By analyzing the fluid inclusion characteristics of the Cambrian Gaotai Formation, it is of great significance to identify the hydrocarbon accumulation of the Gaotai Formation and the Lower Paleozoic gas exploration potential in the eastern Sichuan Basin. The characteristics of fluid inclusions in the study area were analyzed by means of microscope observation combined with the indirect projection dating method of fluid inclusions homogenization temperature-burial history to further determine the hydrocarbon accumulation period. The results show that the host minerals of fluid inclusions in the Cambrian Gaotai Formation are fissure filled calcite and dissolved pore filled calcite, and the homogenization temperature of inclusions ranges from 117. 8 ℃ to 205. 1 ℃ . The charging periods mainly include the initial hydrocarbon generation charging period, the peak oil generation charging period and the crude oil cracking charging period, forming three “episodic” oil and gas charging modes, first low and then high. The three phases of fluid activity correspond to the Late Permian-Early Triassic, Middle Triassic-Early Jurassic and Middle Jurassic respectively. Comprehensive analysis of structural conditions shows that the gas bearing capacity of the Cambrian Gaotai Formation in the eastern Sichuan area is poor, but it is conducive to the migration and accumulation of natural gas in the upper middle and shallow strata, which may be the next exploration direction in this area. [ABSTRACT FROM AUTHOR]

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[Objective] To solve the problems such as unclear lithological variations of the Feixianguan Formation on the western side of the marine trough's southern segment and within the plateau interior of the eastern part of the Sichuan Basin, [Methods] the lithological types and characteristics of the Feixianguan Formation were studied by comprehensively using multiple geological data such as cores, drilling and logging, and an intelligent identification method of lithological logging based on machine learning was proposed, which solved the technical problems of fine identification of lithology in the old area, and revealed the lithology, distribution and evolution of the Feixianguan Formation in the area. [Results and Conclusions] (1) The Feixianguan Formation is mainly composed of lithology such as mudstone, mud crystal limestone, argillary limestone, granule limestone, granulous dolomite, mud crystal dolomite, paste dolomite, and gypsum rock; (2) Comparison found that the improved gradient-boosted decision tree algorithm, namely Stochastic Gradient Boosting Decision Tree (SGBDT), is superior to other algorithms for constructing lithology models, and is more suitable for carbonate rock complex lithology identification; (3) The granule limestone developed intensively in the area south of the Kaijiang-Liangping Sea Trough between Feixianguan I period and Feixianguan III period, and the granulous dolomite was concentrated in the Feixianguan II period and distributed and scattered; (4) The distribution of the oolitic shoal in the area exhibits significant variation. The main development of the plateau ancient geomorphological high point and the edge of the platform in the Feixianguan I period. In the Feixianguan II period, most of the the oolitic shoal at the edge of the platform were developed, and a small number of ancient landforms in the platform were developed .And the main development of the plateau ancient geomorphological high point in the Feixianguan III period. [ABSTRACT FROM AUTHOR]

To investigate the development laws and genesis of the Maokou Formation dolomite in the Fengdu-Shizhu area, we analyzed core, thin section, logging, and geochemical data, and obtained the following understandings: 1. The Maokou Formation dolomite includes layered granular dolomite and leopard-spotted limy dolomite which are fine to medium, and moderately euhedral, and have intergranular pores and intergranular dissolved pores; 2. Vertically, the dolomite is the superimposition of multiple stages, 3–12 m a layer and cumulatively up to 30 m. The distribution of the dolomite is controlled by sedimentary cycles and is commonly found in the middle and upper parts of the cycle; 3. The analysis of rare earth distribution and carbon, oxygen, and strontium isotopes indicates that the dolomitizing fluid is the penecontemporaneous saline seawater, and some pore edges were affected by later hydrothermal dolomitization, resulting in recrystallization of dolomite and cementation of saddle-like dolomite; 4. The relationship between the plane distribution of the dolomite and the paleogeographic pattern during the sedimentary period indicates that the dolomite is concentrated in the granular shoals near geomorphic highlands and slope break zones. In summary, it is proposed that the overlap and migration of granular shoals and isolated seawater promoted the occurrence of reflux infiltration of dolomitizing fluid and dolomitization. Multistage granular shoals on the platform margin provide a good material foundation for the development of dolomite. Karstification is conducive to the occurrence of early dolomitization within the shoals and the preservation of pores. It is found that the early dolomite of the Maokou Formation is best developed in the highlands of faults 15 and 16. The basement faults controlled the sedimentary paleogeomorphology, thereby restricting the distribution of dolomite. This understanding provides a new idea for the exploration of dolomite in the Maokou Formation in the Sichuan Basin. [ABSTRACT FROM AUTHOR]

Tight sandstone and shale oil and gas are the key targets of unconventional oil and gas exploration in the lake-delta sedimentary systems of China. Understanding the coevolutionary diagenesis of sandstone and shale reservoirs is crucial for the prediction of reservoir quality, ahead of drilling, in such systems. Thin-section description, scanning electron microscopy (SEM), X-ray diffraction (XRD), fluid inclusion analysis, porosity and permeability tests, high-pressure mercury intrusion (HPMI) measurements and nuclear magnetic resonance tests (NMR) were used to reveal the coevolutionary diagenetic mechanisms of a sandstone and shale reservoir in the Lianggaoshan Formation of the Eastern Sichuan Basin, China. The thermally mature, organic-matter-rich, dark shale of layer3 is the most important source rock within the Lianggaoshan Formation. It started to generate abundant organic acids at the early stage of mesodiagenesis and produced abundant hydrocarbons in the early Cretaceous. Porewater with high concentrations of Ca2+ and CO32− entered the sandstone reservoir from dark shale as the shale was compacted during burial. Potassium feldspar dissolution at the boundary of the sandstone was more pervasive than at the center of the sandstone. The K+ released by potassium feldspar dissolution migrated from the sandstone into mudstone. Grain-rimming chlorite coats occurred mainly in the center of the sandstone. Some silica exported from the shale was imported by the sandstone boundary and precipitated close to the shale/sandstone boundary. Some intergranular dissolution pores and intercrystal pores were formed in the shale due to dissolution during the early stages of mesodiagenesis. Chlorite coats, which precipitated during eodiagenesis, were beneficial to the protection of primary pore space in the sandstone. Calcite cement, which preferentially precipitated at the boundary of sandstone, was not conducive to reservoir development. Dissolution mainly occurred at the early stage of mesodiagenesis due to organic acids derived from the dark shale. Calcite cement could also protect some primary pores from compaction and release pore space following dissolution. The porosity of sandstone and shale was mainly controlled by the thickness of sandstone and dark shale. [ABSTRACT FROM AUTHOR]

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The relationship between fractures and gas logging in Nanchuan-Wulong area in eastern Sichuan Basin is utilized to study the identification of fractures in shale gas horizontal wells based on geological (core), imaging logging, logging evaluation, seismic prediction, and other data. First, to solve the problems of lack of core and imaging logging and difficulty in identifying natural fractures in shale gas horizontal wells, a core scale shale natural-fracture identification model is established through vertical wells from a geological perspective in this paper, and the development of high angle fractures in the core is consistent with the high values and peaks of total hydrocarbon anomalies. Second, in combination with the fracture identification mode, the fracture development section of non-coring vertical wells can be quickly and qualitatively identified at a low cost based on total hydrocarbon information. There will be a sudden increase in the total hydrocarbon value in the fracture development section, especially in the shale section with low total organic carbon (TOC). The sudden increase in the total hydrocarbon value also represents the existence of fractures. Finally, the plate method of TOC and normalized total hydrocarbon correlation is used to quantitatively identify vertical and horizontal well fracture sections without coring and imaging logging data. The area delineated by a normalized total hydrocarbon value ≥0.4 and TOC ≥0.5% are considered as the fracture development section of a horizontal well. Based on the identified horizontal well fracture section, from the perspective of geological engineering integration, it is possible to avoid or pay attention to these densely developed large or giant fractures in advance, thereby increasing the production of a single well.

The dolostone–evaporite co-occurring system (DECS) is widely found in Precambrian to Quaternary strata. In the eastern Sichuan Basin, DECS is mainly developed in the Cambrian Longwangmiao, Gaotai, and Xixiangchi Formations. In this paper, the controlling factors of sedimentary, sedimentary models, and main controlling factors of reservoir of the DECS of the eastern section of the Sichuan Basin were investigated, through core observations, thin section analysis, and field profiles measurement and sampling, combined with well-logging data, interpretations of geochemical analyses, and cathodoluminescence. The lithological associations of the evaporite and dolostone in the DECS include interbedded dolostone and evaporites, thick-bedded to mass evaporites above the dolomite, dolostone intercalated with evaporite, and evaporite intercalated with dolostones. The reservoir of DECS is mainly a dolomite reservoir in dolomite interbedded evaporite. The lithology mainly consists of oolitic dolomite and siliceous fine crystalline dolomite. The reservoir porosity includes dissolution porosity, porosity, and fractures. The analysis of the geochemical characteristic indicated that the formation of DECS is affected by the closed water body and high salinity (Z > 120), the hot and arid paleoclimate (T = 21.1 ℃), intense evaporation, and the continuous decline of the ancient sea level. According to the paleoenvironmental characteristic, the models of DECS include four dolomite models, evaporite pump (sabkha) dolomite diagenetic model, reflux permeable dolomite diagenesis model, diagenetic model of buried dolomite, and tectonic-age hydrothermal dolomite genesis model, and two evaporite models: Sabkha diagenetic model and underwater concentration diagenetic model. Based on previous studies, the influence of different factors on the deposition of the shoals in the DECS was evaluated. The main controlling factors for the formation of high-quality reservoir units include thermochemical sulfate reduction (TSR) and structure-controlled hydrothermal dolomitization. Sulfides and deep fluids in DECS play a constructive role in reservoirs. [ABSTRACT FROM AUTHOR]

Mixed sedimentation is mainly a phenomenon by which siliciclastic and carbonate are mixed under a similar sedimentary environment. The study of the composition of mixed rock can reflect the sedimentary environment. The mixed sediments of the Canglangpu formation in the eastern Sichuan basin are widely developed and exhibit well exploration prospects. In this study, a classification diagram of mixed sediments suitable for the Canglangpu formation was proposed by combining lithologic characteristics with logging data. Based on the observations of typical outcrop profiles, the identification of thin sections, and drilling and geological logging data, a model was established to identify the electrical responses of the mixed sedimentary rocks in the study area. The results show that (1) the Canglangpu formation can be vertically divided into two complete transgression-regression cycles with the lower part mainly consisting of sedimentary carbonate diamictite and the upper part principally composed of clastic diamictite; (2) mixed sediments in the Canglangpu formation can be divided into six classes, namely carbonate sandstone, argillaceous sandstone, sandy carbonate rock, argillaceous carbonate rock, sandy mudstone, and Carbonate mudstone; (3) the sedimentary environment of the Canglangpu formation be divided into (a) mixed sediments in the shore, which include mixed lagoon and mixed tidal flat, (b) mixed sediments in the shallow marine shelf, which includes mixed beach bar, terrigenous detrital mixed shelf, and carbonate mixed shelf; (4) a mixed shore-mixed shelf sedimentary model has been established; it says that the mixed sediments show obvious characteristics of ring-like development around underwater paleo-heights. The formation mechanism of mixed sedimentation in the eastern Sichuan basin is mainly controlled by the change of sedimentary facies belt, and the development intensity is mainly influenced by early tectonic movement and sea-level change.

It has not been clearly understood whether the hydrocarbon generated from the argillaceous sourcerocks of the Lower Cambrian Qiongzhusi Formation could upward pass through the regional cap rocks of the Lower-Middle Cambrian gypsum rocks and accumulate in the Middle-Upper Cambrian Xixiangchi Group reservoir, namely the cross-formational hydrocarbon accumulation, in the eastern Sichuan Basin. Therefore, the studies on the reservoir solid bitumen in the Xixiangchi Group and the sealing performance of the regional cap rocks of the Lower-Middle Cambrian gypsum rocks were carried out, and the cross-formational hydrocarbon accumulation of the Cambrian oil and gas and its significance for natural gas exploration were discussed. It was shown by synthetic studies that the reservoir solid bitumen in the Xixiangchi Group mainly originated from the argillaceous source rocks of the Lower Cambrian Qiongzhusi Formation. The sealing integrity of the regional cap rocks of the Lower-Middle Cambrian gypsum rocks was damaged to some extent by the reactivated basement faults and the large nascent faults in the Middle Permian to Middle Triassic though it had good sealing performance in geological times, which could provide convenient vertical pathways for the Cambrian cross-formational hydrocarbon accumulation in the eastern Sichuan Basin. Three evolutionary stages were built up about the Cambrian hydrocarbon migration and accumulation as follows. Stage 1 is the formation of the Cambrian pre-salt early paleo-reservoirs in the period ofthe Late Silurian to the Early Permian (S3-P1), stage 2 is the one of the Cambrian cross-formational hydrocarbon accumulation in the period of the Middle Permian to the Middle Triassic (P2-T2), and stage 3 is the formation of the Cambrian late paleo-gas reservoirs in the period of the Late Triassic to the Early Cretaceous (T3-K1). The cross-formational hydrocarbon accumulation is an important feature of hydrocarbon accumulation in multi-cycle superimposed basins, and the reactivated basement faults and the large nascent faults in the period of the MiddlePermian to the Middle Triassic are important clues to the Xixiangchi Group primary paleo-gas reservoirs. Identification and evaluation of the Xixiangchi Group primary paleo-gas reservoirs will be the key points and the main challenges to natural gas exploration in the study area.

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The Xixiangchi Group in eastern Sichuan Basin has great potential for natural gas exploration. However, there is a lack of in-depth studies of the hydrocarbon sources and the formation and evolution of gas reservoirs in this Group. Systematic investigation about the gas reservoir in Pingqiao anticline was consequently carried out in terms of characteristics of reservoir bitumen, the geochemical characteristics of natural gas, diagenetic minerals, and fluid inclusions. Based on this, combined with the reconstruction of the burial history, thermal evolution history and uplifting history of strata, and analysis of the regional tectonic settings, the hydrocarbon sources were identified and the formation and evolutionary processes of the gas reservoirs in Xixiangchi Group was revealed in this study. It was shown that the gas reservoirs have mixed gas sources from the shale source rocks in the Lower Cambrian Qiongzhusi Formation and in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation, and experienced several evolutionary stages, including the paleo-oil reservoir stage from the Late Siliurian to the Middle Permian, the paleo-gas reservoir stage from the Late Permian to the Early Cretaceous, and the superimposed accumulation and mixed-source gas reservoir stage since the Late Cretaceous. The mixed-source gas reservoir is formed by the adjustment of the Xixiangchi Group paleo-gas reservoirs and depressurization of the overpressure Wufeng-Longmaxi shale gas reservoirs and the charging of gas into the Xixiangchi Group reservoir of the Pingqiao anticline since the Late Cretaceous, which show obvious superimposed accumulation characteristics. There are different accumulation patterns in different geological periods. The accumulation pattern of the “old source - young reservoir” (i.e. hydrocarbons generated from older source rocks accumulating in younger reservoirs) dominates before the Late Cretaceous, and that of “juxtaposed young source - old reservoir” (i.e. hydrocarbons generated from younger source rocks accumulating in juxtaposed older reservoirs) dominates after the Early Cretaceous. Moreover, faults acted as critical vertical pathways for hydrocarbon migration during the evolution of the Xixiangchi Group gas reservoirs. This model provides new insights and theoretical basis for evaluation and mapping of the Xixiangchi Group play fairway in eastern Sichuan Basin.

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There is great controversy regarding the origin and source of natural gas in the Lower Triassic Feixianguan Formation in the Eastern Sichuan Basin. This seriously restricts the study of natural gas dynamics in the Feixianguan Formation and thus hampers natural gas exploration in the region, so further study is urgently required. Using experimental tests of natural gas composition, stable isotopes, and noble gas isotopes with gas chromatography (GC) and mass spectrometry (MS) studies of source rock and reservoir asphalt saturated hydrocarbons, the natural gas geochemical characteristics, the genetic identification and a gas-source comparison of the Feixianguan Formation were studied. Then, constrained by the thermal history, the histories of gas generation and expulsion were restored by basin simulation technology. Finally, a gas accumulation model was established for the Feixianguan Formation. The results showed that (1) the H2S-rich and H2S-poor gas reservoirs of the Feixianguan Formation are distributed on the east and west sides of the Kaijiang-Liangping trough in the Eastern Sichuan Basin, respectively. The carbon and hydrogen isotope compositions of the natural gas in the gas reservoirs are generally heavy and have typical characteristics of high-maturity dry gas reservoirs. (2) The natural gas of the Feixianguan Formation is organic thermogenic gas, which is mainly oil-type gas generated by the secondary cracking of crude oil. The gas-generating parent material is mainly type II kerogen. (3) The natural gas of the Feixianguan Formation in the Eastern Sichuan Basin was mainly generated by argillaceous source rocks of the Upper Permian Longtan Formation. (4) Natural gas accumulation occurred as follows: the paleo-structure heights were filled with crude oil in the Early Jurassic, and paleo-oil reservoirs were formed in the Feixianguan Formation; during the Middle-Late Jurassic, the paleo-oil reservoirs were cracked when the reservoir temperatures rose above 160 °C, and paleo-gas reservoirs were formed. Since the end of the Late Jurassic, the paleo-gas reservoirs have been adjusted and reformed to form the present-day natural gas reservoirs. These results provide a basis for studying natural gas accumulation dynamics of the Feixianguan Formation in the Eastern Sichuan Basin. [ABSTRACT FROM AUTHOR]

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Carbon sequestration refers to the technology of capturing carbon and storing it safely instead of directly discharging CO2 into the atmosphere. Rock salt is a promising CO2 reservoir, and the underground rock salt structure formed by salt cavern reservoirs in geological history has a natural sealing function. Storing carbon dioxide in salt caves can ensure that carbon dioxide is stably stored underground for a long time. However, at present, there are few studies on carbon sequestration in abandoned salt caves. This study analyses the characteristics of carbonate-evaporite strata in the high fold zone of eastern Sichuan and qualitatively evaluates Triassic carbonate-evaporite layers formed in the depth range of 800-3000 m in various sedimentary basins in Chongqing. A large set of carbonates-evaporites are developed in Chongqing, and the carbonate rocks near evaporites have low porosity and permeability, which can be used as CO2 storage caprock in salt cavities. Thick evaporated strata can also provide a good cover for CO2 storage in abandoned salt caves, which has great potential space resources.

Microfractures at the Ziliujing lacustrine shale at Fuling in eastern Sichuan Basin were investigated through core observation, SEM image observation and high-pressure mercury injection. and quantifies the impact of micro fracture development to reservoir capacity, leads to the following understanding: (1) fracture types in lacustrine shale reservoir are defined; (2) the contribution of fractures to lacustrine shale reservoir is determined. Results show that the microfractures in the Lower Jurassic Ziliujing lacustrine shale reservoirs in eastern Sichuan Basin can be divided into three types, each with four groups: micro fractures related to sedimentation (bedding fractures), microfractures related to diagenesis (clay–mineral shrinkage fractures), and micro fractures related to hydrocarbon generation (organic-matter shrinkage fractures and organic-matter-cracking fractures). High-pressure mercury injection and plane porosity determination confirm that microfractures in lacustrine shale reservoirs exhibit good reservoir performance, whose contribution to shale reservoir capacity is about 25%.

A shale condensate gas reservoir with a high clay content and a high formation pressure was found in the Jurassic shale of the Dongyuemiao Member in the Fuxing area of the eastern Sichuan Basin. Reservoir characteristics and formation pressure have a significant influence on optimal development. The present study investigated the continental shale of the Dongyuemiao Member in Well F. The petrological properties, physical properties, and pore structure of the Dongyuemiao Member were studied using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), N2 adsorption, and mercury intrusion porosimetry (MIP). The permeability stress sensitivity characteristics of the shale reservoirs are discussed based on the change in shale porosity and permeability under overburden pressure. The tested shale samples yielded total organic carbon (TOC) and S1 + S2 values ranging mainly from 1.0 wt.% to 1.5 wt.% and from 0.39 to 2.28 mg/g, respectively, which was in the high maturity stage of the thermal evolution of organic matter (OM). The shales of the Dongyuemiao Member were found to contain high average clay mineral contents (more than 50%) of calcite and quartz, as well as albite, pyrite, dolomite, and halite. The main developments were identified as silica-rich argillaceous shale lithofacies, argillaceous shale lithofacies, and mixed argillaceous shale lithofacies. The pores were found to mainly be plate-like and flake-like interlayer pores of clay minerals and OM pores with various shapes. The pore size was mainly concentrated below 110 nm, and the pore volume increment increased in flakes with pore diameter. The average porosity and permeability of shale were found to be 4.827% and 0.243 mD, respectively. Clay minerals and quartz are beneficial for improving the porosity and permeability of reservoirs, while carbonate minerals have the opposite effect. The permeability of the shale showed a negative exponential change with increasing effective stress under overburden pressure. When the effective confining pressure was greater than 20 MPa, the decline rate of the shale permeability decreased with increases in the effective stress. The higher the clay mineral and TOC content, the stronger the stress sensitivity of shale permeability. The higher the carbonate mineral content, the weaker the stress sensitivity of shale permeability. The porosity sensitivity exponent indicates that matrix pores and micro-fractures are both developed in the Dongyuemiao Member, and the development of internal fractures is the main factor in the strong stress sensitivity of the shale permeability in the study area. [ABSTRACT FROM AUTHOR]

The petroleum geological elements and main controlling factors of the Middle-Upper Cambrian Xixiangchi Group in the eastern Sichuan Basin were systematically analyzed on the basis of geological outcrops and drilling data. Results showed that the reservoir lithology of the Xixiangchi Group was mainly micritic and silt-crystal dolomites, and the reservoir space was dominated by dolomite inter-crystalline pores, inter-crystalline dissolved pores and karst caves, etc., at the meantime, the reservoir quality is generally poor. The Xixiangchi Group was vertically composed by two sets of source rocks strata: the underlying Lower Cambrian Qiongzhusi Formation and the overlying Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation. The Silurian shale and the Lower Ordovician argillaceous rocks acted as regional and local cap rocks of the Xixiangchi Group. High-quality reservoirs are the necessary prerequisite for hydrocarbon accumulation in the Xixiangchi Group, and the source-reservoir spatial configuration and its effectiveness are the key factors to hydrocarbon accumulation. Though the Xixiangchi Group could have been charged by natural gas generated from the underlying and the overlying source rocks, it is difficult for the natural gas generated from the underlying source rocks to migrate up to the Xixiangchi Group due to the barrier of the Cambrian gypsum layers. However, the faulted anticlines developed over the Cambrian gypsum layer were favorable for hydrocarbon accumulation because they could change and construct a series of effective source-reservoir spatial configuration between the Wufeng-Longmaxi source rocks and the Xixiangchi reservoirs, and finally formed reservoirs with "young source in old reservoir" in the Xixiangchi Group.

Ramp-type bioherm is geologically characterized by lateral migration, small size and discrete distribution. The fine bioherm description is of high difficulty and the benefit development of bioherm gas reservoirs can be hardly realized. To solve these difficulties, this paper put forward a new bioherm identification method based on well-seismic combination after cognizing the geological characteristics of ramp-type bioherms deeply. And this new method plays a successful role in guiding the drilling of Well Y012-X16 in the mode of “two bioherms in one well” in the Damaoping Block of eastern Sichuan Basin. High-yield industrial gas flow of 113.65 × 104 m3/d is obtained during the test and great new progress is achieved in the exploration of bioherm gas reservoirs. And the following research results were obtained. First, the degree of slope at the marginal platform slope of ramp-type bioherm is only 2°–4°. The migration and evolution laws of bioherms are mainly controlled by the change of paleogeomorphology and sea level. As a result, small bioherms of multiple rows and multiple periods are formed and the gas-water contact of ramp-type bioherm gas reservoir is not united, presenting the characteristics of “one bioherm, one reservoir”. The gas bearing property is good at the top of the bioherm. The overall gas bearing property is poorer in the early bioherm of the second member of Upper Permian Changxing Formation and better in the late bioherm of the third member of Changxing Formation. Second, a new bioherm identification method with “double-high” integrated seismic processing technology, data driven “box-shaped volume” perspective technology, remaining thickness method based sedimentary paleogeomorphology restoration technology and three-dimensional visualization technology as the core is developed innovatively, which provides the fine description of ramp-type small bioherms in the three dimensional space. Third, based on the application of the new bioherm identification method, the distribution detail of the bioherms in the Damaoping Block is clarified, the development mode of “two bioherms in one well” for long horizontal sections is established and a PetroChina's new record of bioherm reservoir drilling length is set up in Well Y012-X16, which launches a new means to the benefit development of small bioherm group. Fourth, it is predicted that a number of bioherm development areas can be newly increased in the Damaoping Block, which can provide the supplement productivity of 100 × 104 m3/d for the development of gas reservoirs, so their natural gas exploration and development potential is huge. In conclusion, the fine description technologies for ramp-type small bioherms are conducive to the identification of ramp-type small bioherm group and the efficient exploration and development of bioherm gas reservoirs in the Sichuan Basin.

Gypsum-bearing evaporite is a globally important oil and gas caprock. Sealing of oil and gas reservoirs by gypsum-bearing evaporite is of great importance for oil and gas storage, particularly for natural gas. During the Middle Cambrian, the Gaotaii Formation, gypsum-bearing evaporite was deposited in the eastern Sichuan Basin, which is a key caprock for deep oil and gas resources. Yet little research exists on the gypsum-bearing evaporite of the Gaotai Formation. Based on a field survey, the latest well, lithology, logging and comprehensive analysis of previous data, the lithological division, and genetic type analysis of the gypsum-bearing evaporite, a platform sedimentary model was established. Two sets of gypsum-bearing evaporate associations are developed longitudinally in the Middle Cambrian Gaotai Formation in the eastern Sichuan Basin. The sedimentary environment is divided into normal lagoon and mixed deposition lagoon environments and is influenced by the provenance supply. As sea level decreased, a "bull's eye ring banding" pattern of sedimentary units formed in the normal lagoon. When the sea level rose, thinly interbedded gypsum-bearing evaporite was deposited in both types of lagoons. Overall, gypsum-bearing evaporite has a "broad circular ring" distribution in the northeast direction, indicating that the dry and hot paleoclimate, continuous sea level decrease, incoming materials, and shoal barrier provided favorable conditions for the development of the gypsum-bearing evaporite in the Middle Cambrian Gaotai Formation in the eastern Sichuan Basin. The Jianshen 1 well-Wuxi gypsum-bearing evaporite block and the Taihe 1 well-Shizhu outcrop gypsum-bearing evaporite block were the two primary gypsum-bearing evaporite deposition areas at that time. Increased paleo-temperatures led to the formation of the thinner Gaotai Formation evaporite caprocks overlying the hydrocarbon-bearing reservoirs in the Longwangmiao Formation. [ABSTRACT FROM AUTHOR]

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There exists the Wusheng-Shizhu and Chongqing-Qijiang intra-platform depressions in the eastern Sichuan Basin during the Late Permian to the Early Triassic. The clear understanding of the sedimentary pattern and evolution of reef-shoal around the depression-margin has a practical significance for guiding the exploration and development of the Permian–Triassic reef and shoal gas fields. Based on observation, the systematic sampling and laboratory analysis of typical outcrops of the Changxing Formation (i.e. Juandong, Juhuaba, Lengshuixi, and Taiyuan) are located in the eastern Sichuan Basin. Combined with drilling data and previous studies, the study area is dominated by a ramp sedimentary environment, except for the Kaijiang-Liangping Trough in the Early Changxingian. The main sediments are micrite-containing chert nodules and micrite. With the regression that took place during the Middle-Late Changxingian, there is a distinct sedimentary differentiation that began to appear. There is a “two depressions–one platform–one trough” sedimentary pattern from south to north in the study area in the Changxingian. In essence, there exists the Chongqing-Qijiang and the Wusheng-Shizhu intra-platform depressions (two depressions), platform-flat (one platform) between the Wusheng-Shizhu intra-platform depression and the Kaijiang-Liangping Trough, and the Kaijiang-Liangping Trough (one trough). The reef-shoal of the northern depression-margin distributes between the 2nd Member and lower part of the 3rd Member. By comparison, the reef-shoal of the southern depression-margin distributes in the upper part of the 3rd Member; this indicates that the deposition of reef-shoal shows “earlier in the north and later in the south.” It is considered that the sedimentary palaeogeomorphology of the Changxingian is characterized by “higher in the north-lower in the south.” The clear understanding of the sedimentary pattern and evolution of reef-shoal around depression-margin reveal a new field for exploration and development of the Permian Changxing reef-shoal gas fields in the eastern Sichuan Basin.

Studies of the sedimentary facies and sequence stratigraphy have indicated that the main depositional environments of the Lower Triassic Jialingjiang Formation in the eastern Sichuan Basin were a restricted platforms and evaporite platforms. Two third-order sequences (SQ1, SQ2) and five fourth-order sequences (SSQ1-SSQ5) were recognized. Sediments in the Jialingjiang strata evolved from a restricted platform to an evaporite platform, then to restricted platform again, and finally to an evaporite platform. The main hydrocarbon reservoirs and producing layers appear in the SSQ1-SSQ2 sequence. Sequences SSQ1-SSQ5 are further subdivided into transgressive systems tracts (TST) and highstand systems tracts (HST). Taking these sequences as mapping units, lithofacies paleogeographic maps were prepared for each of the fourth-order tracts. Evidence from the study of the distribution of favorable reservoirs indicates that the restricted platform-to-platform shoal in the SSQ1 and SSQ2 sequences are key areas for future exploration. [ABSTRACT FROM AUTHOR]

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Oil-cracked gas is an important gas resource in petroliferous basins. In theory, oil-cracked gas should be rich in C 2+ (i.e., wet gas). However, gas reservoirs of oil cracking origin in the Sichuan Basin (South China) are mainly composed of dry gas. Fluid inclusions record hydrocarbon generation and evolution. Obviously, identifying wet gas inclusions is the key to solving the above controversy. Here, we calibrate the Raman spectroscopic detection limits of ethane and propane (0.007 and 0.016 MPa, respectively) and demonstrate that Raman spectroscopy is a powerful method to identify C 2+ -bearing inclusions. Based on this technique, C 1 –C 2 –C 3 -bearing wet gas inclusions (C 1 /ƩC i = 0.90–0.93) are identified in calcite cements of the Permian Changxing Formation carbonate reservoir in the Eastern Sichuan Basin. These inclusions are direct geological evidence of oil-cracked gas because the calcite cement precipitated after the formation of pyrobitumen. Furthermore, reconstruction of the temperature-pressure-composition evolutions of calcite-hosted (Changxing Formation) and quartz-hosted gas inclusions in the Ordovician Wufeng–Silurian Longmaxi Formation black shale indicates that high pressure can effectively retard the cracking of C 2+ gas hydrocarbons. Differences in gas composition and δ13CCH 4 signature between reservoir gas and inclusion gas lead us to conclude that large-scale oil cracking occurred during the Early–Middle Jurassic in the Permian Changxing carbonate reservoir. Nevertheless, oil-cracked gas escaped during the evolution of the paleo-gas reservoir. Formation of the current dry gas reservoir can be attributed to the continuous mixing of late-stage kerogen pyrolysis gas. • Raman spectroscopy is a powerful method to identify C 2+ -bearing inclusions. • Pristine signature of oil-cracked wet gas was preserved in fluid inclusions. • Fluid inclusion observations supports that high pressure retards C 2+ cracking. • The reservoir dry gas is a mixture of oil-cracked gas and kerogen pyrolysis gas. [ABSTRACT FROM AUTHOR]

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The thin-skinned fold-and-thrust belt with multiple detachments in the eastern Sichuan Basin is characterized by a series of fault-detachment folds with a backward preferred structural vergence, which is rare in nature and can't be explained well by the classic critically taper theory. Here, three discrete element simulations were conducted to study the influence of multiple detachments on the structural vergence and evolution of the eastern Sichuan thin-skinned fold and thrust belt. The experimental results reveal that multiple detachment layers affected the bulk mechanical strength and facilitated a dually vergent thrust wedge with a backward preferred structural vergence. The addition of intermediate detachment layers influences structural vergence and fault activity of thrust wedges. With the increase in intermediate weak layers, the preferred structural vergence of these model results progressively moved from forward to nearly symmetric and eventually to backward. These results exhibit first-order structural similarities to the structural vergence and fault activity in the eastern Sichuan fold and thrust belt, which indicates that the influence of the internal distribution of weak layers within the eastern Sichuan Basin should not be ignored. The limited Cambrian evaporite layer and the widespread Silurian shale and Lower-Middle Triassic evaporite layers had significant impacts on the deformation of the fold-and-thrust belt in the eastern Sichuan basin. Consequently, we propose a kinematic model for the coupled geodynamic processes of the eastern Sichuan fold-and-thrust belt. The framework of the eastern Sichuan Basin with a number of backward fault-detachment folds was formed by Mesozoic northwestward propagation, and Cenozoic eastward propagation reactivated the folding and thrusting. The Huayingshan fault was affected by the limited Cambrian evaporite layer. • Multiple detachments facilitate a backward preferred structural vergence. • The basal weak layer directly controls the deformation range and structural style. • The Huayingshan fault was affected by the basal detachment layer. • Mechanical properties control the structural vergence of the eastern Sichuan Basin. [ABSTRACT FROM AUTHOR]

The natural gas accumulation zone, where the marine carbonate rock strata are developed, was formed in the eastern Sichuan Basin under the influence of several main tectonic movements (Caledonian Movement, Indosinian Movement, Yanshanian Movement, and Himalayan Movement). Most natural gas reservoirs exhibit the structural-stratigraphic traps together with multistage accumulation, late-stage adjustment and reformation, et cetera. The natural gas accumulation zone (or so-called gas reservoir groups) is controlled by the following main factors: multi-sourced and multi-formed hydrocarbons for marine source rocks (i.e. Lower Silurian Longmaxi Formation, Lower Permian, Upper Permian Longtan Formation), paleo-uplift, paleoslope, and the hinge belt controlled by the steep dip structures, namely the Lower and Middle Triassic high-quality gypsum. Three sets of high-quality source rocks (i.e. S1l, P1, P2l) account for the abundant hydrocarbon supply for natural gas accumulation in the eastern Sichuan area, especially in the destructed oil reservoir formed earlier. The said destructed oil reservoir not only provides the preservation space for natural gas reservoir that will take place later, but it also provides the hydrocarbon source for thermal cracking of hydrocarbons and thermochemical sulfate reduction (TSR). Although the gas reservoirs in the eastern part of the Sichuan Basin experienced multi-stage adjustment and reformation at later times, the thick and high-quality gypsum as well as the mudstone, as available caprocks, have offered a good preservation condition for the underlying gas reservoirs. The paleohighs (e.g. Luzhou paleohigh and Kaijiang paleohigh), the Permian platform margin slope, and the structurally transformed slope under the function of the steep dip anticline in the eastern Sichuan not only form the high-quality carbonate reservoir, but they also became favorable for oil and gas accumulation. The difference in hydrocarbon generation history of the source rock and multistage accumulation of hydrocarbons in the eastern part of the Sichuan Basin caused the diversity in the location of the gas reservoirs, where the gas reservoirs with low H2S contents are distributed in the eastern Sichuan Basin (e.g. Jiannan gas field), while the gas reservoirs with high H2S content are located in the northeastern Sichuan Basin (e.g. Puguang gas field).

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Reef-shoal gas reservoirs of Upper Permian Changxing Formation, which are the important targets for increasing gas reserve and production in Sichuan Basin, are highly heterogeneous in Eastern Sichuan Basin. However, because of the complex tectonic situation, the source and accumulation of natural gas are still being controversial. The lack of systematic research and relevant experimental analysis on controlling factors of hydrocarbon accumulation restrict the effective natural gas exploration. Based on petrographic observation, 3D seismic data and geochemistry, the reservoir spaces, fluid-inclusion of filling mineral, reservoir solid bitumen, and restoration of tectonic evolution are analyzed, which indicates that the reef-shoal gas reservoirs of Changxing Formation are fractured-vuggy type and formed mainly by dolomitization. The reservoir porosity system consists of pores, vugs, and fracture network. The gas reservoir boundary is mainly controlled by reef dolomite reservoirs distributed intermittently along the platform-margin. Both water and gas production wells have undergone two-stage accumulation process. The first stage is the crude oil charging of Wujiaping Formation from the Middle Jurassic to the early of the Late Jurassic; the second stage is that the crude oil of Wujiaping Formation was cracked into gas, accompanied with the formation of a large amount of reservoir bitumen from the middle-late of the Early Cretaceous to the middle of the Late Cretaceous. Tectonic-lithologic traps in both east and west side of the Kaijiang-Liangping Trough are formed in the late period of Indosinian. The faulting activity in the Yanshanian period, which gradually dropped in both east and west side of the Trough and slightly increases the tectonic-lithologic traps, is consistent with the peak period of hydrocarbon charging in the Wujiaping Formation source rock. The direct contact with the active fault above and damage to the regional caprock of the 4th member of Feixianguan Formation jointly result in the destruction of the reef-shoal gas reservoir in Changxing Formation. The Himalayan faulting activity, which further elevated the tectonic amplitude of tectonic-lithologic traps in both east and west side, forms the unique northwest–southeast strike faults in the east side of the Trough. [ABSTRACT FROM AUTHOR]

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Combined elemental and isotopic geochemistry of dolomites in the lower Cambrian Longwangmiao Formation, eastern Sichuan Basin with petrographic observations has been investigated to determine the origin of these dolomites. Our data show that these dolomites are mostly located in the middle and upper parts of the Longwangmiao Formation, and most of them commonly occur as crystalline dolostones, whereas they less commonly occur as a dolograinstone. The dolomite contents of the dolostones range from 64 to 97%. The Mg/Ca ratios of these dolomites vary from 0.6 to 1.2, with an average of 0.8. These criteria divide the Longwangmiao dolomites into two types, according to either a positive correlation (type-A) or negative correlation (type-B), which exist between the MgO and CaO contents. Type-A dolomite consists of micritic-to-fine crystals (< 50 μm) of dull-to-no cathodoluminescence. It occurs in association with evaporites and relicts, and has no direct or indirect evidence of formation from carbonate precursors. These features suggest that the type-A dolomite has a parasyngenetic, evaporitic, and near-surface origin. Type-B dolomite consists mainly of medium-to-coarse (50–250 μm) crystals of zoned to uniform, red, or brown luminescence associated with relict textures. High Na concentrations, low Sr concentrations, and Sr/Ca ratios lower than those of type-A seem to indicate that the salinity of the dolomitizing fluid is similar to that of hypersaline seawater. The strongly positive correlation between δ13C and δ18O values, with the occurrence of karst caves and vugs, shows that the Type-B dolomite has been modified by meteoric water. Estimates from δ18O values suggest precipitation at depths of approximately 308 m. These proxies demonstrate that type-B dolomite formed during burial from seepage–reflux of hypersaline seawater, with a minor influence of meteoric water. Type-A dolomite is characterized by micropores and has a low permeability ranging from 0.00195 to 45.00262 × 10−3 μm2, with an average of 0.03730 × 10−3 μm2. It is volumetrically abundant throughout the region, and thus, it represents a potential gas reservoir. High-quality hydrocarbon reservoirs in the region are closely related to type-B dolomite, which is likely to occur in banks at the margins between open and restricted platforms. [ABSTRACT FROM AUTHOR]

Nanoscale pore characteristics of the Upper Permian Longtan transitional mudrocks and their equivalent strata Wujiaping Formation marine mudrocks in and around the eastern Sichuan Basin was investigated using field emission scanning electron microscopy (FE‐SEM) and low‐pressure N2 adsorption experiments. The results indicate that the Upper Permian mudrock is at a mature stage with total organic carbon (TOC) values ranging between 0.47% and 12.3%. The Longtan mudrocks mainly contain vitrinite, and their mineral composition is primarily clay. In contrast, the Wujiaping mudrocks are dominated by sapropelinite and solid bitumen, and their mineral compositions are mainly quartz and a notably high amount of pyrite. The FE‐SEM reveals that clay mineral pores and microcracks are the common pore types in the Longtan mudrocks. The specific surface area and pore volume depend on the clay content but are negatively correlated with the TOC. The generation of nanometer pores in the Longtan mudrocks is caused by high clay mineral contents. Meanwhile, the Wujiaping mudrock mainly contains OM pores, and the pore parameters are positively correlated with the TOC. The OM pore development exhibits remarkable differences in the Longtan and Wujiaping mudrocks, which might be related to their sedimentary facies and maceral fractions. Vitrinite and inertinite appear as discrete particles in these mudrocks and cannot generate pores during thermal maturation. Sapropelinite often contains many secondary pores, and solid bitumen with large particles, usually with several pores, is not the major contributor to the pore system of the investigated mudrock. [ABSTRACT FROM AUTHOR]

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The exploration of shale gas in Fuling area achieved great success, but the reservoir characteristics and gas content of the lower Jurassic lacustrine in the northern Fuling areas remain unknown. We conducted organic geochemical analyses, Field Emission Scanning Electron Microscope (FE-SEM), X-ray diffraction (XRD) analysis, high-pressure mercury intrusion (MIP) and CH4adsorption experimental methods, as well as NMR logging, to study mineral composition, geochemical, pore structure characteristics of organic-rich shales and their effects on the methane adsorption capacity. The Da’anzhai shale member is generally a set of relatively thick (avg. 75 m) and high carbonate-content (avg. 56.89%) lacustrine sediments with moderate total organic carbon (TOC) (avg. 1.12%) and thermal maturation (Vitrinite reflectance (VR): avg. 1.19%). Five types of lithofacies can be classified: marl (ML), calcareous shale (CS), argillaceous shale (AS), muddy siltstone (MS), and silty shale (SS). CS has good reservoir quality with a high porosity (avg. 4.72%). The small pores with the transverse relaxation time of 0.6–1 ms and 1–3 ms comprised the major part of the porosity in the most lithofacies from Nuclear magnetic resonance (NMR) data, while the large pore (>300 ms) accounts for a small porosity proportion in the CS. The pores mainly constitute of mesopores (avg. 23.2 nm). The clay minerals with a large number of interparticle pores in the SEM contributes most to surface area in the shale lithofacies with a moderate TOC. The adsorption potential of shale samples is huge with an average adsorption capacity of 4.38 mL/g and also has high gas content (avg. 1.04 m3/t). The adsorption capacity of shale samples increases when TOC increases and temperature decreases. Considered reservoir properties and gas properties, CS with the laminated structures in the medium-upper section of the Da’anzhai member is the most advantage lithofacies for shale gas exploitation.

The temperature history of samples and maximum palaeogeothermal profiles of boreholes were reconstructed based on low‐temperature thermochronology and vitrinite reflectance data, and the results provide limits for the timescale and amount of uplift–denudation of the eastern Sichuan Basin. The thermal history showed that the uplifting and cooling of eastern Sichuan Basin began around the Late Cretaceous (approximately 100–80 Ma). The region had experienced a continuous cooling process from the Late Cretaceous until the present, with the geothermal gradient decreasing from 32–36 °C/km to 20–23 °C/km. The amount of denudation at the Puguang region in north‐eastern Sichuan was approximately 2.3 km, whereas that at south‐eastern Sichuan was 1.9 km, and the erosion thickness in the eastern Sichuan fold belt that was revealed via the field samples is 2.3 ± 0.3–2.6 ± 0.3 km. The north‐eastern Sichuan experienced sustained cooling with inconspicuous fluctuations, whereas the thrust belt and the south‐eastern Sichuan Basin presented 2–4 stages with different cooling rates. It may indicate that the eastern Sichuan fold belt experienced a complex structural evolution, characterized by episodic upliftings and deformations since Late Cretaceous, while a different and gentle deformation took place in the northeastern Sichuan Basin. [ABSTRACT FROM AUTHOR]

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