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Large quantities of gas resources have been found in the Paleo-Mesozoic carbonate rocks in the Sichuan Basin. However, many wells cannot obtain high production in deep low porosity-permeability reservoirs. For this contribution, we provide a steerable pyramid method for identifying the fault damage zone in the Kaijiang–Liangping platform margin, which is infeasible by conventional seismic methods. The results show that steerable pyramid processing could enhance the seismic fault imaging and a series of NW-trending strike-slip faults are found along the trend of the carbonate platform margin. The steerable pyramid attribute presents distinct vertical and horizontal boundaries of the fault damage zone, and heterogeneous intensity of an un-through-going damage zone. The width of the fault damage zone is generally varied in the range of 100–500 m, and could be increased to more than 1000 m in the fault overlap zone, intersection area, and fault tips. Further, the fault damage zone plays a constructive role in the high gas production in the deep tight carbonate reservoir. The results suggest the steerable pyramid method is favorable for identifying the weak strike-slip faults and their damage zone. The width of the fault damage zone is closely related to fault displacement, and the much wider damage zone is generally influenced by the fault overlapping and interaction. The fractured reservoirs in the fault damage zone could be a new favorable exploitation domain in the Sichuan Basin. [ABSTRACT FROM AUTHOR]

Based on 2D and 3D seismic data, the latest drilling data and field outcrop data of the northern slope of the Central Sichuan paleo‐uplift, the structural analysis method is used to analyze unconformity development characteristics and fault characteristics during the key structural transformation period, discussing the influence of the structural characteristics on the hydrocarbon accumulation of deep carbonate rocks. The results show that: (1) The two key unconformities of the Tongwan and Caledonian periods were primarily developed in deep carbonate rocks. Firstly, Tongwan's unconformities are characterized by regional disconformities between the second and third members of the Dengying Formation, the top formation of the Sinian and the lower Cambrian, strips of which zigzag through the north and south sides of the study area. Secondly, the Caledonian unconformity is characterized by a regional unconformable contact between the lower Permian and the ower Paleozoic strata. From NE to SW, the age of the strata, which were subject to erosion, changes from new to old, the denudation distribution showing as a nose‐shaped structure which inclines towards the ENE. (2) Boundary fault and transtensional strike‐slip faults developed in the Sinian to Paleozoic strata. In profile, there are three types of structural styles: steep and erect, flower structures, 'Y' and reversed 'Y' type faults. In plane view, the Sinian developed extensional boundary faults extending in an almost NS direction, strike‐slip faults developing and extending linearly in approximately EW, WNW and NE strikes in the Cambrian, with characteristically more in the south and less in the north. (3) The faults in the northern slope show obvious zonal deformations in transverse view as well as significant stages and stratified activity in a longitudinal direction. Among them, the activity of faults in the Sinian was the strongest, followed by the activity in the Cambrian period, the activity intensity of faults in the Permian period being the weakest. This fault activity can be divided into four periods: Sinian, Cambrian‐Permian, the early Indosinian period and the late Indosinian‐Himalayan period, the transtensional strike‐slip faults being the products of oblique extensions of pre‐existing weak zones in the Xingkai and Emei taphrogenesis, with a particular inheritance in the main faults. (4) Combined with hydrocarbon accumulation factors, it is considered that the epigenetic karstification of the Tongwan and Caledonian unconformities in the northern slope controlled the formation and distribution of carbonate karst reservoirs over a large area, also acting as a good pathway for oil and gas migration. The extensional faults developed at the margin of the NS trending rift, controlling the sag‐platform sedimentary pattern in the Dengying Formation of the Sinian. Strike‐slip faults in NE, WNW and ENE directions may control the micro‐geomorphological pattern inside the platform and intensify the differential distribution of grain beach facies. The multi‐stage hereditary activity of strike‐slip faults not only improved the porosity and permeability of the reservoirs, but also acted as the main channel of oil and gas migration, providing favorable conditions for the development of the current multi‐layer gas‐bearing scenario in the northern slope of the Central Sichuan Basin. [ABSTRACT FROM AUTHOR]

The origin of Precambrian dolomites has attracted research interest as a result of their association with petroleum resources. The long diagenetic history of these dolomites and multiple periods of hydrothermal activity mean that dating and identifying different periods of dolomite formation is challenging. This study focuses on dolomites in the Precambrian Dengying Formation of the Sichuan Basin, central China, and presents new petrographic, in situ laser ablation inductively coupled plasma mass spectrometry U–Pb and trace element, and Sr–C–O isotopic data to characterize three types of dolomite and constrain their age and origin. In situ U–Pb dating of type I dolomite yielded a lower intercept age of 637 ± 35 Ma. The rare earth element (REE) patterns, 87Sr/86Sr ratios (0.70827–0.70903), and δ13C VPDB (0.8‰–1.0‰) and δ18O VPDB (−1.5‰ to −1.0‰) values of type I dolomite indicate their derivation from seawater or a precursory source with seawater characteristics. Type II dolomite yielded an age of 636 ± 15 Ma and has lower ΣREE and slightly higher δ13C VPDB (1.2‰–1.5‰) and δ18O VPDB (−0.9‰ to −0.3‰) values than type I, as well as similar 87Sr/86Sr ratios and parallel REE patterns that are indicative of formation by reflux seepage dolomitization. The type III dolomite is characterized by large crystals, unique REE patterns, high 87Sr/86Sr values (0.70942–0.70975), and negative δ18O VPDB values (−6.4‰ to −3.1‰) that indicate a hydrothermal origin. The relationships between hydrothermal dolomite and bitumen indicate that the paleo-oil charging within the Dengying Formation reservoir occurred after the hydrothermal activity at 516 ± 49 Ma. The early hydrothermal activity provided space for the subsequent fluid movement, and oil and gas charging. Our integrated approach, incorporating petrographic, geochemical, isotopic, and geochronological data, constrains the ages of different dolomite phases and an oil-charging event within the Dengying Formation reservoir and provides new insights into the evolution of petroleum systems in general. • We identified three types of dolomites of Dengying Formation matching with the age of diagenesis and fluid activity. • The LA–ICP–MS in situ U–Pb dating of Dengying Formation dolomite yields a lower intercept age of 637 ± 35 Ma. • The dating results suggest an extensional event existed at 516 ± 49 Ma, and accounted for the deposition of hydrocarbon source rocks. • This study outlines potential approaches for furthering our understanding of dolomites and ancient petroleum systems. [ABSTRACT FROM AUTHOR]

The Lower Triassic Jialingjiang Formation, characterised by a carbonate–evaporite succession, is one of the important pay layers in the eastern Sichuan Basin. Several diagenetic events were identified in this study, including meteoric dissolution, dolomitisation, anhydrite/gypsum replacement/cementation, TSR, BSR, and multiple calcite cementation (C1–C3). Dolomitisation and anhydrite/gypsum replacement/cementation are the major diagenetic events affecting reservoir porosity. The dolostone is composed of D1, D2, and D3 dolomite. D1 dolomite is very finely crystalline, dominating in the low-energy, restricted depositional facies. D2 dolomite is finely crystalline, constituting a major portion of the dolostones. D3 dolomite is depositional fabric retentive dolomite, including dolo-wacke-/packstone, dolo-grainstone, and dolo-boundstone. Four types of anhydrites (A1–A4) were identified in the core and thin sections. A1 is stratabound anhydrite. A2 is the anhydrite in dolostones, filling in the interparticle spaces in dolo-grainstone and the intraparticle spaces in dolo-wackestone. A3 is the anhydrite in limestones, which occurs non-fabric-selectively. A4 is the anhydrite filling in the fractures. The diagenetic anhydrites (A2–A4) are interpreted to postdate to major dolomitisation. D1 and D2 dolomites are characterised by δ13C VPDB values of −3.4–+7.3 ‰ and δ18O VPDB values of −4.7−-2.4 ‰. The REE patterns of D1–D3 dolomites are characterised by light rare earth element (LREE) depletion relative to heavy rare earth elements (HREE) ((Pr/Tm) N of 0.54–1.16, 0.81 ± 0.20), middle rare earth elements (MREE) enrichment (BSI values of 1.06–1.38, 1.26 ± 0.10), and negative Ce anomalies (Ce/Ce* ratios of 0.74–0.94, 0.83 ± 0.07). The diagenetic anhydrites (A2–A4) show δ34S V-CDT values of +25.9–+45.7 ‰, similar to those of the stratabound anhydrite (+29.6–+43.0 ‰). The large variations in the δ13C VPDB values of D1 and D2 dolomites are attributed to methanogenesis and BSR. The δ18O VPDB values and REE characteristics of D1–D3 dolomites suggest that the carbonates are dolomitised by the mesohaline fluid. The δ34S V-CDT values of A1–A4 indicate that the diagenetic anhydrites are formed by redistribution in a closed system. The petrographical and geochemical data suggest that the Jialingjiang carbonate evolved in a closed system during burial diagenesis. Primary pores dominate in the Jialingjiang carbonate reservoirs, and the boundstone and grainstone show the best porosities based on the porosityversus lithofacies data. Therefore, we propose that the lithofacies, instead of the diagenetic events, are the key factors of the Jialingjiang carbonate. The resisting compaction of dolomitisation helps preserve the reservoir porosity, and anhydrite is the major diagenetic event destroying the reservoir porosity. • Lithofacies are the key factor of the reservoir qualities. • Petrography and geochemical work suggest the closed diagenetic systems. • Jialingjiang carbonate is dolomitized by mesohaline fluids predating diagenetic anhydrite. • Anhydrite selectively replace calcitic part in Jialingjiang carbonate. [ABSTRACT FROM AUTHOR]