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Determining the present-day stress orientation is essential for the exploration and development of shale gas. The Longmaxi Formation is the most significant shale gas reservoir in the Jiaoshiba Area of the Sichuan Basin, China. However, little is known about the orientation of the maximum horizontal stress (S Hmax) and the factors influencing it within the formation. In this study, the orientation of the S Hmax in the Jiaoshiba Area was determined based on observations of drilling-induced fractures on resistivity image logs. Then, the tectonic activity, the mechanical properties of the rock, and finite element simulations were studied in detail to determine the major factors influencing the S Hmax orientation in the Jiaoshiba Area. The results show that the S Hmax orientation within the study area ranges from nearly EW to NEE-SWW. The vertical variations of the in-situ stress orientation are controlled by the structural strength and the mechanical properties of the rock. In the area that has experienced intense tectonic movements, the deviation in the S Hmax orientation (from 30° to 80°) was caused by the differences in the mechanical properties of the rocks. The finite element simulations indicate that the lateral variations in the S Hmax orientation was mainly controlled by the faults. The stress rotation near the end of the fault is 14°–21° higher than that in the middle of the fault. Furthermore, the stress variation caused by the mechanical properties of the fault zone ranges from 12° to 18°, and this deviation increases as the Young's modulus of the fault zone decreases. In the study area, the azimuth deflection (18°–46°) of the S Hmax orientation influenced by the NE faults is higher than that (12°–36°) influenced by the nearly NS faults, which is the main reason for the lateral stress rotation. • Variations of the S Hmax orientation in shales were determined from DIFs. • Influence factors of the S Hmax orientation in shales were discussed. • FE simulation shows that faults control variations of the S Hmax orientation in the horizontal direction. • Variations of the S Hmax orientation were affected by rock mechanical properties in the vertical direction. [ABSTRACT FROM AUTHOR]

Deep marine carbonate oil and gas resources in the Sichuan Basin are currently key exploration areas. As an important tectonic thermal event, the Emeishan mantle plume has an important influence on the source rocks under the Middle-Lower Permian in the Sichuan Basin. Therefore, the better understanding of the effect of Emeishan basalt eruption on source rock is vital for the exploration of marine oil and gas resources. On the basis of the thermal history study of the Sichuan Basin, this study restored the thermal maturity of the algal dolomite of the Sinian (Ediacaran) Dengying Formation in the Sichuan Basin in normal evolution. Using the hydrocarbon generation potential variation method, the hydrocarbon generation and expulsion characteristics of normally evolved algal dolomite and algal dolomite affected by basalt eruption were calculated. Through comparison, the influence of the Emeishan mantle plume on the algal dolomite of the Sinian Dengying Formation is analyzed. The research indicates that the thermal effect of the Emeishan mantle plume promotes the maturation of algal dolomite, which leads to the shortening of the oil generation period and the early arrival of the gas generation peak. The main hydrocarbon expulsion period of the normally evolved algal dolomite should be from the end of the Triassic to the end of the Jurassic, while the main hydrocarbon expulsion period of the algal dolomite affected by basalt eruption was advanced to the end of Silurian to the end of Triassic. In the current period, due to the high maturity of algal dolomite, the Emeishan mantle plume has little impact on the total resources. • Influence of Emeishan mantle plume on hydrocarbon generation and expulsion characteristics of algal dolomite of Sinian Dengying Formation. [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 Sinian Dengying Formation is a key target for deep oil and gas exploration in the Sichuan Basin, China. Natural gas was mainly distributed in the Ziyang–Weiyuan area (ZWA) and the Gaoshiti–Moxi area (GMA). At present, the genetic types and sources of natural gas are still unclear and controversial. The chemical composition, stable carbon isotopes, and biomarkers were used to analyze geochemical characteristics, genetic types, and gas–source relations. The results showed that the composition and content of hydrocarbon gas in the ZWA and GMA were similar. Natural gas was a typical dry gas with an average dry coefficient (C 1 /C 1 +) of 0.9991, which reflected higher methane (CH 4) content and lower heavy hydrocarbon gas (C 2 +) content. For nonhydrocarbon gas, CO 2 (0.01%–26.34%), N 2 (0.03%–24.95%), H 2 S (0.10%–3.19%), and He (0.01%–0.37%) were mainly distributed. Compared with the GMA, the ZWA was characterized by higher N 2 content and lower CO 2 content. Although the carbon isotopes had not been reversed, the values of δ13C 2 –δ13C 1 were different between the ZWA and GMA, with the GMA having the higher value. The assemblage relation between the natural gas composition and carbon isotopes indicated that the natural gas type was dominated by oil-cracking gas. Mud-shale in the high maturity stage of thermal evolution greatly influenced on the content of N 2 and He. H 2 S mainly belonged to thermochemical sulfate reduction (TSR), and most of CO 2 was formed by deep carbonate metamorphism and TSR. The gas–source relation showed that natural gas in the ZWA was derived from the Cambrian Qiongzhusi Formation and that the gas in the GMA mainly originated from the Sinian Dengying Formation and the Cambrian Qiongzhusi Formation. The established hydrocarbon generation model indicated that the Sinian Dengying Formation is potentially gas-rich. • The Sinian Dengying Formation natural gas was a typical dry gas, dominated by oil-cracking gas. • Source rocks of the Qiongzhusi Formation and Dengying Formation were the mainly gas source for gas reservoirs. • The resource potential was qualitatively evaluated by the established hydrocarbon generation model. [ABSTRACT FROM AUTHOR]

The Neoproterozoic provides an important record of glacial–postglacial paleoenvironments, organic carbon cycling, and hydrocarbon accumulation. Here, to better understand the environmental, sedimentary, and petroleum geological potential of the Neoproterozoic, we summarize and review research on paleoenvironment and hydrocarbon prospectivity during the era for regions worldwide and also present a case study of the Marinoan postglacial Ediacaran Doushantuo Formation in the Sichuan Basin, upper Yangtze region, China. Sediments that were deposited during the Neoproterozoic postglaciations generally have hydrocarbon-generating potential. In some places, the sediments have reached stages of burial and thermal evolution appropriate for hydrocarbon generation, with some sites having been exploited for commercial production. In the Sichuan Basin, the distribution and thickness of Doushantuo Formation sediments were controlled by glacially eroded topography and/or extension-related subsidence. Abundant organic matter was produced through paleo-productivity (Ba/Al and Cu/Al proxies can reach up to 181.08 and 8.94, respectively) and anoxia (U-EF and Mo-EF proxies can reach up to 5.45 and 14.00, respectively) governed by marine transgression. In South China, many depocenters of the Doushantuo Formation sediments have a strip-shaped geometry in the plane and are expected to be prospective for oil and gas. The reported findings can be used as a reference for further studies of glacial and postglacial deposits with organic matter accumulation of the Neoproterozoic and other periods. [ABSTRACT FROM AUTHOR]

The paleo-environment and organic matter accumulation after the Marinoan "Snowball Earth" event is of great significance to understand the Earth system and petroleum geology during this critical period. However, the present studies in South China are not well constrained due to the lack of continuous sedimentary records. Here, we present an integrated petrological and geochemical study of a unique, continuous, and complete outcrop in the northeastern Sichuan Basin, SW China. Results show that the postglacial Doushantuo shales in the study area can be divided into three intervals (from bottom to top: Unit1–Unit2–Unit3) according to geochemistry evolutionary trend. Multiple redox proxies (V EF , U EF , Mo EF , C org /P, Th/U, and U Mo covariation pattern) indicate the bottom redox condition changed from oxic to suboxic, and back to weakly oxic stratigraphically. Ba bio and Ba/Al paleoproductivity proxies suggest that the primary productivity was middle–high and it was slightly higher in Unit 2. The CIA corr values of Doushantuo shales range from 72.2 to 86.8 and gradually increase from Unit 1 to Unit 3, indicating relatively intense weathering conditions and global warm and humid paleoclimatic conditions. The Doushantuo shales in our studied outcrop is approximately 100 m in thickness. They are at over mature stage with average random reflectance of 1.86–2.14%, which makes the recovered original organic matter abundance up to ~2.0 wt%. The kerogen of the shales is type II. The accumulation of organic matter was mainly controlled by the preservation environments, with the primary productivity level as a secondary control. These data were further compared with other outcrops reported in previous studies for a comprehensive understanding. The variation of organic carbon contents in the Doushantuo shales hold similar features across the entire South China, with the middle part producing the highest-quality source rocks with conventional and unconventional hydrocarbon geological significance. This suggests that changes in sea level and paleo-weathering after the Marinoan glaciation controlled the enrichment of organic carbon in shales. The enrichment is a large-scale geological event across basins rather than a local one. This might be general and referenced for the postglacial and/or interglacial shales. [ABSTRACT FROM AUTHOR]

• Major elements trace hydrocarbon sources in over-mature petroleum systems. • The Sinian Dengying Formation gas systems in the Sichuan Basin were investigated. • Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P have variable occurrence states. • Mn/Fe ratio and ternary 10Mn–Fe–P diagram are used to trace hydrocarbon sources. • Lower Cambrian shales are the main sources for natural gases. Identifying hydrocarbon sources in over-mature petroleum systems is important and challenging. Here we present a pilot study using inorganic geochemistry to trace hydrocarbon sources in the giant natural gas accumulations within the Sinian (Neoproterozoic) Dengying Formation in the Sichuan Basin, SW China. We investigated the major elements (Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) as tracers of hydrocarbon sources for the first time. Our results show that the over-mature solid bitumen in the reservoirs contains parts per million amounts of major elements in addition to carbon (i.e., organic matter). Na and K in the bitumen occur mainly in the V–Ni porphyrin form. Al, Fe, and Mn are present in the Fe porphyrin form. The behavior of P is relatively complex, and it may reside mainly in a non-porphyrin organic form. The other elements (Ti, Mg, and Ca) may be present as emulsified "invisible" nano-minerals. The Mn/Fe ratio and ternary 10Mn–Fe–P diagram can be used for hydrocarbon–source rock correlations. Based on the proxies, it is suggested that the lower Cambrian Qiongzhusi Formation was the main source of hydrocarbons in the Dengying Formation. Our data suggest that inorganic geochemistry is an effective method for the study of over-mature hydrocarbons and complements traditional organic geochemical approaches. [ABSTRACT FROM AUTHOR]

Inorganic geochemical approaches (i.e., trace elements) for hydrocarbon–source rock correlations in sedimentary basins are a frontier area of research in petroleum geochemistry. This study used inorganic geochemistry to the over mature petroleum systems in the Sinian (Neoproterozoic) Dengying Formation in the Sichuan Basin of China. Our results show that the Ba N –Ni N –Cu N ternary diagram, V N –Ni N –Mo N ternary diagram and V/Ni–U/Th diagram (related to paleo-productivity and redox conditions), rare earth element (REE)–Y patterns, Ce/Ce*–La/La* diagram, and Ce/Ce*–La/La*–(LREE/HREE) N ternary diagram (related to fluid interactions) can robustly distinguish different source rocks. In particular, V, Ni, and Mo have a strong relationship to organic matter in source rocks, while their occurrence is most likely related to porphyrins in petroleum. Therefore, the V N –Ni N –Mo N ternary diagram can be used for hydrocarbon–source rock correlations. In contrast, Ba, Cu, U, Th, and REE–Y have different and complex geochemical behaviors, limiting the application in hydrocarbon–source rock correlations. The application in the Sichuan Basin shows that the hydrocarbons in the Dengying reservoir were mainly derived from the lower Cambrian Qiongzhusi Formation with minor contribution from the lower Cambrian Maidiping and Sinian Doushantuo formations. Compiled data from Lower Paleozoic crude oils in the Tarim Basin show that the V N –Ni N –Mo N ternary diagram can also be applied to low-maturity crude oil systems. Therefore, V, Ni, and Mo have strong relationship to organic matter in petroleum systems and the potential for hydrocarbon–source correlation, while other elements should be used under given conditions. [ABSTRACT FROM AUTHOR]

The Leikoupo Formation, which was deposited during the Anisian stage in the central Sichuan Basin, has produced significant amounts of conventional gas but there have been few studies on its unconventional resources. Organic-rich shale in the Leikoupo Formation was discovered for the first time in the wells of CT1 and JY1 in the central Sichuan region, but information on its sedimentary environment and climate evolution is scarce. A series of lithofacies, organic geochemistry and inorganic geochemistry analyses were used to analyse core samples of CT1 and JY1 to estimate their vertical changes. The organic matter characteristics, the sedimentary environment and the factors controlling the Leikoupo Formation shale in the Sichuan basin were revealed. The organic-rich shale of the Lei3-2 sub-member was formed in a semi-restricted lagoon environment. The sedimentary environment was dysoxic-anoxic, with high palaeoproductivity, a warm and humid to semi-arid and semi-humid climate. The vertical sea level decreased for a short period, led to an increase in oxygen content in the mixed shale, clayey shale and calcareous shale that were sequentially deposited from bottom to top. In the vertical direction, the palaeo-productivity and aridity decreased, and there was an increase in the salinity and terrigenous debris input. All of these corresponded well with TOC. According to Mo-TOC and Mo–U patterns, the mixed shale was characterised by the highest values of Mo EF , U EF and Mo EF /U EF , followed by clayey shale and calcareous shale, which indicated mixed shale was deposited in a moderately restricted water condition, which progressively became more restricted during the deposition of clayey shale and calcareous shale with the fall of relative sea level. Biomarker compounds (steranes, terpanes), Tmax and other pyrolysis parameters indicated that the organic matter had reached the stage of high-to over-maturity. The gammacerane to C 30 hopane in the ratio indicated that it was the bottom water in which it was deposited was sratified. • The first discovery of Lei3-2 sub-member shale in the Sichuan Basin. • The shale has good original hydrocarbon generation potential. • The shale was formed in a semi-confined environment. • Lithological changes reflect variations in sedimentary environment and sea level. [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]

Carbonate petroliferous basins usually develop fault-reservoir composite migration pathway systems. Accurate estimates of transport efficiency are important for locating oil and gas prospects, which is of considerable interest to oil explorers. The Cambrian Longwangmiao Formation in the Moxi–Gaoshiti area in the Sichuan Basin, the largest single monoblock gas accumulation in China with complicated migration pathway systems, was selected as an example to demonstrate the method. Using a combination of geologic, statistical, experimental data analyses, and quantitative characterization, a quantitative method for calculating the fault-reservoir composite transport efficiency is established for the first time. The results show that fault-reservoir composite migration pathway systems control the hydrocarbon accumulation. The analysis of data from 17 drilling wells using this method indicated a strong correlation between well productivity and composite transport efficiency, and the matching degree of actual and predicted results was as high as 83%. The best locations for oil and gas exploration are near areas with a high composite transport efficiency. Project planners can quickly find oil and gas prospects by matching calculated composite transport efficiencies to observed petroleum occurrences in a basin to avoid costly drilling mistakes. The method will have wide application in other carbonate basins around the world. • The fault-reservoir composite migration pathway controls hydrocarbon accumulation. • Quantitatively method for the composite transport efficiency is first established. • Composite transport efficiency is useful for location of oil and gas prospects. • Prospects for future drilling in the Moxi-Gaoshiti area of China are recommended. [ABSTRACT FROM AUTHOR]

Rhenium–osmium (Re–Os) geochronology is a robust technique for constraining the accurate timing of hydrocarbon evolution, but its effectiveness in over-mature petroleum systems has not been well constrained. Here we conduct a case study in the Neoproterozoic Sichuan Basin, China, i.e., the Sinian Dengying sequence (635–542 Ma). Re–Os isotopic data from this highly mature hydrocarbon reservoir yield a Re–Os isochron ages of 414 ± 44 Ma with an initial 187Os/188Os (Os i) of 1.36 ± 0.19 for bitumen and a 78 ± 37 Ma age with an Os i of 3.21 ± 0.17 for pyrobitumen. Coupled with thermal evolution of possible source rocks and fluid-inclusion homogenization temperatures, the 414 Ma age marks the timing of oil generation, and the 78 Ma age indicates the timing of gas generation associated with the thermal cracking of oil, and the formation of pyrobitumen in the reservoir. A comparison of the Os i of the 414 Ma bitumen and that of possible source rocks at the time of bitumen formation reveal that it was primarily sourced from shale of the lower Cambrian Qiongzhusi Formation. The results of this study provide new insights to better understand the over-mature oil–gas system in the Sinian Sichuan Basin, and show that Dengying Formation reservoirs in the depocenter areas of the basin are potentially favorable for oil and gas exploration. The results further highlight the utility of the Re–Os geochronometer to constrain the accurate age and source of hydrocarbon evolution. • Hydrocarbon evolution in Sinian Sichuan Basin was accurately dated by Re–Os geochronology. • Two Re–Os isochron ages were obtained at 414 Ma and 78 Ma. • The two ages mark the timing of oil generation and thermal cracking of oil, respectively. • The reservoir bitumens were sourced mainly from the lower Cambrian Qiongzhusi shales. • The Dengying Formation in the depocenter areas are potential reservoirs. [ABSTRACT FROM AUTHOR]