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Deep-seated faults and folds of foreland basin systems have become important exploration targets in the recent years because they are crucial in controlling fluid migration and hydrocarbon accumulation. In this study, we analyzed the characteristics and formation history of these structures in the northwestern Sichuan Basin using recently acquired two-dimensional (2D) and three-dimensional (3D) seismic data. The seismic interpretation revealed that the thrust sheets, tectonic wedges, and foredeep were well developed in the northwestern Sichuan Basin from the mountain to the basin. Forward thrusts, fault-bend folds, and wedges are the main types of structures in the thrust sheets and tectonic wedges. The deep-seated faults and folds were easily recognized in the high-resolution 3D seismic data. The imbricate thrust faults that merged into detachment layers of the Lower Cambrian are the main types of structures in the foredeep, and they show a prominent strike-slip influence in the horizontal direction. The formation of these structures in the foredeep in the northwestern Sichuan Basin mainly endured two stages of thrusting, including those during the Middle-to-Late Triassic and Cenozoic. Based on the tectonic evolution and seismic data, we infer that these deep-seated faults and folds in the foredeep may have formed earlier than the northern Longmen Shan fold-and-thrust belts and they may have been initially active in the late of Early Triassic and reactive during the Cenozoic. Furthermore, evaporites in the Lower and Middle Triassic were crucial in forming these structures. The petroleum exploration data suggested that the deep-seated faults can facilitate hydrocarbon accumulation. The thrust faults in the foredeep were more likely to act as migration pathways for fluids instead of sealing barriers along the horizontal direction. The interconnected reservoirs of deep-seated folds possess a great potential to allow large-scale hydrocarbon accumulation. Our study provides a good example for evaluating the hydrocarbon exploration potential in the deeply buried area in the sedimentary basin. [ABSTRACT FROM AUTHOR]

The exploration of high production, natural gas wells (>106 m3/d) has been extremely successful in the Gaoshiti and Moxi areas of the Leshan–Longnvsi paleo‐uplift, in the west‐central Sichuan Basin. This paper focuses on the Sinian and Cambrian reservoirs from the Gaoshiti and Moxi areas, where a dataset of new trace elements in bitumen and potential source rock is reported and where hydrocarbon‐source correlation is attempted based on trace elements characteristics of the 12 potential source rocks and 22 bitumen samples. Results show that the bitumen from the three reservoirs can be distinguished from each other using trace element distribution patterns and redox‐ and provenance‐sensitive elemental ratios. Bitumen in the second and fourth members of the Sinian Dengying Formation were derived primarily from the third member of the source rocks of the Dengying Formation, which represents a self‐sourced paleo‐petroleum system. In contrast, bitumen in the reservoirs of the Cambrian Longwangmiao Formation is derived from the Cambrian Qiongzhusi Formation and third member of Dengying Formation. The trace element method used in this study improves our understanding of the complex hydrocarbon–source relation and confirm the potential target for petroleum exploration in the study area. [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 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]

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]