Oil & Gas Geology ›› 2023, Vol. 44 ›› Issue (6): 1555-1567.doi: 10.11743/ogg20230617

• Petroleum Geology • Previous Articles     Next Articles

Hydrocarbon accumulation conditions and exploration targets of the Triassic subsalt ultra-deep sequences in the western Sichuan Depression, Sichuan Basin

Shuangjian LI1,2(), Zhi LI1,2, Lei ZHANG1, Yingqiang LI1,2, Xianwu MENG3, Haijun WANG3   

  1. 1.Petroleum Exploration and Production Research Institute,SINOPEC,Beijing 102206,China
    2.Key Laboratory of Geology and Resources in Deep Stratum,SINOPEC,Beijing 102206,China
    3.Southwest Oil and Gas Company,SINOPEC,Chengdu,Sichuan 610041,China
  • Received:2023-07-20 Revised:2023-10-20 Online:2023-12-01 Published:2023-12-20

Abstract:

The Triassic subsalt marine sequences in the western Sichuan Depression, with burial depths generally exceeding 7 000 m, have been subjected to limited oil and gas exploration. Nonetheless, these sequences boast superior conditions for hydrocarbon accumulation, showing great exploration potential. Based on the latest drilling and seismic data, we conduct a comprehensive analysis of the ultra-deep source rocks, reservoirs, and structural deformation styles, as well as their effect on dynamic hydrocarbon accumulation in the western Sichuan Depression. Accordingly, we delineate favorable exploration targets in the depression. The key findings of this study are as follows: (1) Three suites of regional source rocks, namely the Lower Cambrian Qiongzhusi Formation, the Middle Permian Maokou Formation and the Upper Permian Longtan Formation, are developed in the ultra-deep layers of the western Sichuan Depression. Three suites exhibit hydrocarbon-generating intensities exceeding 2.0×109 m3/km2, enabling the western Sichuan Depression to have the resource potential to form large- or medium-sized oil and gas fields; (2) The depression hosts the platform-margin mound shoals of the Sinian Dengying Formation and the grain shoal dolomite reservoirs of the Permian Qixia and Maokou formations, which are influenced by high-energy facies tracts and multi-stage karstification. All these lay the foundation for the formation of large-scale reservoirs; (3) Subsalt, parautochthonous anticline structures are developed in the footwall of nappes in the piedmont zone. Meanwhile, highly-steep faults and strike-slip transpressional structures are found in the piedmont depressed zone. All these structures act as favorable traps for hydrocarbon accumulation; (4) The piedmont buried structures and the tectono-lithologic composite traps within the depression were both formed during the Indosinian, matching well with the main hydrocarbon-generating period of the Cambrian source rocks and the basement fault activity, suggesting favorable conditions for early hydrocarbon accumulation. As revealed by a comprehensive assessment, favorable targets for future oil and gas exploration in the western Sichuan Depression include the platform-margin zone of the Sinian Dengying Formation and the Permian high-energy facies tracts connected to highly-steep basement faults in the depression, as well as the autochthonous structures in the footwall of nappes in the piedmont zone.

Key words: Qiongzhusi Formation, Longtan Formation, Dengying Formation, Qixia Formation, Maokou Formation, subsalt deep sequence, accumulation condition, western Sichuan Depression, Sichuan Basin

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