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Progress in shale gas accumulation theory and exploration of the Cambrian Qiongzhusi Formation in the Sichuan Basin, China
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DOI：10.1016/s1876-3804(26)60720-3.png)
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Abstract
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Taking the Cambrian Qiongzhusi Formation in the Ziyang–Jingyan area of the southwestern Sichuan Basin as the research object, this study investigates the reservoir characteristics, enrichment mechanism and accumulation model of new-type shale gas by comprehensively using core and thin section observation, geochemical testing, and production dynamic analysis. Two types of shales, organic-rich shale and organic-lean shale, are developed in the study area. The organic-lean shale can receive gas supply from source rock cracking in adjacent areas, possessing the foundation of multi-source hydrocarbon generation and gas supply. Multiple sets of tuffaceous shale were developed in the Qiongzhusi Formation of southern Sichuan Basin. Multi-stage volcanic-hydrothermal activities promoted the development of inorganic pores, microfractures and reservoir space. The reservoirs are characterized by high inorganic pore content, high brittle mineral content and high free gas proportion, and the ultra-deep shale presents favorable reservoir fracturing property and gas-bearing potential. Breaking the traditional understanding that shale gas only migrates over a short distance and accumulates in-situ merely in organic-rich shale, a new mixed-source enrichment model of in-situ generation + external migration charging for organic-lean shale is established. It is clarified that natural gas in the central Sichuan Basin follows a composite accumulation evolution path of “source rock cracking – in-situ generation – migration replenishment”. Three types of gas reservoirs are formed successively, including the early in-situ cracked conventional gas in the Cambrian Longwangmiao Formation of the Gaoshiti – Moxi area, the middle-stage in-situ enriched shale gas in the Qiongzhusi Formation of the Ziyang area, and the mixed-source shale gas formed by in-situ generation superimposed with late migration replenishment in the Qiongzhusi Formation of the Jingyan area.
Keywords:
Sichuan Basin
Cambrian Qiongzhusi Formation
organic-lean shale
tuffaceous shale
silty shale
mixed-source shale gas
in-situ and migration accumulation
ultra-deep shale gas
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