Superimposed hydrocarbon accumulation through multi-source and multi-stage evolution in the Cambrian Xixiangchi Group of eastern Sichuan Basin: A case study of the Pingqiao gas-bearing anticline

The Xixiangchi Group in eastern Sichuan Basin has great potential for natural gas exploration. However, there is a lack of in-depth studies of the hydrocarbon sources and the formation and evolution of gas reservoirs in this Group. Systematic investigation about the gas reservoir in Pingqiao anticline was consequently carried out in terms of characteristics of reservoir bitumen, the geochemical characteristics of natural gas, diagenetic minerals, and fluid inclusions. Based on this, combined with the reconstruction of the burial history, thermal evolution history and uplifting history of strata, and analysis of the regional tectonic settings, the hydrocarbon sources were identified and the formation and evolutionary processes of the gas reservoirs in Xixiangchi Group was revealed in this study. It was shown that the gas reservoirs have mixed gas sources from the shale source rocks in the Lower Cambrian Qiongzhusi Formation and in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation, and experienced several evolutionary stages, including the paleo-oil reservoir stage from the Late Siliurian to the Middle Permian, the paleo-gas reservoir stage from the Late Permian to the Early Cretaceous, and the superimposed accumulation and mixed-source gas reservoir stage since the Late Cretaceous. The mixed-source gas reservoir is formed by the adjustment of the Xixiangchi Group paleo-gas reservoirs and depressurization of the overpressure Wufeng-Longmaxi shale gas reservoirs and the charging of gas into the Xixiangchi Group reservoir of the Pingqiao anticline since the Late Cretaceous, which show obvious superimposed accumulation characteristics. There are different accumulation patterns in different geological periods. The accumulation pattern of the “old source - young reservoir” (i.e. hydrocarbons generated from older source rocks accumulating in younger reservoirs) dominates before the Late Cretaceous, and that of “juxtaposed young source - old reservoir” (i.e. hydrocarbons generated from younger source rocks accumulating in juxtaposed older reservoirs) dominates after the Early Cretaceous. Moreover, faults acted as critical vertical pathways for hydrocarbon migration during the evolution of the Xixiangchi Group gas reservoirs. This model provides new insights and theoretical basis for evaluation and mapping of the Xixiangchi Group play fairway in eastern Sichuan Basin.