Reservoir-forming mechanism and enrichment conditions of the extensive Xujiahe Formation gas reservoirs, central Sichuan Basin

Abstract Using the Guang'an and Hechuan reservoirs as a basis, reservoir-forming conditions and mechanisms have been studied in the central Sichuan Basin. Reservoirs in the Xujiahe Formation form as extensive patch-like accumulations. The gentle structural setting and the sandwich-type source-reservoir-cap association provide a fundamental basis for gas accumulation across a large area. Gas-generating intensity is horizontally variable, and the heterogeneity of reservoirs is laterally strong, which makes it difficult to form a large connected area of reservoir, but favorable for forming patch-like reservoirs in a large area. The uplifting of the Sichuan Basin since the end of Cretaceous has caused hydrocarbon expulsion and gas reservoir-forming to take place on a large scale, and the effective storage space left after the diagenesis and tectonic action is a “patch” type rather than “sheet” type, which causes commercial gas reservoirs to be “extensive patch-like accumulations” rather than “extensive continuous gas accumulations”. Gas accumulation in the Xujiahe Formation reservoir is determined jointly by source kitchen, major sand body, structural setting, and fractures. Further research on source kitchen should be conducted to find co-developed areas of sandbodies, fractures, and high-intensity gas generation. The evaluation and research of sedimentary sandbodies in Members I, III and V should be intensified.