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

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.

摘要以广安、合川等典型气藏解剖研究为基础，对川中地区天然气成藏条件与机理等进行研究。川中须家河组天然气为“大范围斑块式”成藏：平缓构造背景和生储盖组合“三明治”结构为大面积成藏提供了基础，但源灶生气强度平面分布不均衡和储集层横向非均质性决定不能大面积连片式成藏，而是大范围斑块式成藏；白垩纪末期以来的整体抬升是一次重要的卸载排烃、成藏事件，成藏是大范围的，成岩和构造作用以后保留下来的有效储集空间是“斑块状”而非“席状”分布的，决定经济性的天然气成藏是“大范围斑块状”而非“大面积连片式”的。须家河组天然气成藏具有源控性，天然气富集主要受气源灶、主砂体、构造背景和裂缝发育的联合控制，应细化气源灶研究，在高生气强度区内，寻找主砂体和裂缝共生发育带。此外，加强与须家河组一、三、五段源岩同期沉积砂体的评价研究，应有新气藏发现。