川西坳陷什邡地区蓬莱镇组天然气藏特征及成藏过程分析

川西坳陷什邡地区蓬莱镇组气藏属于浅层常规—致密砂岩次生气藏,其形成机制不同于下伏须家河组储层的致密砂岩气。研究区内主要发育岩性圈闭,天然气来自于下伏须家河组煤系烃源岩及部分受到调整和改造的须家河组原生气藏,经长距离运移后在蓬莱镇组圈闭中聚集成藏。气层集中在蓬莱镇组中上部,天然气干燥系数大,以煤型气为主。流体包裹体分析表明,研究区内蓬莱镇组共经历3期热流体活动,前两期分别为液态烃充注及气液混合烃充注,但并未形成规模油藏;第3期为大规模天然气充注,对应成藏时间为晚白垩世晚期至古近纪早期(78~34Ma)。在此基础上,结合地质构造演化、烃源岩生排烃期、储层致密化窗口及生储盖组合特征,确定了蓬莱镇组天然气藏的形成过程及成藏特征:蓬莱镇组下部储层在晚白垩世中期达到致密化,已不利于天然气聚集成藏;喜山运动时期,下伏须家河组气藏受到调整和改造,早期发育的深大断裂得以扩展,大量深部气源沿断层向上运移至蓬莱镇组中上部,形成了浅层次生气藏。

Accumulation process and characteristics of gas reservoirs in the Penglaizhen Formation of Shifang area, Western Sichuan Depression

Abstract: Gas reservoirs in the Penglaizhen Formation within Shifang area of Western Sichuan Depression are shallow sub-tight sandstone secondary ones, whose formation mechanism differs from that of tight sandstone gas reservoirs in the underlying Xujiahe Formation. Lithologic gas reservoirs occur mainly in the study area. Gas is concentrated in the middle and upper Penglaizhen Formation with high dry coefficients. Gas is mainly from coal-measure source rock in the underlying Xujiahe Formation, with a bit originating from the original reservoirs suffering from adjustment and transformation. Anyhow, it is dominated by coal-type gas. Fluid inclusion analysis shows that there are three stages of thermal fluid flow in the Penglaizhen Formation, with the first two stages being respectively liquid hydrocarbons and gas liquid mixture of hydrocarbons charge without large-sized reservoirs and the third being large-scale natural gas filling whose corresponding accumulation time was from late Cretaceous to early Paleogene (78-34 Ma). On such a basis and in combination with the tectonic evolution, the stages of hydrocarbon generation and expulsion, reservoirs densification and source-reservoir-cap assemblages, the authors determined the formation and accumulation characteristics of gas reservoirs in Penglaizhen Formation. The lower part reservoir of Penglaizhen achieved densification during the Late Cretaceous. During the Himalayan Movement, Xujiahe gas reservoir was adjusted and renovated, and the earlier-formed deep faults were extended. A lot of deep gas source migrated upward to the upper part of Penglaizhen along faults, thus forming the shallow secondary gas reservoirs.