川中北斜坡中二叠统茅口组多期流体活动与成藏过程

Multi-stage fluid activity and accumulation process of the Middle Permian Maokou Formation in the northern slope of central Sichuan Basin

川中北斜坡中二叠统茅口组岩性主要为生屑灰岩,发育岩溶缝洞型储层。通过岩心观察、镜下薄片鉴定,结合阴极发光特征,确定了茅口组溶蚀孔洞及裂缝中的矿物充填序列。在此基础上对不同期次成岩矿物中捕获的流体包裹体特征进行分析,结合川中北斜坡二叠系埋藏热演化史及成岩矿物U—Pb定年数据,明确茅口组油气成藏过程。结果表明:茅口组储层储集空间以残余溶蚀孔洞及裂缝、溶蚀缝为主;孔洞中见粉—细晶方解石→细—中晶方解石→粗—巨晶方解石3期充填。第二期方解石U—Pb年代学定年时间为晚二叠世(244.1±6.3 Ma),流体包裹体特征表现为高成熟度液烃包裹体与盐水包裹体混生,测得均一温度峰值主要集中在120~130℃之间,第三期粗—巨晶方解石发育高密度甲烷包裹体及沥青包裹体,其均一温度峰值主要集中在165~170℃之间。川中地区中二叠统茅口组经历了3期成藏事件:第一期为晚二叠世—早三叠世下寒武统筇竹寺组生成的原油充注形成古油藏;第二期为中侏罗世—早白垩世油藏裂解形成气藏;第三期为晚白垩世喜马拉雅期构造抬升气藏调整形成现今混合气藏。中二叠统天然气主要为筇竹寺组烃源岩生成的原油裂解气,混有少量中二叠统自身烃源岩的贡献,川中地区发育的继承性断裂具有较好的沟通源储作用。

The lithology of Middle Permian Maokou Formation in the northern slope area of central Sichuan Basin is mainly bioclastic limestone,and the reservoir type is karst fracture and cave.Through core observation,microscopic thin section identification and cathodoluminescence characteristics,the mineral filling sequences of Maokou Formation are determined.On this basis,the characteristics of fluid inclusions captured in different stages of diagenetic minerals are analyzed,and combined with Permian burial thermal evolution history and diagenetic mineral U-Pb dating data in central Sichuan Basin,the process and time of oil and gas accumulation in Maokou Formation are determined.The results show that the reservoir space of the second member of Maokou Formation is mainly composed of residual dissolution pores,fractures and dissolution fractures.The reservoir space of the Maokou Formation is dominated by residual dissolution cavity,fracture and dissolution fracture.Three stages of calcite filling were observed in the pores,with the sequence of powder-fine crystal calcite→fine-medium crystal calcite→coarse-giant crystal calcite filling.The U-Pb chronological dating of the second stage calcite is the Late Permian(244.1±6.3 Ma).The fluid inclusions are characterized by the mixing of high maturity liquid hydrocarbon inclusions and brine inclusions.The peak homogenization temperature is mainly between 120℃and 130℃.In the third stage of coarse-giant calcite,there are high-density methane inclusions and asphalt inclusions,and their homogenization temperature peaks are mainly concentrated in the range of 165-170℃.It is revealed The Maokou Formation of Middle Permian in central Sichuan Basin has three stages of hydrocarbon accumulation,the first stage is the filling of oil from Late Permian to Early Triassic Lower Cambrian Qiangzhusi Formation to form fossil oil reservoir,the second stage is in the Middle Jurassic-Early Cretaceous fossil oil reservoir cracking to form gas reservoir,and the third stage is the adjustment of Late Cretaceous Himalayan structural uplift gas reservoir to form present mixed gas reservoir.The Middle Permian natural gas is mainly crude oil cracking gas generated by the source rocks of the Qiongzhusi Formation,mixed with a small amount of Middle Permian source rocks,and the inherited faults developed in central Sichuan Basin play a good role in communicating source and storage.