西湖凹陷西次凹古近系花港组—平湖组深层油气成藏过程

西湖凹陷西次凹古近系花港组和平湖组深层油气资源丰富，是东海陆架盆地勘探开发新领域，受储层差异致密化影响，油气成藏机理复杂。为了厘清西次凹深层油气成藏过程，本文通过流体包裹体岩相学特征、显微测温、激光拉曼光谱、包裹体所含油气地球化学等实验分析，结合构造演化、地层埋藏史、热史等，开展了系统的储层特征、油气成藏期次、成藏过程研究。结果表明，花港组深层（>4000 m）和平湖组储层均已致密化，溶蚀孔隙是主要的储集孔隙类型；花港组和平湖组均发育两期油气包裹体，早期含油包裹体较多，晚期以天然气包裹体为主，成藏时间分别为龙井运动期和冲绳运动期，以晚期天然气成藏最为关键。包裹体中油气地球化学特征类似，反映生烃母质以高等植物生源为主，低等生物为辅，与平湖组煤系烃源岩特征一致。冲绳运动叠加平湖组生气增压是晚期天然气成藏的主要动力，此时H10段及以上储层未致密化。根据储层致密化与成藏时序匹配关系可将西次凹M构造深层油气藏类型划分为常规型、先成藏后致密型、先致密后成藏型和边成藏边致密型4种。本文成果可为研究区下一步致密砂岩气的勘探开发部署提供重要的理论依据。

Hydrocarbon accumulation process in the deep of Paleogene Huagang Formation and Pinghu Formation in the western subsag of Xihu sag

The deep Paleogene Huagang Formation and Pinghu Formation in the western subsag of Xihu sag are rich in oil and gas resources, making them a new field for exploration and development in the East China Sea shelf basin. However, the hydrocarbon accumulation mechanism is complicated due to the influence of differential densification of reservoirs. In order to clarify the hydrocarbon accumulation process in the deep reservoirs of the western subsag, we conducted systematic research on reservoir characteristics, hydrocarbon accumulation stages, and accumulation process. This was done through the experimental analysis of petrographic characteristics, microscopic temperature measurement, laser Raman spectroscopy, and geochemistry of the oil and gas contained in the fluid inclusions. This research was combined with an examination of structural evolution, stratigraphic burial history, and thermal history. The results of our study showed that the deep of Huagang Formation (>4000 m) and Pinghu Formation have both undergone densification, with dissolution pores being the main type of reservoir pores. Two stages of hydrocarbon inclusions were identified in both formations, with early stages dominated by oil- bearing inclusions and late stages dominated by natural gas inclusions. These findings indicate that the accumulation time corresponds to the Longjing movement and the Okinawa movement, respectively, with the late period being the most important. The geochemical characteristics of hydrocarbons contained in inclusions suggest that the hydrocarbon- generating material is mainly derived from higher plants and supplemented by lower organisms, which is consistent with the characteristics of coal- measure source rocks in the Pinghu Formation. The Okinawa movement, combined with pressurization caused by gas generation in the Pinghu Formation, served as the main driving force for gas accumulation in the late stage, particularly when the reservoirs in the H10 member and above were not densified. According to the relationship between the time sequence of reservoir densification and hydrocarbon accumulation, four types of reservoirs could be determined: conventional type, densification followed by accumulation, accumulation followed by densification and accumulation while densification. The research results provide a theoretical basis for the next step in the exploration and development of tight sandstone gas in the study area.