博兴洼陷下第三系沙四上亚段和沙三段岩性油藏形成机制

Forming mechanism of lithologic reservoir in the upper Es_4 and Es_3 of the Palaeogene in Boxing Sag

博兴洼陷下第三系岩性油藏的形成具有“复式排烃、排聚耦合和四元控藏”的总体特征,而其独特的烃源岩结构和地层压力系统决定了沙四上亚段和沙三段烃源岩在排烃方式、排烃机制以及岩性油藏成藏特点上的不同。沙四上亚段烃源岩有机质富集,地层压力较高,多为微裂缝排烃;其排烃效率高、范围大,油气顺层侧向运移至洼陷南斜坡鼻状构造的储层发育带聚集成藏。而沙三段烃源岩有机质较分散,地层压力低,多为差异突破压力排烃;其排烃效率低、范围小,只有在有效排烃体内的储集体才可能聚集成藏。断层排烃可以提高其排烃效率和成藏有效性,沙三段岩性油藏主要分布在洼陷中心附近。

The forming of lithologic reservoirs in the Palaeogene of Boxing Sag is generally characterized by multiple hydrocarbon expulsion, expulsion and accumulation coupling, and fourfold genesis controlling reservoir-forming. The upper Es4 are different from Es3 in mode, mechanism of hydrocarbon expulsion and the forming features of lithologic reservoir. These differences are caused by the unique structure of source rocks and formation pressure. The source rocks of the upper Es4 are rich in organic matter （OM） and have higher formation pressure. The hydrocarbon expulsion is controlled by micro fractures and has a higher efficiency and wider scope. Oil and gas are migrated laterally along the formation to the structural nose belts with developed reservoir and accumulated in the southern slope of Boxing Sag. Whereas the source rocks of Es3 are characterized by distracting OM and lower formation pressure. The hydrocarbon expulsion depends on the differential breakthrough pressure and gets a lower efficiency and less scope. Only hydrocarbon accumulation within the efficient hydrocarbon expulsion body can make the sand body form reservoirs. Faulting may improve the efficiency of hydrocarbon expulsion and effectiveness of reservoir-forming. The lithologic reservoirs of Es3 are mainly distributed nearby the center of Boxing Sag.