渤海湾盆地东营凹陷沙四下亚段地层压力演化与天然气成藏

Pressure Evolution and Gas Accumulation of the Fourth Member of the Shahejie Formation in Dongying Depression,Bohai Bay Basin

通过实测压力、泥岩声波时差、流体包裹体古压力恢复,对东营凹陷民丰地区沙四下亚段地层压力的演化特征进行了分析。研究表明,民丰地区地层压力纵向上存在"常压—超压—常压"三段式结构;时间上,沙四下亚段地层压力具有"二旋回波动模式",即存在"常压—弱超压—常压"和"常压—超高压—常压(弱超压)"的演化过程,其中两次地层超压的形成时间与中深层天然气藏的两期成藏时间相对应,为油气藏的形成提供了动力条件。研究区地层压力的动态演化过程是多因素作用的结果,第一次高压的形成是地层快速沉积产生欠压实的结果,生烃贡献相对较小;第二次超压的形成是烃源岩大量生烃和原油裂解成天然气造成的,沉积作用为辅。由于地温梯度降低、断裂—砂体泄压、饱和气藏深埋等作用的影响,现今民丰地区沙四下储层表现为以常压为主,伴生部分超压的分布特征。

After the paleo-pressure build-up by means of measured pressure,mudstone sonic log interval and fluid inclusions,the pressure evolution model of the fourth member of the Shahejie Formation is analyzed in Minfeng area,Dongying depression.The results show that,different periods of reservoir formation correspond to different pressure system in Es4 reservoir,Minfeng area： the hydrocarbon accumulation of Sha 2-last stage of Dongying was accomplished in a higher pressure system;the hydrocarbon accumulation environment of the last stage of Ng was normal pressure;and after the middle and late period of Nm the cracking gas in the central area of sub sag was formed in a high pressure system.The dynamic evolution process of formation pressure is the results of integrated effect of multiple factors,the first high pressure was due to uncompaction of the high-rate deposition of the formation,and the hydrocarbon-generation made a little contribution;a large amount of hydrocarbon played an important part in the form of the second high pressure,in addition,the sedimentation acted as the auxiliary role.Under the influence of the decrease of geothermal gradient,fracture,sand body-pressure discharge and the deep-burial of gas reservoir,the gas reservoir founded in Sha 4 reservoir,Minfeng area is mainly normal pressure.