辽河坳陷西部凹陷南段古近系沙河街组致密砂岩特征与成因机制*

渤海湾盆地辽河坳陷西部凹陷南段古近系沙河街组中发现了低产油气流和工业油气流,储集层具有低—特低孔隙度、特低—超低渗透率、低压的特征,属于典型的致密砂岩储集层。应用大量的薄片、扫描电镜、粒度分析、压汞和物性等测试资料,研究了致密砂岩的微观特征,探讨了致密砂岩的形成机理。研究结果表明,致密砂岩(孔隙度小于10.0%)大多发育在凹陷区;矿物成分、粒度、分选、磨圆等沉积学参数与致密砂岩的形成关系不大;致密砂岩的孔隙类型主要为次生孔隙,孔隙结构整体上具有中孔径、微细喉以及喉道分选不均匀等特征;机械压实作用和胶结作用导致了致密砂岩的形成,该致密砂岩属于次生型致密砂岩。

Characteristics and forming mechanism of tight sandstones in the Paleogene Shahejie Formation of southern West Sag,Liaohe Depression

Low-yield and commercial oil flows have been discovered in the Paleogene Shahejie Formation,southern West Sag,Liaohe Depression,Bohai Bay Basin. The reservoirs are typical tight sandstone reservoirs characterized by low-ultralow porosity,extra-low-ultralow permeability and low pressure. Based on data of thin sections,SEM,grain size,mercury pressure analysis and physical property,the micro-characteristics and formation mechanisms of tight sandstones were analysed. The results show that:The tight reservoirs(with porosity value lower than 10%)are mainly distributed in the sag area;The formation of tight reservoirs are not controlled by factors such as mineral composition,grain size,grain sorting and roundness;Pore types mainly consist of secondary pores and pore structures are characterized by medium pore diameter,tiny throat and heterogeneous throat sorting;The tight sandstones are mainly formed through compactions and cementations and are thus categorized as diagenetic tight sandstones.