西湖凹陷西斜坡平湖组储层特征及致密化过程分析

Reservoir characteristics and densification process of Pinghu Formation in western slope of Xihu Sag

综合应用铸体薄片、扫描电镜、阴极发光、恒速压汞、流体包裹体和X衍射等分析技术,对储层岩石学、孔喉结构和成岩作用类型及特征进行深入研究,探讨储层致密化过程和物性演化。结果表明:平湖组储层砂岩类型主要为长石岩屑质石英砂岩,以细-中粒结构为主,分选性中到好;孔隙类型以溶蚀粒间孔为主,孔隙半径主要集中于130~190μm,喉道半径主要集中于0.2~10μm;埋藏压实是导致平湖组储层低渗-特低渗的主因,埋深、粒度及泥质决定了压实作用的强弱,而后期次生溶蚀及胶结作用的差异加剧了储层的非均值性。中成岩A期平湖组储层次生溶蚀规模受限于流体环境,细粒沉积不利于后期溶蚀是造成储层致密化的主因;进入中成岩B期,成岩环境呈碱性且逐渐封闭,大量含铁碳酸盐、呈丝状或弯曲片状伊利石等富集堵塞喉道,致使储层大规模致密。

By means of casting thin section, scanning electron microscope, cathodoluminescence, constant-rate mercury injection, fluid inclusions, X-ray diffraction, we studied the reservoir petrology, pore throat structure,types, and characteristics of diagenesis, and discussed the densification process and physical evolution of the reservoir of Pinghu Formation in Xihu Sag, East China Basin. Results show that the reservoir sandstone of the Pinghu Formation is mainly feldspar lithic quartz sandstone, the structure is mainly fine-medium grain with medium to good sorting. Intergranular pores formed the main reservoir space, and the pore radius ranged 130~190 μm,with throat radius of 0.2~10 μm. Burial compaction is the main cause of low permeability and density of PF reservoir. Depth, grain size, and argillaceous content determined the strength of compaction. In addition, the difference of secondary corrosion and cementation in later stage intensified the heterogeneity of reservoirs. In diagenetic stage, the scale of bio-dissolution in the Pinghu Formation reservoir was limited by fluid environment, and fine-grain particle deposition was not conducive to late dissolution, which are the two main reasons for reservoir densification. In diagenetic stage, the diagenetic environment was alkaline and gradually closed, after which a large number of iron-bearing carbonates, filamentous, and curved illite were built up and block the throat, resulting in the large-scale compaction of the reservoir.