Origin of Quartz Cement in the Paleogene Sandstone Reservoir of Shahejie Formation, Bohai Bay Basin, China

The precipitation of authigenic quartz plays a significant role to reduce the reservoir characteristics and enhance the stiffness of the rock. The Es1 sandstone of Shahejie Formation is acting as a significant hydrocarbon producing rock in the Nanpu Sag. Various methods like thin section petrography, cathodoluminescence(CL), scanning electron microscope(SEM, with EDS), and electron microprobe analysis has been used to reveal the origin of quartz cement as well as to evaluate the effect of quartz cement on reservoir quality. The studied sandstone is classified as immature to mature feldspathic litharenite and lithic arkose and consists of quartz, feldspar, rock fragments and micas. Petrographic studies and SEM analysis shows that the authigenic quartz is acting a significant cement that reduces the reservoir quality. Whereas clay minerals(kaolinite and mixed layer illite to smectite) are dominant in the Es1 sandstone, that can reduce the reservoir quality. SEM, CL and thin section analysis reveal that there are two stages of quartz cement in the studied samples; that are pore filling authigenic cement and quartz overgrowth cement. Fluid inclusion homogenization temperatures shows that stages of quartz cement were developed with continuous process from 70℃ to 130℃. Quartz cements were generally originated from I/S reaction, feldspar dissolution, conversion of rock fragments and pressure solution. Feldspar dissolution(K-feldspar) and kaolinite to illite reaction is an insignificant silica source for the silica cement which is internally precipitated in a close system with diffusion transporting mechanism. Overall, quartz cement significantly enhance the rock strengthen and brittleness effectively as well as it reduce the overall reservoir quality.