The pore structures of different lithofacies in low-permeability sandy conglomerate reservoirs and their diagenetic impacts: A case study from the Es4 member of the northern steep slope in Dongying Depression, Bohai Bay Basin, NE China

Abstract

Sandy conglomerates are an important reservoir type in the Bohai Bay Basin, China. These reservoirs are characterized by complex lithofacies distribution, low permeability and heterogeneous pore structures that significantly impact reservoir quality and field development strategies. This study evaluated the Es4 member of the Eocene Shahejie Formation based on cores from three wells in the Dongying Depression. The analytical methods include thin section analyses, scanning electron microscopy (SEM), pressure-controlled mercury injection (PMI), rate-controlled mercury injection (RMI) and X-ray computed tomography (CT). The results show that the microscopic pore structures among various lithofacies of the Es4 sandy conglomerate reservoirs are controlled by different levels of diagenetic alteration, resulting in the heterogeneity of macroscopic reservoir quality. The high content of matrix in Mc (matrix-supported conglomerate) reservoirs leads to low primary porosity. Due to the poor grain sorting, Gc (grain-supported fine conglomerate) and Ps (pebbly sandstone) reservoirs lose a great number of primary pores during mechanical compaction process, whereas Ms (massive medium sandstone) reservoirs preserves abundant intergranular pores. Highly mineralized pore fluids, resulting from massive dissolution of volcanic rock fragments and carbonate fragments, contributes to extensive ferroan carbonate cementation during the late diagenetic stage. Powder crystal ankerite, with a grain radius of about 0.02 mm, blocks most pore throats within Gc reservoirs. Authigenic illite and mixed-layer illite/smectite bridges the throats and reduces the pore throat radius exponentially. Moderate dissolution of rock fragments and feldspars generates a number of secondary pores in Gc, Ps and Ms reservoirs, slightly enhancing permeability. With greater residual primary intergranular porosity and more large-size throats, Ms reservoirs possess the best reservoir quality, followed by Ps reservoirs. These research results are crucial to reservoir evaluation and predication in similar sandy conglomerate reservoirs.