Investigation of pore structure and reservoir quality of Eocene beach-bar sandstones in Dongying Depression, Bohai Bay Basin, East China

Abstract

The Eocene beach-bar sandstones are among the most important petroleum exploration targets of Dongying Depression, China. A little is known about the heterogeneous pore structure characteristics, contributing factors and their relations with highly variable macroscopic petrophysical properties. In this study, various testing techniques and methods have been conducted to investigate the depositional and diagenetic features, petrophysical properties and pore structure characters. Three major lithologies, namely fine sandstones, siltstones and argillaceous siltstones, are identified with distinct sandstones texture, diagenetic components and petrophysical properties. Relatively good sorting and large particle size are the dominant sedimentological factors that result in well-preserved primary porosity of fine sandstones. The exceptionally high permeability of argillaceous siltstones with low porosity is largely related with micro-fractures. Compaction is the main porosity-reducing factor of beach-bar sandstones, in addition to cementation of carbonate, quartz overgrowth and clay. Dissolution process crucially depends on the preservation of primary porosity, and it is often limited due to strong compaction. The reconstructed pore network and MICP experiments both suggest that the pore space connectivity is closed related with pore and throat size, and large pore throats are more likely to be connected by neighboring throats and form connected paths in the sandstones. The large pore systems determine the reservoir quality by contributing the vast majority of permeability, even if they may occupy small proportion of pore volume. The pore structures of fine sandstones mainly consist of large pore systems, which cause favorable high reservoir quality and strong correlations between porosity and permeability. The unfavorable reservoir quality, great fractal dimensions and poor correlations between porosity and permeability of argillaceous siltstones samples are largely attributed to the dominating micro pore throats.