Reservoir diversity and its genetic mechanism in the Es4 abnormally high-porosity zone of deep sandstone in the Bonan step-fault zone, Bonan Sag, Bohai Bay Basin, China

Abstract

Abnormally high-porosity zones in deep reservoirs are widely developed in the fourth member of the Eocene Shahejie Formation (Es4) in the fault-step zone of the Bonan Sag, Jiyang Depression, Bohai Bay Basin, China. However, research on reservoir diversity in these abnormally high-porosity zones has focused on the shallow parts of the reservoirs, and their genetic mechanisms remain unclear, restricting further exploration and development. In this study, comprehensive experimental analysis methods, including casting thin-section observation, scanning electron microscopy and digital core analysis were used to quantitatively characterise the pore structure of the different types of reservoirs in the Es4 abnormally high-porosity zone and analyse the genetic mechanism of these relatively high-quality reservoirs. The results indicate great reservoir diversity in the Es4 abnormally high-porosity zone. The main reservoir rocks are fine-to-coarse feldspar lithic or lithic feldspar sandstone with low porosity and low permeability and can be subdivided into three types (A–C). Four three-dimensional pore-structure models were constructed via the Avizo software to quantitatively reveal the reservoir diversity. The results suggest that ‘macropores’ and ‘mesopores’ are the most significant contributors to both the total pore volume and the amount of connected pores. Many ‘macropores’ and ‘mesopores’ are connected by many pore throats in Type A reservoirs, but these are significantly reduced in Type B reservoirs. Type C reservoirs hardly contain any (connected) ‘macropores’ and ‘mesopores’. Comprehensive analyses of the reservoir physical properties, secondary mineral composition and pore structure demonstrate that primary sedimentary conditions, calcite cementation and dissolution are the main factors that determine reservoir diversity in the Es4 abnormally high-porosity zone. Strong dissolution and effective removal of secondary products are the main mechanisms of the formation of Type A reservoirs, whereas strong dissolution failing to remove dissolution products has led to the formation of Type B reservoirs. Primary sedimentary conditions, strong calcite cementation and lack of dissolution are the main causes of failure to form effective pore networks in Type C reservoirs.