Elaborated Porosity and High-Quality Reservoirs in Deeply Buried Coarse-Grained Sediment: Insight of the Sublacustrine Sandy Conglomerates in the Eocene Shehejie Formation (Es4), Dongying Subbasin, Bohai Bay Basin, China

Abstract

Reservoir porosity evaluation is crucial for successful prediction of reservoir quality in deeply buried heterogeneous strata. Recent studies have demonstrated that high-quality reservoirs occur in deeply buried strata. However, little is known about the details of pores related to the good reservoirs. The northern Dongying steep slope sandy conglomerate was investigated to understand the porosity related to high-quality reservoir formation in sandy conglomerate using seismic data, well-logs, SEM-EDS, cathodoluminescence, and optical microscopy. The result reveals three genetic pores: primary, secondary, and mixed pores. The dissolution porosity consists of intergranular pores, intragranular pores, intercrystalline pores, and moldic pores. The intergranular dissolution and enlarged pores are the main contributors to good reservoir quality among the different pores. The index of compaction indicates that compaction is the crucial factor diminishing the reservoir quality in the progradation sequences, while cementation stands as the critical factor for porosity reduction in the retrogradation sequences. Comparing the reservoir properties reveals that the dissolution porosity is more significant in the Es4s than the Es4x, which renders the Es4s reservoir relatively good compared to the Es4x. This study demonstrated that for oil exploration in the fourth member of the Shahejie Formation, the Es4s should be prioritized.