Differential diagenetic controls on petrophysical heterogeneity of lacustrine beach-bar tight sandstones: Insights from the Boxing Sag, eastern China

Abstract

The Bohai Bay Basin is rich in petroleum resources, and the lacustrine beach-bar tight sandstone facies in the upper part of the fourth Member of the Paleogene Shahejie Formation (Es4s) of the Boxing Sag is a key exploration target. However, the heterogeneity in petrophysical properties caused by differential diagenesis during burial impacts its development. In this study, an integrated approach is used that includes core and thin section descriptions, scanning electron microscopy (SEM), carbon and oxygen isotope analysis, fluid inclusion analysis, rock physical properties, and high-pressure mercury injection analyses to investigate the differential diagenesis of the beach-bar tight sandstone reservoirs and its effects on petrophysical heterogeneity. Key findings include: (1) The sequence of diagenetic evolution of the reservoirs is: mechanical compaction and spherical pyrite cementation → calcite and dolomite cementation → feldspar dissolution, authigenic kaolinite precipitation, and quartz overgrowth → ferro-calcite and ankerite cementation → secondary dissolution and quartz overgrowth → late agglomerate pyrite cementation, reflecting alternating alkaline and acidic diagenetic environments. (2) Mechanical compaction and cementation are the dominant factors responsible for the reduction in reservoir quality. (3) Four distinct diagenetic facies are identified: an intense compaction with intergranular dissolution diagenetic facies (DF-I), an intense compaction with residual intergranular porosity diagenetic facies (DF-II), a weak-moderate compaction with intergranular cementation diagenetic facies (DF-Ⅲ), an intense compaction with matrix occlusion diagenetic facies (DF-Ⅳ). DF-I and DF-II exhibit superior reservoir quality (porosity >10 %, permeability >0.1 mD), making them effective reservoirs. (4) The genesis and spatial distributions of these facies are controlled by the petrological structure, petrographic composition, sand-mud interface, mudstone thickness, and distance from oil-source faults. By integrating multiple factors, this study elucidates the genetic mechanisms of the four diagenetic facies and establishes their vertical distribution patterns. This provides a basis for predicting favorable reservoir distribution and guiding future exploration.