Depositional–diagenetic–fault coupling and the genesis of differential-quality deep lacustrine reservoirs, Bohai Bay Basin

Deep lacustrine sandstones commonly show strong reservoir-quality heterogeneity, yet the coupled controls of deposition, diagenesis, and fault-related fluids remain insufficiently constrained. Integrated petrographic, geochemical, and petrophysical data from the Lower 3rd sub-member of the Eocene Shahejie Formation in the Raoyang Sag, Bohai Bay Basin are used to clarify the origin of high-quality reservoirs in deeply buried lacustrine sandstones. The reservoirs are generally tight, with porosity of 1.9–15.8% and permeability of 0.01–70.5 mD, although reservoir quality is locally enhanced near 3,900 m depth. Pore evolution was governed by compaction, carbonate and chlorite cementation, feldspar dissolution, and late ankerite replacement, producing three genetic reservoir types: I) early-cemented tight reservoirs in outer lacustrine-fan subfacies, II) dissolution-dominated reservoirs with relatively high porosity but low permeability in delta-front facies, and III) fault-associated high-quality reservoirs in inner-lacustrine-fan sand bodies. These differences reflect depositional–diagenetic coupling, in which sand-body thickness and grain size influenced the initial reservoir framework and diffusion-controlled carbonate cementation near sandstone–mudstone interfaces. Fault-related fluid systems further modified reservoir quality: relatively closed fluid systems led to carbonate precipitation and pore loss, whereas pore development was favored by dissolution in relatively open fluid systems and inhibition of late carbonate cementation by hydrocarbon charge. These findings highlight depositional–diagenetic–fault coupling as a predictive framework for high-quality deep lacustrine sandstone reservoirs in rift basins.