Reservoir Characteristics and Controlling Factors of Shale Oil in Continental Saline Lake Basins—A Case Study of the Shahejie Formation, Jiyang Depression, Bohai Bay Basin, China

Saline lacustrine basins represent a key target for shale oil and gas exploration. The Shahejie Formation in the Jiyang Depression of the Bohai Bay Basin, China, is a typical example of saline lacustrine shale oil. However, its pore-fracture system characteristics and the factors controlling reservoir structure (porosity and pore-network development) remain poorly understood, limiting efficient exploration. This study focuses on shale intervals spanning the Lower Es3 to Upper Es4 of the Paleogene Shahejie Formation in the Jiyang Depression of the Bohai Bay Basin. By integrating core analysis, thin-section examination, field-emission scanning electron microscopy (FE-SEM), XRD, total organic carbon (TOC), and Ro data, we investigate the reservoir characteristics and the factors controlling reservoir quality of saline lacustrine shales, with the ultimate goal of predicting shale-oil sweet spots. Results show that Shahejie Formation mainly consists of laminated calcareous shale, laminated dolomitic shale, laminated mixed shale, and laminated clayey shale. The spatial configuration between minerals and organic matter formed during deposition exerts a primary control on reservoir properties. In laminated calcareous shale, microcrystalline calcite closely coexists with organic matter, forming micrometer-scale “source-reservoir” coupled pairs, where intercrystalline calcite pores and edge fractures along organic matter create an initial pore network. In laminated dolomitic shale, subhedral to euhedral dolomite crystals form a rigid framework, resulting in well-developed intercrystalline pores and stronger resistance to compaction. The carbonate-rich laminae of laminated calcareous shale exhibit porosities of 8–15%, whereas Fabric B shows porosities 3–5 times higher than Fabric A under comparable maturity conditions. The primary controlling factor for reservoir quality is lamina-fabric type, followed by carbonate mineral abundance. Diagenetic fluid alternations between acidic and alkaline conditions exert nonlinear modifications on pore systems. At Ro ≈ 0.7–0.9%, carbonate dissolution and associated pore enlargement are enhanced, preferentially dissolving calcite and further widening dolomite intercrystalline pores. When Ro > 1.0%, alkaline conditions promote carbonate cementation, reducing porosity, while overpressure-induced pore preservation and fracture generation effects also shift. Based on this, we propose a “fabric-constrained nonlinear reservoir evolution model,” indicating that sweet spots result from the spatial superposition of carbonate-organic matter coupled depositional architecture and moderate diagenetic modification (Ro = 0.8–1.0%). Because calcite is more soluble, it is preferentially dissolved by organic acids during the early oil window, generating dissolution pores; dolomite is more chemically stable, so its rigid crystal framework resists compaction and preserves intercrystalline pores. When these two minerals alternate in thin laminae, calcite laminae provide dissolution-enhanced pores while dolomite laminae maintain a rigid pore framework—This complementary coexistence provides a favorable mineral–fabric configuration for sweet-spot development. This study provides geological criteria for identifying intervals with favorable pore development and potential storage capacity in lacustrine saline-lake shales.