A review of the controlling mechanisms of carbonate laminae on shale oil ‘sweet spots’ in saline lacustrine basins

Carbonate laminae in saline lacustrine shale sequences are widely developed, and their diverse combinations and spatial stacking patterns represent a key geological factor controlling the differential enrichment of lacustrine shale oil. These laminae, composed of micro- to millimeter-scale thin layers of minerals such as calcite and dolomite, serve as preferred targets for saline lacustrine shale oil “sweet spots” due to their significant economic potential and development prospects. The laminae exhibit various structures, including cryptocrystalline, sparitic, and fibrous structures, which originate from diverse formation mechanisms—mechanical, physicochemical, and biochemical processes—leading to distinct mineralogical and structural heterogeneities. In terms of pore-fracture systems, carbonate laminae develop advantageous pore types such as intercrystalline, dissolved, and intergranular pores. Their high brittleness promotes the formation of heterogeneous fracture networks, including bedding-parallel and structural fractures. During diagenesis, organic acid dissolution and recrystallization further enhance pore connectivity, significantly improving in storage capacity and permeability. This facilitates the micro-migration and enrichment of light hydrocarbons within the laminae, forming movable oil. The control of carbonate laminae on shale oil sweet spots is manifested in four aspects: extensive distribution facilitates segmental and areal sweet spot development; high organic matter content provides the material basis for hydrocarbon generation; high-quality source-reservoir configurations promote near-source migration and enrichment; high brittleness enhances reservoir fracability and development potential. In conclusion, detailed study of carbonate laminae provides critical guidance for shale oil exploration and sweet spot prediction in saline lacustrine basins.