Characteristics and genesis of tuffaceous shale oil lithofacies:A case study of Lucaogou Formation in Santanghu Basin

Lucaogou Formation in the Santanghu Basin is a special lacustrine fine-grained sedimentary deposits rich in volcanic debris and carbonate, with abundant shale oil resources. However, understanding of shale oil characteristics and genesis remains unclear. Lithofacies, sedimentary environment, and formation mechanisms of tuffaceous shale oil are investigated based on core and thin section observations, X-ray diffraction, field emission SEM and geochemical analysis. Results show that three mixed lithofacies types are developed in Lucaogou Formation: blocky tuff, laminated tuffaceous dolomite, and laminated dolomitic tuff. These lithofacies types are characterized by high content of felsic and dolostone, widespread organic matter, and low clay content. Formation of tuffaceous shale oil sweet spots is primarily influenced by four factors: inputting of volcanic ash as a high-quality source rocks and reservoirs provides good material basis; devitrification of volcanic glass, calcitization, and dissolution are crucial for formation of reservoirs; expulsion of source rocks with high-abundance organic matter expulsion facilitates migration and accumulation of hydrocarbon in adjacent reservoirs; fracture development improves reservoir permeability to form highly productive sweet spots. By analyzing characteristics and genesis of tuffaceous shale oil, the main controlling factors of reservoir physical property and oil saturation are clarified, which is of great significance for selection of shale oil exploration zones.