Characteristics and exploration potential of shale oil reservoir in the Upper Member of the Xiaganchaigou Formation in the Ganchaigou area, western Qaidam Basin

Objective Well C902, located in the Ganchaigou area of the western Qaidam Basin, achieved a breakthrough in shale oil exploration as indicated by the high-production oil flow in the Ⅳ-Ⅵ layers of the Upper Member of the Palaeogene Xiaganchaigou Formation. However, ambiguities in evaluation criteria and resource potential limit the efficiency of shale oil development in the Upper Member of the Xiaganchaigou Formation in the Ganchaigou area. Methods Thus, the exploration potential of shale oil in the Ⅳ-Ⅵ layers of the Upper Member of the Xiaganchaigou Formation in the Ganchaigou area was comprehensively evaluated by comparing the lithology, physical properties, pore structure, and organic geochemistry between Well C902 and other wells. Results The complicated lithofacies of laminar/lamellar dolostone, laminar/lamellar limestone, laminar clay shale, and lamellar mudstone suggest the mixed carbonate and siliciclastic deposition of the Ⅳ-Ⅵ layers. The high brittleness index implies the good fracability of the Ⅳ-Ⅵ layers. The intergranular pores and laminar fractures, with a total proportion of 85% in the pore system, were dominant in the Ⅳ-Ⅵ layers that can be roughly classified as low-porosity and ultralow-permeability reservoirs. The organic matter, dominated by type Ⅰ-Ⅱ1, shows an average TOC content of more than 0.9% in the Ⅳ-Ⅵ layers. Both chloroform asphalt "A" and S1+S2 indicate a high hydrocarbon generation capacity. Conclusion Summarily, the widely deposited the Ⅳ-Ⅵ layers of the Upper Member of the Xiaganchaigou Formation have the characteristic of source-reservoir integration and show not only good quality of source rock but also strong fracability of the shale oil reservoir. The excellent source-reservoir-caprock assemblage is also widely developed, which ensures good sealing ability and favors improving oil and gas recovery after fracturing. This study can theoretically improve the development efficiency of shale oil in the Ganchaigou area.