Reassessment of exploration directions of continental shale oil in Lower Jurassic Da'anzhai Member in northern Sichuan Basin

The Da'anzhai Member of the Lower Jurassic in northern Sichuan Basin is one of the major oil-bearing formations in the basin. In recent years, with the development of unconventional exploration theories, exploration efforts have shifted towards the shale reservoirs in the Da'anzhai Member, enhancing the understanding of the exploration directions for continental shale oil in northern Sichuan. Based on the data from shale oil exploration wells LY1 and YY2, including core samples, experiments, drilling, and fracturing data, a reassessment of the oil and gas formation conditions, resource potential, and engineering geological conditions was conducted. The results indicate that: (1) The second submember of the Da'anzhai Member in Langzhong has a good resource base and formation conditions, indicating great exploration potential. The second submember mainly develops three lithologies: shale, shell limestone, and shale interbedded with shell limestone. The shale exhibits good hydro- carbon source quality and is generally in a middle to high-mature stage. Its hydrocarbon production intensity is (20-90)×104 t/km2, and the retained hydrocarbon accounts for 66% to 78% of the total hydrocarbon production, with a shale oil resource amounting to 3.26×108 t. (2) The shale samples from the second submember in the study area have poorly developed fracture networks, characterized by poor permeability conditions, difficult drilling and compressibility conditions. The matrix-type shale oil has high crude oil viscosity and high wax content. The coupling configuration relationship between the permeability of the shale samples and the fluidity of the shale oil is key to achieving high production in target layers. (3) Given the current poor production results in matrix-type shale exploration, it is recommended to adopt a combined conventional and unconventional exploration method to study the formation system of the shale and limestone interlayer reservoirs in the research area, considering high-quality shale and fracture-porosity type limestone as favorable exploration targets. Through the integration of geological and engineering research methods, efforts should be made to tackle technical challenges related to drillability, compressibility, and production, thereby improving engineering and technological levels.