Objective Promising gas shows have been observed in shale gas wells drilled in the organic−rich Uralik shale along the western margin of the Ordos Basin, marking a breakthrough in shale gas exploration and revealing significant exploration potential. To enhance the geological understanding of the sedimentary characteristics of organic−rich shale and clarify the distribution and development patterns of these shales, we conducted sedimentary facies division and analyzed favorable facies zones for the Uralik shale.

Methods Based on core observation, drilling, logging, and experimental tests, a comprehensive study of the sedimentary characteristics of the Uralik shale was conducted, leading to a refined division of sedimentary facies. We also optimized favorable sedimentary facies zones in conjunction with shale gas enrichment characteristics.

Results The study concluded that the Uralik shale is widely and relatively stably distributed in the study area, exhibiting characteristics of 'thicker deposits in the northern region and thinner in the southern region', and 'thicker in the western region and thinner in the eastern region.' There is a significant difference in burial depth between the north and south, with strong structural disturbance in the east and west. The Uralik shale was deposited in a semi−deep water environment, predominantly consisting of graptolite−rich black shale. The sedimentary facies are arranged in a north−south belt, with water depth gradually increasing from east to west. This progression developed platform margin slope facies, broad−shelf sea facies, and basin facies. The western part of the basin is dominated by broad−shelf sea facies, which can be subdivided into three microfacies based on mineral composition: siliceous mud shed, calcareous mud shed, and mixed mud shed. Among these, the siliceous mud shed shale, which has a high silicon content (>50%), high organic matter abundance (TOC >1%), and good reservoir physical properties, is identified as the most favorable sedimentary microfacies.

Conclusion Therefore, based on the research, it is concluded that the high−quality Uralik shale is a siliceous shale deposited at the base of the Uralik Formation.