Analysis of favorable sedimentary facies belt of shale gas in Uralik Formation in western Ordos Basin

The characteristics of organic-rich Uralik shale in the western margin of the Ordos Basin include low organic matter abundance, poor hydrocarbon generation potential, and dense physical properties. There are good gas shows from the shale gas wells drilled to this shale, showing good shale gas exploration potential. In order to improve the geological understanding of the sedimentary characteristics of the organic-rich shale and clarify the development characteristics and distribution rules of the organic-rich shale, the division of sedimentary facies and the analysis of favorable facies zones were carried out for the Uralik shale. It is concluded that the Uralik shale is widely and relatively stably distributed in the study area, with the characteristics of ' thick in the northern region and thin in the southern region, thick in the western region and thin in the eastern region ', ' large difference in burial depth between north and south, and strong structural disturbance in the east and west '. The Uralik shale was deposited under a semi-deep water environment, mainly composed of graptolite - rich black shale. The sedimentary facies are distributed in a north-south belt. The water body was gradually deepened from west to east, and respectively developed the basin facies, the broad-sea shelf facies and the platform margin slope facies. The western part of the basin is dominated by the broad-sea shelf facies. According to the difference of mineral composition, the broad-sea shelf facies can be subdivided into three microfacies : siliceous mud shed, calcareous mud shed and mixed mud shed. Among them, the silicon content is high ( > 50 % ), organic matter abundance is high ( TOC > 1 % ), and reservoir physical properties is good in the siliceous mud shed shale, which are considered as the favorable sedimentary microfacies. Therefore, based on the above research, it is believed that the high-quality Uralik shale is a set of siliceous shale deposited at the Uralik Formation bottom.