Sedimentary facies distribution and the characteristics of transitional shale in the upper Palaeozoic in northeast Ordos basin, China

Abstract

The Taiyuan formation that was formed in the late Permian period of the upper Palaeozoic in the Linxing area, north-eastern of Ordos basin, China, shows the marine and continental interdepositional systems, as well as the resource potential of shale gas. Through a field outcrop, core observation, and well logging data, the Taiyuan formation can be divided into Tai1 (p1t1) and Tai2 formations (p1t2). The main depositional environments of the region in Taiyuan formation included tidal flats, swamp, and lagoon which is also a good depositional environment for high-quality shale development. During the Taiyuan formation period, the sedimentary sea level in the study area rose and fell frequently, and the barrier coastal sedimentary system is mainly developed. In addition, the shale has superior development characteristics: The Taiyuan formation shale has a considerable amount of material, with an accumulated thickness of greater than 30 m; the average organic carbon content in the Taiyuan group is 3.34%, and the content in the Tai2 formation (p1t2) is slightly higher than in the Tai1 formation (p1t1); a high thermal evolution content is developed with an average vitrinite reflectance (Ro) of 1.33%; the Kerogen type is the III model, in which gases are mainly developed; the brittle mineral content in the rock mineral components is high, and the brittleness index is generally greater than 50%, which indicates a high rate of compressibility. Therefore, the Taiyuan formation indicates a marine and continental interdeposition and has a high exploration and development potential for shale gas.