Characteristics and controlling factors of transitional shale gas reservoirs: An example from Permian Shanxi Formation, Daning-Jixian block, Ordos Basin, China

The shales in the 2^nd Member of Permian Shanxi Formation (Shan2 Member) at the Daning-Jixian block at the eastern margin of the Ordos Basin were deposited in a marine–continental transitional environment. The Shan₂³ sub-member, which is the target interval for transitional shale gas exploration and development in China, is characterized by substantial shale thickness, few and thin interlayers. This interval, however, has been rarely investigated for reservoir characteristics, especially the controlling factors of high-quality reservoirs. In this paper, the reservoir characteristics and main controlling factors of high-quality reservoirs in Shan₂³ in the Daning-Jixian block were systematically discussed by organic geochemical analysis, mineral compositional analysis, and microscopic reservoir characterization. The results indicate that the lagoon facies shale interval in the upper part of the Shan₂³ sub-member has a high total organic carbon (TOC) content, a high brittle mineral content, and high BET surface area and BJH pore volume values, indicating that this is the sweet spot for shale gas exploration and development. Pore space in the Shan₂³ sub-member shale is dominated by clay mineral intercrystalline pores (76.9%), which also contains organic matter pores (18.7%). The SEM observations reveal that the mineral components are different in pore size distribution (PSD) and the organic matter pores and calcite dissolved pores are mainly in meso-scale, the pores in clay minerals and quartz are in meso- and macro-scale, while the pores in feldspar and pyrite exhibit a wide and uniform size distribution. The single-factor analysis shows that the clay mineral content is the dominant factor controlling pore development in the Shan₂³ sub-member transitional shales, the TOC content has a certain effect on pore development, and the kerogen type has no discernible effect on pore development.