Characteristics and Controlling Factors of Tight Gas Sandstones from the Upper Shanxi and Lower Shihezi Formations in the Northern Sulige Area, Ordos Basin, China

Tight sandstone gas reservoirs with ultralow porosity (<10%) and permeability (<1.0 mD) in the Ordos Basin exhibit enormous resource potential. In this work, the characteristics of high-quality tight sandstone reservoirs in the eighth member of the Shihezi Formation (He 8) and the first member of the Shanxi Formation (Shan 1) of the northern Sulige area are investigated. To study the characteristics and controlling factors of the tight sandstone, various technical methods, including casting thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), mercury injection capillary pressure (MICP), and spontaneous imbibition, are conducted on typical sandstone samples. The results show that the He 8 and Shan 1 sandstones predominantly consist of medium- to fine-grained sandstones with high compositional maturity levels. These sandstone reservoirs have low porosity (mainly <12%) and ultralow permeability (mainly <0.5 mD), with a good correlation between porosity and permeability. Intragranular dissolved pores are predominant in both the He 8 and Shan 1 sandstones. In comparison to the He 8 Member, sandstones from the Shan 1 Member are much tighter, resulting in lower porosity and poor pore connectivity. Sedimentation and diagenesis play crucial roles in the development of high-quality reservoirs in the study area. High-quality reservoirs, mainly developed in the point bar and braided bar, are fine-medium sandstones with good sorting, followed by natural levee. Destructive diagenesis, including compaction and cementation, has significantly reduced pore volumes and reservoir physical properties, while extensive dissolution pores are generated by dissolution to improve reservoir space. Moreover, the presence of chlorite rims with an appropriate content can enhance the physical properties of sandstone reservoirs. Therefore, the findings of this work are beneficial for the predication and identification of high-quality reservoirs in the Ordos Basin, thus providing a foundation for further exploration of favorable reservoirs.