Study on the characteristics and influencing factors of Chang 7 ultralow-porosity and low-permeability reservoirs in the Heshui area, Ordos Basin

Abstract Recently, the exploration and development of the Chang 7 shale oil reserves and production have increased in the Ordos Basin. However, the characteristics of the Chang 7 reservoir vary greatly in different areas, which affect the exploration and development of shale oil. We used various analytical methods such as core observation, casting section, high-pressure mercury injection, nuclear magnetic resonance, X-ray diffraction (XRD), and logging interpretation to study characteristics of the reservoir in the Heshui area. The lithology of the Chang 7 member is mainly feldspathic quartz sandstone suggesting that the content of quartz is higher than that of feldspar, and it has relatively low carbonates. In addition, the kinds of feldspar are mainly plagioclase and potassium feldspar, and the concentration of clay minerals is 80%, mainly comprising illite and chlorite. The reservoir of the Chang 7 member is chiefly comprised of submicron pores, such as feldspar dissolved pores, intergranular pores, dissolved pores, microfractures, and intergranular pores. The porosity ranges from 6% to 12%, whereas permeability is less than 0.2 × 10−3 μm2. The Chang 7 reservoir has a strong heterogeneity. Specifically, the heterogeneity of Chang 71 is weaker than that of Chang 72. There is a complex diagenesis such as compaction, dissolution, and cementation, and the compaction and cementation are relatively more and the dissolution is dominated by constructive diagenesis of feldspar dissolution. Sedimentary microfacies are one of the main factors controlling reservoir development. The physical properties of the reservoir in the branch channel are better than the edges of the branch channel and the lacustrine. The diagenesis affects the development and distribution of good reservoirs, and destructive diagenesis such as compaction and cementation reduces porosity and permeability. Conversely, dissolution increases the physical properties of the reservoir. Tectonics has no obvious effects on reservoir but plays a positive role in the migration and accumulation of hydrocarbon when microfractures developed.