Controlling factor analysis and prediction of the quality of tight sandstone reservoirs: A case study of the He8 Member in the eastern Sulige Gas Field, Ordos Basin, China

The He8 Member is the main producing zone in the Permian rocks of the eastern Sulige Gas Field in the Ordos Basin. It is characterized by low(10%≤Φ<15%) to ultralow(<10%) porosity and ultralow permeability(K < 1mD). To realize efficient exploration and development in this gas field, it is necessary to study the genesis of tight sandstone reservoirs and clarify the factors controlling reservoir development, in order to establish effective methods for reservoir quality prediction. The composition, pore structure, diagenetic type and strength of sandstone reservoirs, as well as their authigenic mineral contents and types, were analyzed. The techniques including casting thin sections, SEM, X-ray diffraction, high-pressure mercury intrusion, porosity-permeability tests and fluid inclusion homogenization temperature tests were used, in order to reconstruct the diagenetic and pore evolution history of the sandstone reservoirs. The effects of 12 sedimentary and diagenetic factors on porosity and permeability were analyzed using the stepwise multiple linear regression method, and a prediction model of reservoir quality (including both porosity and permeability) was constructed. The results show that the reservoir rock is dominated by litharenite, which accounts for 69.76%, followed by sublitharenite, which accounts for 26.15%. The reservoirs are characterized by low to ultralow porosity (porosity ranges from 0.29 to 16.6%, with an average of 7.51%), ultralow permeability (horizontal permeability ranges from 0.0065 mD to 5.21 mD, with most values being less than 0.4 mD). Displacement pressure is high (which ranges from 0.48 to 4.2 MPa, with an average of 1.48 MPa). Sandstone reservoirs are reformed by moderate compaction, moderate to strong cementation and weak dissolution. Cementation mainly occurs as quartz overgrowths, authigenic quartz, ferrocalcite, nonferrous calcite and clay minerals. Clay minerals are dominated by grain-coating chlorite, kaolinite and illite. Dissolution has little influence on reservoir quality. Reservoir porosity is mainly controlled by the contents of kaolinite, siliceous cement, illite, chlorite, and feldspar, as well as the degree of compaction. On the other hand, reservoir permeability is mainly controlled by the amount of siliceous cement, quartz, illite, and kaolinite, as well as the degree of compaction. The porosity and permeability were predicted using the equation established by multiple linear stepwise regression analysis, and good results were achieved. In this paper, the factors that influence the quality of the sandstone reservoir of the He8 Member in the eastern Sulige Gas Field were clarified, and a corresponding prediction model of reservoir quality was constructed. This model provides a new concept and method for quantitatively predicting the quality of tight sandstone reservoirs in other areas.