Quantitative evolution of pores in tight sandstone reservoirs: a case study of late Triassic Chang 6 member, Western Ordos Basin, China.

In order to understand pore evolution in relation to sedimentary facies, a study has been conducted on the Chang 6 reservoir in Western Ordos Basin. Tight reservoir space comprises primary intergranular pores with an average original porosity is 36.29%, and secondary pores related to feldspar dissolution. Reservoir porosity is 8.03–11.43% and permeability is 0.12 × 10−3–1.31 × 10−3 μm2. Cementation and mechanical compaction are the main reasons for the quality of the reservoir. Dissolution of feldspar grains improved the reservoir quality. The reservoir has undergone four evolutionary stages: ① Syn-sedimentary compaction which decreased the porosity by 21.27%, to an average remaining porosity was 15.12%. ② Early diagenetic cementation which decreased the porosity by 6.23% to an average residual porosity was 8.89%. ③ Early stage of mesodiagenesis: involving dissolution and formation of micro-fractures that led to an increase in porosity by 5.47% resulting in an average porosity of 14.36%. ④ Compaction and cementation during the late mesodiagenetic stage resulting in an average porosity of 10.87%. Quantitative calculations reveal an average reservoir porosity is 10.68% with an error of 1.75%. The results contribute to a better understanding of the main controlling factors and pore evolution characteristics of tight reservoir development. [ABSTRACT FROM AUTHOR]