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 x 10(-3) - 1.31 x 10(-3) mu m(2). 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.