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

Abstract

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.

Keywords:

• Chang 6
• delta front
• pore evolution
• tight reservoir
• Western Ordos Basin

Acknowledgments

We thank Professor Kenneth A. Eriksson for his guidance and help in the process of writing the paper.

Additional information

Funding

This research was supported by the NSFC (No. 41302076) and Shaanxi Natural Science Foundation (No. 14JS081).