Diagenesis and pore evolution model of tight sandstone gas reservoirs of Benxi Formation in the south-central Ordos Basin, China

Abstract

ABSTRACT In recent years, gas tests in Benxi Formation in the south-central Ordos Basin have shown good oil and gas displays. In 2009, a gas reservoir was encountered in the Benxi Formation during drilling by the Shanxi Yanchang Petroleum Company, and the open flow potential was estimated at over a million cubic meter per day. Thus, studying the reservoirs in this area is of great exploration and guidance significance. In the paper, reservoirs were studied by casting thin sections, scanning electron microscopy, and high-pressure mercury intrusion experiments, and then, a diagenetic evolution sequence was established. Combined with the recovery model of the initial porosity of the sandstone and a quantitative pore analysis technique, the quantitative porosity evolution model of the Benxi Formation reservoir was established. The results show that i) the Benxi Formation mainly includes quartz sandstone, with less lithic quartz sandstone and lithic sandstone; ii) In general, five types of pores are generally developed in the Benxi Formation, namely, residual intergranular pores, intergranular dissolution pores, intercrystalline pores, micropores and microfractures; iii) Reservoir development was mainly affected by sedimentation and diagenesis; iv) At present, the main reservoir has currently reached the end of the mid-diagenesis of the B stage or late diagenesis stage. The average primary porosity of the sandstone was approximately 38.1%. The reservoir underwent a porosity reduction stage due to strong compaction, a porosity reduction stage due to early cementation, a porosity enhancement stage due to dissolution and a porosity reduction stage due to late cementation. The average porosity of the reservoir is now 4.89%.