Diagenesis Evolution and Pore Types in Tight Sandstone of Shanxi Formation Reservoir in Hangjinqi Area, Ordos Basin, Northern China

Abstract

Diagenesis and pore evolution of tight sandstone reservoir is one of the most important issues surrounding clastic reservoirs. The tight sandstone of the Shanxi Formation is an important oil and gas producing layer of the Upper Paleozoic in Ordos Basin, and its densification process has an important impact on reservoir quality. This study determined the physical properties and diagenetic evolution of Shanxi Formation sandstones and quantitatively calculated the pore loss in the diagenetic process. Microscopic identification, cathodoluminescence, and a scanning electron microscope were used identify diagenesis, and the diagenesis evolution process was clarified along with inclusion analysis. In addition, reservoir quality was determined based on the identification of pore types and physical porosity. Results show that rock types are mainly sublitharenite and litharenite. The reservoir has numerous secondary pores after experiencing compaction, cementation, and dissolution. We obtained insight into the relationship between homogenous temperature and two hydrocarbon charges. The results indicated that there were two hydrocarbon charges in the Late Triassic–Early Jurassic (70–90 °C) and Middle Jurassic–Early Cretaceous (110–130 °C) before reservoir densification. The quantitative calculation of pore loss shows that the average apparent compaction, cementation, and dissolution rates are 67.36%, 22.24%, and 80.76%, respectively. Compaction directly affected the reservoir tightness, and intense dissolution was beneficial to improve the physical properties of the reservoir.