Influence of Diagenesis on Pore Evolution of Tight Sandstone Gas Reservoir: A Case Study of He1 Reservoir in the J72 Well Area, Dongsheng Gas Field, Ordos Basin

In order to clarify the controlling effect of different types of diagenesis on pore evolution in the J72 well area of Dongsheng gas field, the petrological characteristics, pore types, diagenetic characteristics and their key roles in pore evolution were carefully analyzed based on casting thin section, scanning electron microscope, cathode luminescence, and grain size analysis data. Results show that the lithology of the He 1 reservoir in the J72 well area is mainly medium and coarse-grained lithic sandstone and lithic quartz sandstone. And the pore types are mainly intergranular dissolved pores and mold pores, their proportion is about 68.16%. The diagenetic stage of the reservoir is the middle diagenetic stage A, and the strong compaction is the essential reason for the decrease in the porosity of the reservoir. The cement is mainly filled and cemented by clay minerals and calcite, and the dissolution pores formed by dissolution account for 80.17% of the whole pore space, occupying the dominant position in the pore space. According to the empirical formula, the initial porosity recovered is 32.76%, and the porosity change rates of the reservoirs undergoing compaction, cementation, and dissolution are 47.39%, 39.48%, and 13.03%, respectively. The existence of a large number of intergranular dissolved pores and mold pores is the fundamental reason for the development of high-quality reservoirs in the study area. The absolute error between the results of the quantitative calculation of pore evolution and the actual porosity measurement is 0.39%. There is good consistency between the two. This work can provide an important reference for the study of pore evolution of tight sandstone gas reservoirs with a similar geological background and the prediction of high-quality reservoirs.