In the Late Paleozoic, both the southern North China Basin and the Ordos Basin formed part of the North China Craton basin, and Carboniferous-Permian coal measure strata were widely developed. The Taiyuan Formation is not only an important source rock, but also has good conditions for the accumulation of unconventional natural gas such as tight sandstone gas. However, research into the sandstone reservoir is still at an early stage, and a series of key issues, such as sandstone reservoir characteristics, tight genesis and gas charging characteristics, still lack detailed research. By comprehensive utilization of a series of experimental methods including casting thin sections, scanning electron microscopy (SEM), X?ray diffraction, petrophysical property analysis and fluid inclusion observations, this work studies the petrology characteristics, diagenetic features and porosity evolution history under diagenesis of the tight sandstone reservoir of the Taiyuan Formation in the Taikang uplift, southern North China Basin. The tightening time and genesis of the sandstone reservoir are also determined, and the charging process of natural gas in the tight sandstone reservoir is deduced in combination with the burial history and thermal evolution history of the formation, and the coupling relationship between the sandstone reservoir tightness and the gas charging process is finally clarified. The results show that the tight sandstone reservoir of the Taiyuan Formation has undergone a variety of diagenesis, mainly including compaction, cementation and dissolution, and currently entered phase B of the middle diagenetic stage. The pore types are dominated by secondary dissolution pores, intergranular micropores associated with clay minerals and microfractures. Through quantitative analysis of the influence of different diagenesis on porosity, it is concluded that compaction is the most important factor leading to the density of the reservoir which results in the decrease of the original reservoir porosity (38.60%) by 31.01%, and only 7.27% loss of porosity due to the clay minerals, carbonate minerals and siliceous cementation. The feldspar and debris particles have limited ability (only 3.28%) to increase porosity by dissolution. The porosity evolution results show that the Taiyuan Formation sandstone reservoir entered the densification stage in the late stage of middle diagenetic A (Late Triassic). By analyzing the type and composition of fluid inclusions, it is confirmed that the Taiyuan Formation sandstone experienced a single gas charging process. The comparison of homogenization temperature of hydrocarbon-bearing saline inclusions with burial history and thermal history shows that the gas charging time was 229~213 Ma (corresponding to the Late Triassic), which is basically consistent with the time of reservoir densification, indicating that the densification process and the gas accumulation process may have occurred simultaneously.
Key words：
gas charging; tightening genesis; tight sandstone; Taiyuan Formation; Taikang uplift; southern North China Basin
