Effects of diagenetic evolution on gas-bearing properties of deep tight sandstone reservoirs: A case study of reservoirs in the 1^st member of the Permian Shanxi Formation in the Qingyang gas field, southwestern Ordos Basin

This study aims to analyze the effects of diagenetic evolution on the gas-bearing properties of reservoirs. Using data from casting thin section observations, scanning electron microscopy (SEM) images, physical property tests, and comprehensive log interpretation, as well as previous research results, we investigate the diagenetic characteristics of deep tight sandstone reservoirs in the 1^st member of the Permian Shanxi Formation (hereafter referred to as the Shan 1 Member) in the Qingyang gas field. Key findings are as follows: (1) The multi-stage thick-bedded sandstones in the middle tributary channels of the Shan 1 Member in the Qingyang gas field exhibit two types of superimposition relationships: continuous and intermittent deposition. The pore assemblages in the sandstone reservoirs of the continuous deposition type are dominated by dissolution pores, intergranular pores and intercrystalline micropores, while those in the sandstone reservoirs of the intermittent deposition type predominantly include dissolution pores and intercrystalline micropores. Compared to the intermittent deposition type, the sandstone reservoirs of the continuous deposition type exhibit lower content of fillings, more favorable physical properties, and enhanced gas-bearing properties; (2) Compaction is identified as the most significant destructive diagenesis that undermines the gas-bearing properties of reservoirs in the Shan 1 Member. The sandstone reservoirs of the continuous and intermittent deposition types exhibit compaction rates of 55.4% and 61.2%, respectively. Dissolution acts as the most significant constructive diagenesis that enhances the gas-bearing properties of these reservoirs. The sandstone reservoirs of the continuous and intermittent deposition types present dissolution rates of 27.9% and 19.7%, respectively. The sandstone reservoirs of the continuous deposition type underwent a diagenetic evolution characterized by moderate compaction, followed by moderate cementation, and then moderate dissolution, indicating weak destructive diagenesis. In contrast, the reservoirs of the intermittent deposition type experienced a diagenetic evolution from moderate to high compaction, to weak to moderate cementation, and finally weak to moderate dissolution, suggesting strong destructive diagenesis; (3) The sandstone reservoirs in the Shan 1 Member experienced reservoir tightening first and then hydrocarbon charging. Due to the influence of diagenetic evolution, the sandstone reservoirs of the continuous deposition type exhibit lower-level tightness, larger-scale natural gas charging, and more favorable gas-bearing properties compared to the intermittent deposition type; (4) The diagenetic intensity of reservoirs in the Shan 1 Member was quantitatively determined using the comprehensive diagenetic index. Regarding the planar distribution, the trunk channels to the west of the Qingyang area primarily consist of sandstones of the intermittent deposition type, with reservoirs exhibiting low comprehensive diagenetic index and poor gas-bearing properties. In contrast, in the trunk channels to the east of the Qingyang area, sandstones of the continuous deposition type are predominant, with reservoirs featuring high comprehensive diagenetic index and favorable gas-bearing properties. Therefore, gas-bearing reservoirs primarily occur to the east of the Qingyang area.