The Upper Paleozoic in the southwestern Ordos Basin has significant potential for natural gas exploration. This study investigated the diagenetic fluid evolution and hydrocarbon accumulation characteristics of He 8 section from Permian Lower Shihezi formation and Shan 1 section from Shanxi formation tight sandstone reservoirs by petrographic observation, scanning electron microscope imaging, fluid inclusion study, and laser Raman spectrum analysis. The results show that He 8 section and Shan 1 section reservoirs are mainly composed of quartz sandstone, subordinate arkose quartz sandstone, and lithic quartz sandstone, with minor lithic sandstone and lithic arkose sandstone. The major pores are intergranular dissolved pores. The main diagenetic minerals include quartz overgrowth, siliceous cement, carbonate cement, illite, montmorillonite, and mixed-layer clay minerals. The overall diagenetic features show strong compaction, multistage siliceous and calcareous cements, and abundant clay minerals, strong dissolution, and well-developed fractures. Two stages of fluid inclusions developed in the He 8 and Shan 1 sections recorded the migration and accumulation of the early-stage and late-stage natural gas, respectively. The reservoir in the study area experienced early and late diagenetic stages, and its formation was simultaneous with or after its densification. The diagenetic environment changed from alkaline to acidic and again into alkaline. There are two stages of fluid activities in the study area, namely, the early diagenetic stage corresponding to hydrocarbon generation and migration and the late diagenetic stage corresponding to hydrocarbon accumulation. This study suggests that Upper Paleozoic natural gas migrated into the reservoir in Weibei Uplift, Yishan Slope, and Tianhuan Depression tectonic units during 220-197 Ma, and the large-scale migration and accumulation occurred in these tectonic units at different times. No natural gas was generated in the west margin of the basin because the temperatures of the hydrocarbon source rocks in the Upper Paleozoic were below the gas window.

中文翻译：

---

鄂尔多斯盆地西南部上古生界致密砂岩储层成岩流体演化及成藏特征

鄂尔多斯盆地西南部上古生界天然气勘探潜力巨大。本研究通过岩相学观察、扫描电镜成像、流​​体包裹体研究、激光拉曼光谱分析等手段，研究了二叠系下石河子组河8段和山西组致密砂岩油藏山1段成岩流体演化及油气成藏特征。结果表明，河8段和山1段储层以石英砂岩、次生长石石英砂岩和岩屑石英砂岩为主，少量岩屑砂岩和岩屑长石砂岩。主要孔隙为粒间溶孔。主要成岩矿物有石英增生、硅质胶结物、碳酸盐胶结物、伊利石、蒙脱石、和混合层粘土矿物。整体成岩特征表现为压实性强，多级硅质、钙质胶结物，粘土矿物丰富，溶蚀作用强，裂缝发育。河8段和山1段发育的两期流体包裹体分别记录了早期和晚期天然气的运移和聚集。研究区储层经历了早、晚成岩阶段，其形成与致密化同时或之后。成岩环境由碱性变为酸性，再变为碱性。研究区流体活动分为两个阶段，即与生烃运移相对应的早期成岩阶段和与油气聚集相对应的晚期成岩阶段。本研究表明，上古生界天然气在220-197 Ma期间运移到渭北隆起、伊山斜坡和天环坳陷构造单元储层中，并且在这些构造单元中不同时期发生了大规模运移和聚集。由于上古生界烃源岩温度低于气窗，盆地西缘未产天然气。