EXPERIMENTAL STUDY ON THE INFLUENCE OF PORE STRUCTURES AND DIAGENESIS ON THE PETROPHYSICAL PROPERTIES OF TIGHT SANDSTONE RESERVOIRS

The microscopic pore structures and their diagenetic evolution process of tight reservoirs have a significant impact on hydrocarbon enrichment. In this paper, the micro-geological properties of tight sandstone reservoirs in the Yanchang Formation in the northwestern Ordos Basin were systematically studied, using a large number of cores, thin sections, particle size tests, petrophysical properties tests, and mercury intrusion experiments. The research results show that the lithology of the target layer is mainly medium-fine sandstone and siltstone. The particle size of sandstone is between 0.04-0.24 mm, and the content of quartz particles is between 25% and 75%, with an average of 50%. The main types of feldspar minerals arc potash feldspar and albite, with a content ranging from 25% to 75%, with an average of 38%. The particle size probability curve shows that the curves of most samples show jumping and suspension characteristics. The particle size probability test also shows that the total suspended content of the Chang 8 Member from low to top gradually decreases, and the overall jump gradually increases, so the overall hydrodynamic conditions gradually weaken. Block bedding, parallel bedding, horizontal bedding, and cross bedding are often observed in the tight sandstone of Chang 8 Member. The main diagenesis of the Chang 8 Member are: compaction-pressure solution, cementation, dissolution, and elastic clayization. According to the mercury intrusion experiment, the tight sandstone reservoir types of the Yanchang Formation were divided into three types. Different types of sedimentary microfacies have different pore structures, which in turn affect the petrophysical properties of the reservoir. The comparison results show that the pore structures and reservoir petrophysical properties of the underwater distributary channel microfacies are the best, followed by the estuary bar microfacies and the sheet sand microfacies. Early compaction and cementation have a destructive effect on the petrophysical properties of the target layer, and the dissolution has significantly improved the petrophysical properties of the reservoir. The dissolution rate is mainly distributed in 2%-7%.