关键词: 延长组, 流体包裹体, 油气充注, 成藏动力, 鄂尔多斯盆地

Abstract: With the deepening of petroleum exploration, new discoveries have been made in tight reservoirs in the eighth member of the Triassic Yanchang Formation (Chang 8 member) in Pengyang area in the southwestern margin of Ordos Basin, indicating a great exploration potential. However, the overall research level is low, such as the extremely complex oil-water contact and the unclear law of hydrocarbon charging. The fluid inclusions provide a basis for analyzing the process of hydrocarbon charging. By using fluid inclusion testing methods such as microscopic observation, homogenization temperature measurement and PVT simulation, as well as the quantitative grain fluorescence (QGF) experimental technology, the process of hydrocarbon accumulation and adjustment in Chang 8 member is discussed and the difference of hydrocarbon accumulation in the interior of the basin and basin margin is analyzed. The study results show that oil and gas inclusions are relatively developed in the study area, the maturity of crude oil is low, the oil display grade and modern/paleo oil saturation are lower than those in the interior of the lake basin, showing an overall low intensity of hydrocarbon charging. The large-scale hydrocarbon charging events mainly occurred at the end of Early Cretaceous and the early stage of Late Cretaceous. The limited pressure difference between source rock and reservoir provided driving force for the short-distance hydrocarbon migration, but it was lower in the study area than that in blocks in the main basin, which led to a low oil saturation in the basin marginal area. The relationship between quantitative grain fluorescence (QGF) and quantitative grain extract fluorescence (QGF-E) indexes shows an inconsistent trend, indicating the change of the paleo oil-water contact or the adjustment of oil reservoir in the study area. The later tectonic movements greatly adjusted/destroyed oil reservoir near the faults in the southwestern basin margin, which further decreased the scale of oil charging in Chang 8 member. The deep research on oil charging and accumulation laws and evolution process of oil reservoir in the basin margin has a great guiding significance for identifying the exploration potential of low-permeability tight oil and gas sandstone reservoir in various tectonic settings.