Characteristics and genetic mechanism of large granitic buried-hill reservoir, a case study from PengLai oil field of Bohai Bay Basin, north China

Abstract In recent years, a major breakthrough has been made in granite hydrocarbon exploration in the Bohai Bay Basin. With the proven reserves of more than 2 × 108 t, the Penglai (PL) oilfield is the largest granite oilfield discovered in China. Although the reservoir scale is large, the reservoir heterogeneity is also strong, which brings challenges to its exploration and evaluation. Based on the data of cores, thin sections, whole rock and physical property, the reservoir characteristics and control factors of reservoir-forming in the study area are discussed through the fault analysis and the recovery of tectonic uplift history. The results show that the lithology of PL structure are mainly Mesozoic intrusive granite and Proterozoic mica quartz schist, which develops reservoirs related to the weathered crust and internal fractures. Due to the influence of weathering intensity and fault activity, the porous reservoirs in weathered glutenite zone, pore-fractured reservoirs in weathered fracture zone and fractured reservoirs in the inner fracture zone were distributed orderly from top to bottom vertically. The scale of the high quality reservoir on the plane is controlled by lithology, faults, weathering and buried depth. Granite is the main reservoir-forming lithology for more brittle and less resistant to weathering than mica quartz schist, which forms a special phenomenon that the granite in saddle is rich for hydrocarbon while the mica quartz schist in high position is poor for hydrocarbon accumulation. The northwest Miaoxi fault is the main controlling-fault in the study area, and there is an obvious negative correlation between reservoir thickness and distance from this fault. The long-term uplift of Cenozoic provides conditions for strong weathering, while the shallow burial in the late stage provides effective protection for weathered crust reservoirs. The coupling of the above conditions is the main reason of large scale reservoir development in the study area, and among them, the long-term uplift is the key factor for reservoir-forming.