Reservoir forming mechanism of Penglai 9-1 granite buried-hills and its oil geology significance in Bohai Sea

The Penglai 9-1 oilfield is the largest granite buried-hill oilfield in China at present, but the reservoir forming mechanism and its evolution is complex, it causes the reservoir distribution varies in large ranges. Based upon geochemical, geophysical, experimental simulation and other methods, and combined with field geological observation, this paper has discussed the reservoir forming mechanism of Penglai9-1 granite buried-hills and its hydrocarbon accumulation model. The results show that the granite buried-hills intruded along deep faults under North China plate crack background, the intrusion period for the Jurassic 160-170 Ma, and the Middle-Late Mesozoic granite belongs to Circum-Pacific Tectonic Region. In the paper, we propose that reservoir forming are controlled mainly by epigenic karstification and tectonic reconstruction; the weathering and denudation in Jurassic affects the reservoir macroscopic development; the faults and joints formed in Cenozoic strengthen the transformation of granite reservoir. Laterally, there is a positive correlation between the fracture density and granite reservoir thickness; Vertically, the granite rock can be divided into five zones:soil zone, sand weathering zone, broken zone, fracture zone and base-rock zone, The upper-middle parts (sand weathering zone, broken zone, fracture zone)of the granite buried-hill are the high-quality reservoir and the main oil reservoir. Penglai9-1is the near-beded reservoir model, the successful discovery of the oil-field has enriched the oil and gas exploration in the field of volcanic rock, and also have important reference value for other granite buried-hills research and exploration in similar areas.