Genetic mechanism of the granite buried-hill reservoir of the Penglai 9-1 oilfield in Bohai Sea

The Penglai 9-1 oilfield is the largest granite buried-hill oilfield in China presently, genesis and evolution of the granite buried-hill reservoir is complex. Based on geochemical, geophysical, experimental simulation and other methods, and combined with field geological observation, genetic mechanism of the granite buried-hill reservoir of the Penglai 9-1 oilfield and its hydrocarbon accumulation mode were well investigated. Results showed that the granite was formed by magmatic intrusion along deep faults under intraplate breakup of North China Plate, it was the product of magmatic activities of Yanshan tectonic episode of Circum-Pacific Tectonic Region, and the intrusion time was 160-170 Ma of the Jurassic. Formation of the granite buried-hill reservoir was mainly controlled by the epigenic karstification and tectonic reconstruction, the Yanshanian weathering and denudation controlled macroscopic development characteristics of the granite buried-hill reservoir, and faults and joints formed by Cenozoic tectonic movement promoted modification of the granite buried-hill reservoir. Laterally, thickness of the granite buried-hill reservoir had a positive correlation with fracture density. Vertically, the granite rocks could be divided into five zones: soil zone, sandy zone, broken zone, fracture zone and base rock zone. The upper-middle part (sandy zone, broken zone, fracture zone) of the granite buried hill was the high-quality favorable reservoir zone and the main oil-bearing interval.