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Key Factors for Oil Accumulation of Mesozoic Granite Buried Hill in Penglai 9-1 Oil Field, Bohai Bay Basin


Penglai9-1 buried-hill oil field locates at Miao Xi Uplift, and has proved reserves of more than 2×108 m3 with a large oil-bearing area(more than 80 km2) and a high oil column height(maximum height over 230 m). It is so far the single largest and the only Mesozoic granite oil field in Bohai Bay Basin. Restricting by the degree of reservoir development, the Oil-water interface along the buried hill was dome-shaped. Oil accumulation region mainly in the saddle of hill where the granite distributing, while only local hydrocarbon show on both sides of the higher quartz schist hills. Oil and source correlation shows that the lacustrine source rocks in the first and the third member of Shahejie formation in Bozhong and Miaoxi sag around the hill provide hydrocarbon for the large reservoir. A thick layer of weathering crust leaching by atmospheric water is the main reservoir space and conducting layer. Shallow buried depth of weathering body is a key factor for weathering crust to maintain good physical properties. Efficient faults connecting oil source and the good lateral interconnection in thick weathering crust make up the good hydrocarbon migration and accumulation system. Overlying shallow lake mudstones of Guantao Formation forming regional cap rocks, the reason that thin shallow lake mudstones in early diagenesis phase can seal the oil column height over 200 meters is the density difference between the oil in Penglai 9-1 oil field(0.987g/cm3) and water is small. The bigger fault throw of boundary faults separated the granite and the quartz schist on both sides of the hills, and avoided the docking of weathering crust between granite rocks and quartz schist, then prevented the migration of oil to both sides of the quartz schist hills. In addition, the weathering crust of the metamorphic buried hills was thin with a limited distribution for mainly quartz schist constitution has strong resistance to weathering, and is difficult to reserve for steep occurrence, restricting the formation of the horizontal channel layer for oil migration on both sides Metamorphic hills. All of the above may be the main reasons for oil accumulation on the saddle of granite hill, but not on the both higher quartz schist hills.