Oil & Gas Geology ›› 2018, Vol. 39 ›› Issue (2): 330-339.doi: 10.11743/ogg20180212

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Factors controlling medium-to-deep coarse siliciclastic reservoirs of high quality at the southeastern margin of Qinnan Sag, Bohai Bay Basin,China

Wang Guanmin1,2, Zhang Jie1, Wang Qingbin3, Li Jiawei1   

  1. 1. School of Geosciences, China University of Petroleum(East China), Qingdao, Shandong 266580, China;
    2. Laboratory of Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao, Shandong 266071, China;
    3. Tianjin Branch of CNOOC Ltd., Tanggu, Tianjin 300457, China
  • Received:2016-03-25 Revised:2018-01-20 Online:2018-04-28 Published:2018-05-16

Abstract: Coarse siliciclastic reservoirs with low porosity and low permeability of fan-delta facies are common in the medium-to-deep first and second members of the Eocene Shahejie Formation (Es1-2) at the southeastern margin of Qinnan Sag in the Bohai Bay Basin,but there are also some zones with high porosity and high permeability in some intervals.The reservoir spaces are largely dissolution pores and biological coelomopores,and the mechanisms of favorable reservoir development are different from that of the common tight coarse siliciclastic reservoirs.So we integrated data from core samples,casting thin sections,burial history,and fluid inclusion to investigate the characteristics and controlling factors of the high-quality coarse siliciclastic reservoirs in the Es1-2.The results indicate that spiral-shell-bearing coarse siliciclastic interbeds are common in the Es1-2 coarse siliciclastic reservoirs of fan-delta facies in the study area and are interpreted as wave-modified,intermittently abandoned underwater distributary channel deposits.Intensive calcite cementation plays a major role in the formation of tight reservoirs.The spiral-shell-bearing coarse siliciclastic rock is the major rock type for the development of high-quality reservoirs.Both lithological factors and diagenetic factors contributed to the development of high-quality reservoirs.The former include particle supporting,good sorting,coelomopores being rich in spiral shell fossils and early cryptocrystalline dolomite cementation,while the latter are mainly poor compaction and infiltration of diagenetic fluids due to particle supporting,strong dissolution of intermediate-acidic volcanic detritus,dolomite cements and feldspars by acidic pore-space waters as well as protection of pores by early charged hydrocarbons.This mechanism of favorable reservoir formation is of great referential significance for petroleum exploration in tight coarse siliciclastic reservoirs formed in brackish settings on steep-slope zones.

Key words: brackish lake, coarse siliciclastic rock, medium-to-deep strata, high-quality reservoir, Qinnan Sag

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