Assemblages and patterns of forming oil and gas reservoirs in Lishui Seg of East China Sea Basin

Abnormal formation pressure is closely related to the migration, accumulation and distribution of oil and gas and abnormal pressure phenomenon is very obvious in Lishui Seg. Through analyzing the characteristics of abnormally high formation pressure distribution, the relation between abnormally high formation pressure and reservoir formation is discussed and the assemblages and patterns of reservoir formation related to abnormally high formation pressures in the region are summarized in the paper. In light of the research results it is indicated that the abnormal formation pressure in Lishui Seg belongs to a top-sealed and stagnantly expelling abnormal pressure system; the pressure gradients in the overpressure zone are obviously different from each other; and the overpressure zone may be vertically divided into three abnormal formation pressure zones, i.e. confining-bed overpressure zone, pressure transitional zone and superhigh pressure zone, etc. According to oil and gas distribution and the characteristics of forming oil and gas reservoirs, three reservoir formation patterns may be classified in the region, i.e. reservoir formation in the confining bed, reservoir formation above the confining bed and reservoir formation below the confining bed in the abnormal pressure system.