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Characteristics of formation pressure and their relationship with hydrocarbon distribution in Pinghu tectonic belt of Xihu sag,East China Sea

LIU Jin-shui;Shanghai Branch of CNOOC (China)Ltd.;  
The abnormal pressure develops commonly in Paleogene Pinghu Formation of the Pinghu tectonic belt,East China Sea shelf basin.This paper discusses the characteristics of the abnormal high pressure developing in strata and their relationship with the distribution of oil and gas in order to provide geological bases for oil and gas exploration in the study area.Based on the acoustic travel time data and combined with the logging,geological,geochemical and formation test data,the paper uses the Bowers method to calculate the pore fluid pressure of the single well and restores the single well pressure profile of the Pinghu tectonic belt.The measured formation pressure is used to analyses the characteristics of the distribution of formation pressure.The result shows the abnormal high pressurein this tectonic belt is mainly distributed over the deeper buried Pinghu Formation,with the increase of depth,the overpressure increases as ladder-shape;The burial depths of the reservoirs with abnormal high pressure in different tectonic units of the Pinghu tectonic belt are obviously different.The burial depth of the reservoirs with abnormal high pressure in the Pingzhong area is 3.3km,and the that of the reservoirs with abnormal high pressure in the Pingbei area is 3.7km,indicating that the burial depths of the reservoirs with abnormal high pressure,from the Pingzhong area to the Pingbei area will become deeper and deeper.The vertical superimposed pressure field determines the distribution of oil and gas.The coupling relationship of the overpressure and the distribution of hydrocarbon zones can be divided into four types:the double seal type with high pressure,the double seal type with transitional pressure,the single seal type with transitional pressure,and the single seal type with normal pressure.The double seal type with transitional pressure and the single seal type with normal pressure are more common in the study area.The top of the zone of the single seal type with normal pressure,the top of the zone of the double seal type with transitional pressure and the top of the zone of the double seal type with high pressure are the favorable accumulation areas of oil and gas.
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