Hydrocarbon migration caused by rupture of pressure compartment in south Liaozhong sag, Bohai Bay Basin, Offshore China

Abstract

Abstract Based on the study of the abnormal pressure of mudstone and reservoirs of south Liaozhong sag, Bohai Bay Basin, the structure of the abnormal pressure compartment in the Paleogene of south Liaozhong sag and its impact on oil and gas migration have been analyzed. The pressure compartment in the study area has a “sandwich structure”, with the mudstone of Paleogene Dongying Formation as cap rock, the reservoirs of the upper Shahejie Formation of the Paleogene as the transport interlayers, and the mudstone from the lower Shahejie Formation as the hydrocarbon source rock. Based on overpressure formation mechanism and basin-modeling, the pre-neotectonic movement pressure of the transport interlayers was reconstructed, then the rupture of the pressure compartment and pressure differentiation caused by Tanlu fault zone during the neo-tectonic movement were investigated. The hydrocarbon migration was simulated under different pressure conditions before and after the neo-tectonic movement, to find out the relationship between the neo-tectonic movement of Tanlu fault, abnormal pressure and oil and gas migration. The simulation results show that the differential activity within Tanlu fault zone caused the differentiation of pressure and increase of fluid potential gradient; and the strike-slip pull-apart area with low potential caused by pump-sucking effect became the main direction of hydrocarbon migration in Tanlu fault zone. The comprehensive research of the late-stage hydrocarbon accumulation shows that, there are inner and outer compartment hydrocarbon accumulation systems in the south Liaodong sag, and the transport interlayer in the inner compartment is the “hydrocarbon transfer station” of the shallow outer compartment, and determines the hydrocarbon plane distribution.