Research on cap rock, pressure seal and abnormal pressure system

Based on studies of the Mesozoic unconventional tight sandstone gas reservoirs in the western depression of the Sichuan Basin,it was found that sealing capability of caprocks depends on not only micropores,but also fractures and pore fluid pressures.Researches of the Tertiary oil and gas reservoirs in the western depression of the Liaohe Basin proved that undercompacted mudstones worked as effective overpressure sealing for hydrocarbon accumulations.Therefore,caprock seals were divided into 3 types:micropores,micofractures and undercompacted ones with overpressure sealing.A new parameter(the ratio of oil saturation to its corresponding capillary pressure)was used to distinguish microfracture caprocks from micropore ones.Abnormal pressure sealing of pore fluid was usually caused by mudstone undercompaction,which was radically different from normal micropore sealing mechanism of compacted mudstone.The undercompacted mudstone sealing played an important role in shallow-medium buried accumulations,especially in shallow gas accumulations.Development of micropore caprocks might bring about the formation of compartments which took place in medium-deep buried horizons,and affected hydrocarbon migration and accumulation greatly.According to their origin,abnormal pressure systems were divided into 4 types:the sealed(compartments),the expulsion-delayed(overpressure zones related to undercompacted mudstones),the top-sealed and expulsion-delayed,and the invaded.Based on researches of abnormal pressure and overpressure sealing,it was concluded that the optimal targets of exploration were the compartments with huge volume,high pressure coefficient and low pressure gradient,as well as the traps immediately below main overpressure sealing surfaces of undercompacted zones.These concepts are summarized as abnormal pressure sealing controlling hydrocarbon accumulations.