Analysis on the different sealing conditions required by mudstone caprock at different fault evolution stages:A case study on the 1st member of Qingshankou Formation in the Sanzhao Depression and 2nd member of Dongying Formation in the structure No.5 of the Nanpu Sag

Abstract

To decipher the hydrocarbon distribution pattern of oil bearing basins with hydrocarbon generating in the deeper formations and then migrating along faults upwards to the shallower reservoirs, we have studied hydrocarbon migration characteristics along faults and mudstone caprock sealing conditions required for accumulating hydrocarbon. Fault characteristics at different evolution stages were studied and corresponding pressure conditions were depicted. The results showed that to seal hydrocarbons migrating along faults, different conditions were required at various stages. The hydrocarbon migration along active faults was driven by both buoyancy and formation overpressure. While the carrier system is associated cracks and induced fractures, the corresponding porosity and permeability were relatively good. In this case, the hydrocarbon migration resistance crossing through the mudstone caprock was lower. Therefore, only when faults above the caprock are disconnected with those below, can the caprock seal the migrating hydrocarbon. In contrast, the hydrocarbon migration along static faults was driven only by buoyancy, while the carrier system was of fault rock pores. In this case, the corresponding porosity is relatively poor with higher migration resistance for the hydrocarbon to break through the mudstone caprock. Therefore, to seal hydrocarbon migrating along faults, the displacement pressure of fault rock should be equal to or greater than the reservoir overpressure.