Risk analysis of natural hydraulic fracturing in an overpressured basin with mud diapirs: A case study from the Yinggehai Basin, south China sea

Abstract The Yinggehai Basin is a strongly overpressured basin without large-scale faults in the Central Depression Zone. The hydraulic seal capacity of the caprocks over geological history has been the key factor controlling the vertical distribution characteristics of oil and gas. Based on leak-off data, rock mechanics tests and measured pore pressure data, this paper analyses the potential of natural hydraulic fracturing with the fault analysis seal technology (FAST) methodology for caprocks C1 (caprock of the Huangliu Formation) and C2 (caprock of the Meishan Formation) of the diapir belt in the north Central Depression Zone and determines the failure mode and gas sealing capacity of the two caprocks. The results show that gas leakage occurred continuously in both C1 and C2 in the diapir zone (DZ) throughout geological time. However, far from the diapir zone (FDZ), hydraulic leakage occurred only in caprock C2 while no leakage occurred in caprock C1 over geological time. These results are consistent with the gas accumulation that occurred in both the Yinggehai Formation and Huangliu Formation in the DZ but only in the Huangliu Formation in the FDZ. A relatively reasonable explanation is that diapirism promoted natural hydraulic fracture development via the following effects: (i) Diapirism transfers deep overpressure to shallow layers and increases the pressure gradient; (ii) The high-angle fractures associated with diapirism reduce the critical stress condition for the occurrence of natural hydraulic fracturing; and (iii) Diapirism results in the formation of dome structures in shallow layers, causing more shallow burial of caprock in the DZ than the FDZ. Shallowly buried caprocks, which have a relatively lower hydraulic fracture pressure, facilitate gas migration. These effects promoted natural hydraulic fracturing in the DZ, which is the main reason for the difference in the vertical distribution of gas reservoirs between the DZ and FDZ.