Interaction between diagenesis and faulting in sandstone reservoirs: Inferences from the Groningen gas field

Abstract

We investigated the progressive diagenetic evolution of fault zones in the Rotliegend reservoir of the Groningen gas field (NE Netherlands) through microstructural and petrological analysis of cataclastic bands and the only retrieved fault gouge from the reservoir. Our observations show that authigenic quartz, ferroan dolomite, siderite and anhydrite are spatially associated with the presence of faults in the reservoir. The uneven distribution of syntaxial quartz overgrowths in the host rock suggests that silica was readily available in the reservoir, but that quartz precipitation was inhibited by coatings and slow-growing euhedral faces. Quartz cementation was preferentially concentrated in cataclastic bands, where fresh fracture surfaces provided favourable nucleation sites. Anhydrite and siderite cements are concentrated in dilatant fractures in cataclastic bands or in the host rock adjacent to it. Based on their association with dilatant fractures and the required sources for sulphates and reducing fluids, respectively, we interpret that they were introduced by extrinsic, fault associated fluids, during a late stage of the diagenetic history. The preferential quartz cementation in cataclastic zones suggests that many Groningen faults experienced cementation-related strengthening, while this process was inhibited in the clay-rich fault cores as observed in the BIR-01 well, implying that fault reactivation is most likely to occur within the weak fault core.