Multi-scale characterization of fault zones in the Groningen gas field: Implications for fault reactivation

The exploitation of georesources and subsurface storage may lead to various hazards including subsidence and induced seismicity. The need for à priori knowledge of fault-rock properties in subsurface exploitation is clearly illustrated by the Groningen gas field, where gas extraction has led to induced seismicity on a complex network of pre-existing faults. Despite the advanced state of knowledge on the Groningen field in terms of depositional facies, host rock composition, and fault distribution at the reservoir level, kinematic definitions and structural observations of these faults are missing at all observational scales. In particular, the internal structure of fault zones and composition of fault rocks require significantly more understanding. In this study, we present an integrated dataset that synthesizes fault gouge (micro-)structural observations and experimental properties from samples of one Groningen reservoir fault, complemented by meso-scale interpretations derived from reflection seismics and well data. We identified two genetically different types of fault rocks at the reservoir level. Quartzofeldspathic gouges with interstitial clays result from cataclastic grain size reduction followed by pressure solution-related grain indentations, which infers time-dependent healing by pressure solution as well as cementation. Clay-supported gouges were formed by lithology mixing and deformation is accommodated by slip on phyllosilicate foliation planes. We infer that fault reactivation in clay-rich fault cores promotes aseismic slip, whereas reactivation of cemented quartzofeldspathic faults may involve cataclasis outside the pre-existing fault core, promoting seismic slip. Our core-scale and microscale observations provide quantitative data on the intrinsic properties of fault rocks and their distribution in fault zones in the Groningen Rotliegend reservoir. Together with the reservoir scale structural fault interpretations and inferences from mechanical experiments, they provide a geological context for the present-day observed induced seismicity.