Microstructural Characterization of Fault Rocks from the Groningen Gas Field

Abstract

Human activities in the subsurface such as geothermal energy production, CO2- and hydrogen storage, and gas extraction can affect the regional stress field and lead to induced seismicity. Gas production from the Groningen gas field in the northeast of the Netherlands has led to more than 300 shallow earthquakes with local magnitudes ML > 1.5 and up to a maximum magnitude of ML 3.6, resulting in substantial damage to buildings. Recent earthquake localization studies show that seismicity dominantly occurs on complex normal fault systems, at the depth of the Permian (Rotliegend) reservoir. These faults were formed during multiple tectonic phases from the Late Paleozoic to Early Cenozoic and may comprise breccias, cataclasites, fault gouges and clay smears. The fault strength and slip behaviour are controlled by its composition and microstructural state (porosity, grain size and shape, and presence of foliation within the fault core). Fluid-rock interactions and diagenetic processes during and after fault activity may have altered these characteristics and, hence, the strength and slip behaviour of the fault. Knowledge on the state and composition is thus required to reliably predict the maximum stress drop and seismic energy release upon fault reactivation. However, such knowledge is still lacking at present day. With this study, we aim at characterizing the microstructures of fault gouges in the Groningen faults. We assess the mineralogy, porosity, and grain size distribution of natural samples from faulted core samples derived from the Groningen gas field. Well-log data is presented to show the representativeness of these samples in the larger context of the gas field. The observations on natural microstructures are then used to define simplified geometrical representations or scenarios that can be used as input for microphysical models. Microstructural characterization involves optical microscopy for quantitative petrography of both bulk rock and selected regions of interest (ROI) within the fault zone. Scanning Electron Microscopy (SEM) with Backscattered Electron (BSE), Cathodoluminescence (CL), and Energy-Dispersive X-ray Spectroscopy (EDX) is employed to analyse porosity, grain size, shape, and mineralogy of faulted regions. Preliminary results show that the compositions of fault rocks differ from the host rock and that along-fault variability in mineralogy, cementation, and grain size are important to consider. We distinguish between four main types of fault gouges in the Groningen Rotliegend, based on their microstructural characteristics: (1) gouges consisting of quartz and feldspar grains embedded in a very fine clay matrix, (2) very fine-grained quartz-rich gouges, (3) quartz-cemented gouges, and (4) anhydrite-cemented gouges. We expect that induced fault movement in the first two gouges occurs by reactivation of the earlier produced fault gouges. Since quartz and anhydrite cementation is concentrated in the faults, reactivation of the latter two presumably occurs by cataclastic processes and gouge formation from the adjacent bulk rock rather than the cemented gouge. This suggests that a well constrained fault diagenetic history is required to infer which components of the fault material governs its frictional behaviour and hence the related seismic hazards.