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Fault Zone Development, Architecture and Fluid Migration Properties in Heterogeneous Clastic & Carbonate Strata

Abstract

Faults & fault zones control many processes within the brittle upper crust & their individual properties contribute towards the fluid transport capabilities of their protoliths. Outcrop studies exploring faults show that the mechanical properties of protoliths are strong controls on resultant fault architecture. Despite a large volume of work, many features of fault structure & along-strike fault heterogeneity are poorly constrained. In particular, the 3D properties of faults & fault zones within cyclical, mixed clastic & carbonate strata at or below the seismic scale are currently poorly understood, despite being vital considerations for trap identification, exploration risk & reservoir quality. We present observations of outcrop-scale fault zones within the Carboniferous deltaic successions of the Midland Valley, Scotland, UK. The dataset consists of detailed, 3D fault observations collected using photogrammetrically generated models, surveyed coal surfaces, LiDAR, geophysical wireline logs & field observations. Using this data, we examine how rheological differences within the host rock stratigraphy impact fault zone development & properties such as the fault core & damage-zone distribution. The complexity of faults & fault zones shows a positive correlation with the volume of interbedded shales in a faulted succession. Heterogeneous mechanical stratigraphy has a strong control on early fault growth, generating multiple low displacement fault surfaces that combine displacement via relay zones. As growth increases, such zones are breached, & displacement is taken up on single, high-displacement slip surfaces. Breached & linked relays form large lenses, generating anomalously large fault core-width to displacement ratios compared to exclusively clastic successions. Conversely, because movement is focused on multiple slip surfaces during early fault growth, brittle structures like fractures, joints & deformation bands, often found in clastic damage zones, are limited & faults contain anomalously low damage zone to fault-displacement ratios. We use these observations to model how observed differences in damage zone & fault core width, linkage & 3D connectivity that are sub-seismic scale influence structural trapping & intra-reservoir flow within the Carboniferous strata of the southern North Sea. Initial results show large variability & anisotropy in hydraulic-connectivity that contradict the sealing capability calculated using shale gauge ratio.