The Effect of Authigenic Clays on Fault Zone Permeability

• Published in: Journal of geophysical research. Solid earth Vol. 126; no. 10
• Main Authors: Farrell, N. J. C., Debenham, N., Wilson, L., Wilson, M. J., Healy, D., King, R. C., Holford, S. P., Taylor, C. W.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-10-2021
• Subjects: Abrasion; Anisotropy; authigenic clay; Authigenic minerals; Capillary pressure; capillary threshold pressure; Carbon dioxide; Carbon dioxide fixation; Chlorite; Clay; Clay minerals; Deformation; Drainage; Energy resources; Energy sources; Fault lines; fault seal; Fault zones; Faults; Flow; Fluid flow; Fluids; Geological faults; Geologists; Geophysics; Grains; Hydrocarbons; Kaolinite; Laboratories; Mathematical models; Membrane permeability; Mercury; Microscopic analysis; Minerals; Morphology; Parameter robustness; Permeability; Radioactive wastes; Renewable energy; Reservoirs; Rock; Rocks; Sandstone; Sustainability; Sustainable development; Sustainable energy; Waste storage
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2021JB022615

Clays are understood to form the majority of fluid‐flow barriers in faulted reservoirs and numerous fault gouge and fault seal studies have quantified the volumes of smeared and abraded clays create fluid‐flow barriers along fault surfaces. However, clay‐related permeability adjacent to the fault surface, including in the fault damage zone, has largely been neglected. Previous studies have shown the morphology and distribution of unfaulted authigenic clays, and not just clay volume, exert a significant control on the magnitude of permeability. However, fault‐related studies have neither characterized deformed authigenic clays nor addressed their influence on fluid‐flow. In this study laboratory permeabilities of faulted, authigenic clay bearing sandstones sampled from the Otway basin (Australia) and the Orcadian basin (UK) present trends which; (a) do not correspond to expected patterns of fluid‐flow in faulted clay‐bearing sandstones and, (b) cannot be explained using published models of permeability related to changing clay volume. Microscopic analysis shows that faulting has disaggregated authigenic clays and, similarly to framework grain deformation, comminuted and sheared clay grains. However, instead of impeding fluid‐flow, analysis of pore networks (using mercury injection porosimetry) showed that faulting of authigenic clays has increased pore connectivity, contributing to increased magnitude of permeability and development of permeability anisotropy. Contrary to published results of faulting and fluid‐flow in impure sandstones, our results show that fault related processes involving the formation of clays in the fault zone can increase permeability and reduce the capillary threshold pressures of fault rocks relative to the unfaulted host rock.
Plain Language Summary
Predictions of fluid leakage in geological reservoirs are essential for energy extraction (e.g., geothermal, hydrocarbons), development and maintenance of sustainable energy resources (e.g., carbon dioxide sequestration and nuclear waste storage). Reservoir‐bounding faults commonly form fluid‐flow barriers as the process of faulting breaks grains and smears clays along fault surfaces creating a seal. Therefore, the volume of clay in faulted rocks is an important parameter in relation to fault sealing capacity. Authigenic clays are minerals that commonly grow in spaces between grains, choking fluid pathways and reducing flow. In this study, microscopic analysis of two faulted sandstones containing authigenic minerals (chlorite and kaolinite) showed that faulting also breaks authigenic clay grains. However, instead of impeding fluid‐flow, laboratory measurements of faulted sandstones showed that deformation of authigenic minerals can influence flow direction and even increase the magnitude of fluid‐flow. Consequently, rather than representing fluid barriers, some faults may actually form important fluid draining structures. These novel results prompt questions about the robustness of parameters currently used to model fluid‐flow around faults and, with global need for sustainable energy demanding that we can safely store fluids in subsurface reservoirs, should motivate geologists to take a fresh look at fault seal analysis.
Key Points
Tectonically deformed authigenic clays have been identified in faulted sandstones
The morphology, aggregate structure and distribution of authigenic clay can contribute to increased permeability in faulted sandstones
In contrast to accepted fault seal models, this study shows that clay‐rich fault zone rocks can be the locus of better fluid‐flow