The evolution of fault zones in basalt: predicting internal structure, petrophysical properties and effect on fluid flow

Interest in the architecture and fluid flow potential of fault zones in crystalline rock has intensified over recent years, due to their importance to the hydrocarbon industry, carbon storage, and radioactive waste storage. Although some 30% of hydrocarbon plays within igneous rocks are contained within basalt, and basalt has been proposed as a host rock for CO2 storage, little work has focused on the evolution of faults in basalt. Constraining the mechanisms of fault growth and evolution allows us to place constraints on the likely across- and along- fault flow properties of the bulk fault zone. Here we present a new detailed field-data derived model for sub-surface fault rock evolution in basalts, based on fieldwork in the North Atlantic Igneous Province (Scotland, the Faroe Islands and Iceland). The faults have offsets from <1m to >100m, and cut basaltic lava flows, and interbeds of volcaniclastic and sedimentary rocks. Faults with offset of <1m are observed to have grown by fracture linkage and propagation, with narrow fracture-bound breccia zones forming in situ. At higher displacements (1-24m), multiple slip surfaces develop and breccia zones break down into micro breccias and cataclasites. Fracturing in the fault walls create coarse protobreccias that are consumed by the fault with increasing growth, leading to a widening of the overall fault zone. Larger displacements (>25m) are accommodated on dominant through-going slip surfaces with the development of foliated cataclasites. Continued breakdown of the wall rock leads to increasing fault zone width and continuing evolution of fault rocks along subsidiary slip surfaces. In parallel with the mechanical breakdown of the host rock, chemical breakdown of the basalt assists in fault rock formation. In the lowest offset faults, groundmass in the wallrock is altered to clays and fracture surfaces are coated with clay. As deformation progresses, feldspar phenocrysts increasingly alter to clays. These clays form the matrix to breccias and cataclasites and concentrate along fractures and minor slip surfaces. This fault evolution model implies that with increasing displacements, the system evolves from a conduit to conduit-barrier system. At lower (0-1m) displacements, the predominance of open fracture development and limited clay formation would result in faults having a higher permeability than the host rock. Clay formation could block original flow pathways (i.e. slip surfaces and fractures), but flow could still occur in higher porosity zones such as unsealed fractures and breccia zones within the fault. At high displacements (>25m), the presence of highly comminuted, clay-rich fault rocks and clay-coated slip surfaces will tend to progressively reduce the across-fault bulk permeability. Progressive brecciation of the wall rocks and continuous production of slip surfaces could potentially provide high permeability pathways through this increasingly complex fault zone and increase along-fault permeability. This detailed work on the mechanisms of fault rock development will ultimately allow us to make predictions of fault zone permeability at depth.