| This research has shown that the hydraulic conductance of a fault, a quantitative measure of the flow attributes of a fault, is a function of shear-zone permeability, thickness, mudstone fraction and stratigraphic throw. This was accomplished through the characterization of the structure and permeability of shear zones from a fault known to impede fluid-flow within a siliciclastic aquifer in central Texas. The objectives of this research have been to: (1) characterize the structure of 29 shear zones from continuous drill core recovered from the faulted aquifer, (2) characterize the permeability structure of the shear zones, (3) define relationships between the shear-zone structure, stratigraphic-throw, lithology, structural position, and permeability, and (4) infer how the structure and the permeability of the shear zones evolve with displacement.; The types of features identified within the shear zones include: (1) internal zones of slip or zones of concentrated shear (slip surfaces); (2) outer boundaries of the shear zone; (3) internal layers of sandstone and clay-rich mudstone with differing degrees of deformation.; The average shear zone permeability ranges from 0.2–7.9 md which is at least 71 to 2815 times less than the average protolith permeability (563 md). The mechanism responsible for reduction in shear zone permeability is grain size reduction by fracture, and cataclasis to reduce porosity. Post-faulting diagenesis is not a major factor.; Shear zone hydraulic conductance ranges from 0.1384 to .0006 day−1 and the shear zones with the smallest conductance value exhibit the largest head changes across the shear zone.; The shear zone permeability and hydraulic conductance show a dependence on shear-zone stratigraphic throw, thickness and mudstone fraction. However, the hydraulic conductance exhibits a stronger correlation than permeability. The shear-zone mudstone fraction is approximately twice the mudstone fraction of the displaced section for these shear zones. Since thickness correlates directly with displacement, the fluid-flow properties of a fault can be related to two easily determined and measurable values, the displacement of a fault and the mudstone fraction in the shear zone. The fluid flow properties of a fault rely on multiple parameters including shear-zone permeability, thickness, throw, and mudstone fraction. |
