The Effects of Fault-Zone Cementation on Groundwater Flow at the Field Scale

Fault-zones are an important control on fluid flow, affecting groundwater supply, hydrocarbon/contaminant migration, and waste/carbon storage. However, most current models of fault seal do not consider fault-zone cementation despite the recognition that it is common and can dramatically reduce permeability. We study field-scale fault-zone permeability and cementation by examining the variably cemented Loma Blanca fault, a normal fault in the Rio Grande Rift. We collected electrical resistivity data from transects crossing the fault, developed a geologic model constrained by borehole/outcrop samples and the resistivity transects, and conducted a series of pumping tests in groundwater wells installed within tens of meters of the cemented fault. Inversions of the resistivity data, ground-truthed by geological data, indicate a low resistivity anomaly in the cemented portions of the fault and within the adjacent footwall in the unsaturated zone. This low resistivity may be an indication of a higher degree of fluid saturation resulting from greater capillary forces in the cemented fault. This mechanism for generating low resistivity anomalies in the cemented fault zone suggests that the anomalies likely correspond to regions with low permeability. In addition, borehole samples and inversions of the resistivity data delineate an erosional surface which truncates the cemented fault zone and laterally separates hydrostratigraphic units with an approximately 10X difference in permeability. Constant rate pumping tests demonstrate that the cemented fault acts as a barrier to groundwater flow. The cemented fault is not an ideal barrier to flow (i.e. drawdown deviates from an image well solution), as there is likely some fluid flow laterally around the cemented section and/or cross-fault fluid flow. The strong lateral permeability heterogeneity across the erosional surface imposed an additional control on the local flow field. This permeability discontinuity acts as either pseudo-barrier or pseudo-constant head boundary, depending on the location of pumping.