New Mexico Geological Society Annual Spring Meeting — Abstracts

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. As part of a study of field-scale fault-zone permeability and cementation, we examine the variably cemented Loma Blanca fault, a normal fault in the Rio Grande Rift. We collected electrical resistivity data from 15 parallel two-dimensional transects orthogonally crossing the fault, centered on exposures of the fault at the land surface. Inversions of the resistivity data indicate a low resistivity anomaly in the cemented portions of the fault and within the adjacent footwall; these anomalies are present in the unsaturated zone. This low resistivity signature may be an indication of a higher degree of fluid saturation resulting from greater capillary forces, both in the cemented fault (due to reduced pore sizes within the cemented material) and in the footwall (possibly due to smaller grain size). These mechanisms for generating low resistivity anomalies in both the cemented fault zone and in the footwall, suggest that the low resistivity anomalies likely correspond to regions with low permeability. The ability to characterize spatial variations in the degree of fault zone cementation with resistivity has exciting implications for improving predictive models of the hydrogeologic impacts of cementation within faults.