WELL-TESTING INVESTIGATION OF CEMENTED-FAULT CONTROLS ON SUBSURFACE FLUID FLOW

Geologic faults can impact subsurface fluid flow by acting as conduits, barriers, or combined conduit-barrier systems. The ability of a fault to connect or compartmentalize flow between different geologic formations is important for groundwater, petroleum, and solute transport processes. Conventional barrier fault representations (e.g. image well theory) preclude fault dip angle, fault thickness, and permeability. Traditional means of evaluating fault sealing potential also neglect cement as a limitation on permeability. We have completed an initial hydrogeologic field study at the north-striking Loma Blanca Fault in central New Mexico’s Rio Grande Rift. A key objective is to evaluate how pumping-induced drawdown and flow near a complex barrier-fault differs from simplified, traditional flow barrier representations. The Loma Blanca is an east-dipping normal barrier fault discontinuously cemented with calcium carbonate, which has much lower permeability than the surrounding unlithified host sediments. Three constant-rate cross-fault pumping tests performed in the local unconfined aquifer were integrated with concurrent geological and geophysical studies to develop a more robust conceptual model of the site. Collaborative electrical resistivity surveys and lithologic sampling revealed that the fault cement terminates in the northern area of the study along with an abrupt north-south lateral change in lithofacies, creating a permeability (k) discontinuity between the north and south areas. The hypothesis for this transition is a previous fluvial erosional/depositional event from a neighboring ephemeral stream. The combination of the discontinuous fault cement and k discontinuity manifest in well test results with competing flow-boundary effects. Test interpretations reveal that cross-fault flow is limited by the cement, while the k discontinuity creates pseudo barrier or pseudo constant head effects depending on the pumping location.