ABSTRACT

Fluid pressure, temperature, and formation-water salinity mapped in the Rio Grande Embayment are consistent with the conceptual model of episodic fluid expulsion from extremely overpressured sediments along regional growth-fault zones. A large geothermal anomaly exists in an arcuate trend coincident with the Wilcox Fault Zone. Heat conduction and production processes do not appear to be viable mechanisms for producing the anomaly. The anomaly is centered on the most basinward and deepest-seated of the Wilcox faults. The geometry of the Wilcox faults in the Rio Grande Embayment results in a well-defined flow path for more efficient heat advection relative to Wilcox faults in the Houston Embayment to the north and the basinward Frio faults where there is no thermal anomaly. Data indicate relatively high fluid pressures above the thermal anomaly, consistent with the model of recent fluid upwelling along the faults creating a pressure high. Salinity inversions (high over low) occur above the top of fluid overpressure (0.465 of the lithostatic gradient) along both the Wilcox and Frio Fault Zones. This is consistent with the model of fluids "burping" up out of the overpressured sediments along both of the fault zones. Fluid pressures in the area approach "extreme overpressures," (defined as pressures near the limiting pressure gradient for the area of about 0.8-0.9 of the lithostatic pressure) at depths of 3-4 km resulting in 6 to 9 km of extremely overpressured sediments. This high fluid pressure results in a low differential stress required for the initiation of hydraulic fracturing. Hydraulic fracturing in the prevailing extensional stress regime produces vertical fractures oriented parallel to regional strike of the faults which are presumed to be the conduits for flow.