Geologic and hydrologic constraints on fluid and heat flow in overpressured rocks of the Rio Grande Embayment, Gulf of Mexico Basin

Abstract

Fluids are episodically expulsed from extremely overpressured sediments during natural hydrofracturing events and discharge vertically along regional fault zones in the Rio Grande Embayment. The basal 9 km (29,500 ft) of the stratigraphic section is extremely overpressured with fluid pressures at 80 to 90% of the overburden pressure, close to the minimum pressure needed for the onset of hydraulic fracturing. Small increases in fluid pressure in this extremely overpressured regime trigger hydraulic fracturing and episodes of fluid discharge. The most recent pulse of fluid discharge was along the Wilcox Fault Zone where the largest thermal anomaly in the Gulf of Mexico Basin occurs coincident with a positive fluid pressure anomaly and salinity inversions. Heat conduction is not a viable mechanism for producing the anomaly. Heterogeneous salinity distributions along the Wilcox, Vicksburg, and Frio Fault Zones represent the cumulative effect of expulsion events in the fault zones. Neither distinct thermal nor pressure anomalies are evident in the Vicksburg or Frio Fault Zones, suggesting that fluid expulsion events had a shallower fluid source, were of smaller magnitude, were more diffuse than in the Wilcox Fault Zone, or that similar thermal and pressure anomalies have already dissipated. The thermal conductivity of 83 Wilcox and Frio sandstones from the Embayment were measured and conductivity was correlated to petrographic variables. Thermal conductivity ranges from 2.06 to 5.73 W/m/K over a porosity range of 2.4 to 29.6 %. For a given porosity, because of a higher quartz content, Wilcox sandstones are more conductive than Frio sandstones. Thermal conductivities of clean (< 25% clay) sandstones can be described by a multilinear function of decreasing thermal conductivity with increasing porosity and increasing thermal conductivity with quartz content. Radiogenic heat production within the sedimentary section of the Embayment is a significant source of heat, contributing up to 26% of the overall surface heat-flow density in the Embayment. Heat production rates range from a low of 0.07 ± 0.01 μW/m3 in clean Stuart City limestones to 2.21 ± .24 μW/m3 in Frio mudrocks. Mean heat production rates for Wilcox sandstones, Frio sandstones, Wilcox mudrocks, and Frio mudrocks are 0.88, 1.19, 1.50, 1.72, and μW/m3, respectively. In general, the mudrocks produce about 30-40% more heat than stratigraphically equivalent sandstones. Frio rocks produce about 15% more heat than Wilcox rocks per unit volume of clastic rock (sandstone/mudrock).