ABSTRACT

Local groundwater temperatures and bottom-hole temperatures (BHTs) for oil and gas wells present two lines of evidence indicating positive regional geothermal anomalies along the Balcones/Ouachita trend in Central Texas. Analysis of the variables in the heat-flow equation, however, indicates that these anomalies are probably not due to conductive heat flow; most of the rock units for which data exist are limestones and sandstones, and thus, should have high thermal conductivities (and low geothermal gradients). Measurements of heat flow are few along this trend, but because the strata for which BHT data exist generally contain fluids, it is reasonable to assume that hydrodynamics plays a role in creating these apparent thermal anomalies. In short, Darcy's Law, not the heat-flow equation, may control thermal conditions: rocks having high thermal conductivities generally also have high hydraulic conductivities, so that upwelling warm waters may account for the observed thermal anomalies. Since upwelling waters also maybe important conveyors of hydrocarbons, these geothermal/hydrodynamic anomalies also indicate promising areas for petroleum exploration.

Detailed investigations, however, demonstrate that these regional anomalies have high-frequency perturbations; local areas within a regional high may have anomalously low temperatures. Local faulting not discernible on a regional scale may control detailed hydrodynamic conditions, forming structural traps for hydrothermal fluids as well as for hydrocarbons. But they can also localize downwelling recharging waters that impart a low thermal anomaly. The radius of influence within which any well "senses" the ambient thermal regime is probably small within a fault zone, dictated by detailed stratigraphic dislocations. Although complex perturbations affect the prevailing thermal regime in ways not yet completely understood, some of these geothermal anomalies indicate general areas of long-term upwelling from deep within the Gulf Coast Basin. Thermal anomalies may prove to be indicators of economic geothermal resources. They also may indicate hydrodynamic petroleum traps, in which warm waters might have filtered through a trap zone during the process of petroleum accumulation. These thermal anomalies may point toward hydrocarbons in a downstructure direction.