Geothermal Gradient Map of Texas (and Generalized Tectonic Features)

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1984

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Abstract

Geothermal gradient contours are commonly used to show presumed variations in the earth's internal heat. Many researchers apply the heat-flow equation literally (see inset table 1), assuming that geothermal gradients change as a function of heat flow. But the underlying premise of the heat-flow equation is that radiogenic heat flows from high to low-potential areas by means of solid-state conduction. Rarely do such conditions occur in nature owing to the interaction of hydrodynamics. Moreover, thermal conductivity is a factor in the heat flow equation. Thermal conductivity varies inversely with geothermal gradient. Clearly, for any correlation between geothermal gradient and heat flow to be meaningful, one must allow for the thermal conductivity of rock in which the geothermal gradient is measured. It is also important for users of this map to be aware that many of the apparent areal perturbations of geothermal gradient may be due to subsurface water flow; thus, Darcy's Law, as discussed below and as shown in table 1, is important.

Within these constraints, this map shows geothermal gradients contoured for a few specific geologic horizons across Texas. Uniformity of rock type was the main criterion used in choosing the horizons presented here. This ensures a more or less constant thermal conductivity, thus avoiding perturbations of geothermal gradients owing to areal variations in thermal conductivities.

It would have been desirable to depict statewide geothermal gradients as a single set of contours representing a single rock type and, preferably, a single rock unit. The best possible situation would be a statewide sampling of, for example, granitic basement, because a crystalline basement complex would yield gradient values that (theoretically) indicate thermal conditions in the crust. In fact, this ideal case (readings confined to granitic rock) cannot be attained because of (1) geologic realities—granitic basement probably does not occur throughout Texas; and (2) data constraints—the paucity of wells penetrating basement across the state. Given these geologic and data constraints, we collected readings on bottom-hole temperature (BHT) and depth for a uniform rock type within as few geologic units as possible but allowing for adequate well control over a broad area. Sedimentary carbonate rocks (limestone and dolomite units) are the best compromise on the basis of these criteria. Carbonate rock units are generally thick and widespread and have little internal lithic variation. Moreover, many limestone and dolomite units are petroleum reservoirs, and thus there is widespread electric log control for use in computing geothermal gradients.

Four carbonate rock units provide data for contouring geothermal gradients for most of Texas inland of the Stuart City/Sligo Reef Trend (Lower Cretaceous Shelf Edge). These include three Mesozoic formations beneath the inner Gulf Coastal Plain: the Jurassic Smackover Formation and the Sligo/Pettet and Edwards Formations, both of Cretaceous age. For the remainder of the state west of the Balcones/Ouachita Trend, we employed data mostly from the Ellenburger Group of Ordovician age. Coastward of the Lower Cretaceous Shelf Edge, however, fundamental geologic changes occur. There, few wells are completed in carbonate rocks owing to radical facies changes and excessive depths to correlative Mesozoic units. Most of the BHT/depth (bottom-hole temperature) data exist for Tertiary and Quaternary clastic rock units, and within these units, there is no assurance of lateral or vertical lithic continuity. Hence, for this region (the Tertiary Gulf Coast Basin), we present a separate set of contours based on a moving average of the gradient data derived from the 1:1,000,000-scale version of the Geothermal Gradient Map of North America (American Association of Petroleum Geologists and US Geological Survey, 1976). In this way, we were able to smooth contours that otherwise would suggest geothermal anomalies but that may in fact merely be due to penetration of sandstone and mudstone in adjacent wells. In short, five sets of data are contoured on this map: the moving-average contours for the Tertiary Gulf Coast Basin and separate contours for each of the four discrete geologic units. The carbonate rock units are depicted as separate sets of contours so that each set will be internally consistent while still being broadly comparable to one another. The local overlapping of contours for different units indicates local, apparent geothermal perturbations that probably result from hydrodynamic conditions within the rock unit and not from variations in heat flow. This mosaic of contours thus allows better resolution of certain controls on local anomalies and allows direct comparison of geothermal gradients to subsurface structures mapped on key horizons (namely the Ellenburger and the Edwards, compare this map to that by Sellards and Hendricks, 1948; also an updated depiction of statewide structures is forthcoming [Ewing and others, in progress]). However, the segregation of the gradient map by rock unit promotes a fragmented view of statewide geothermal trends: because of uneven well control there are significant blank areas on this map; compare this depiction to the inset figure 1. Gaps in contouring occur along the Balcones/Ouachita Trend, along the Llano Uplift, in parts of Trans-Pecos Texas west of the Delaware Basin, and in West Texas along parts of the Amarillo Uplift and the Matador Arch. These gaps appear where there are no wells penetrating the designated horizons or correlative strata, or, as occurs along parts of the Balcones trend, the target horizons do not lie within the proper depth range. We attempted to obtain all readings from points deeper than 2,000 ft (to avoid near-surface hydrologic perturbations) but shallower than the range of geopressured conditions (because of the abrupt discontinuities of geothermal gradients in that zone).

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