Figure 1 : Location of study area, depicting geothermal fairways and the "bad-water lines" of the Edwards Limestone. San Antonio is the square in Bexar County. From Woodruff and Foley (1985).
Figure 2 : Geothermal gradients in the Edwards and other limestones. From Woodruff and Foley (1985), modified after Woodruff and others (1984).
Figure 3 : Temperatures from wells in the Edwards Limestone, in degrees C. See Woodruff and others (1982b) for further data.
Figure 4 : Hosston Formation isopach map, Bexar County. From Zeisloft and Foley (1983).
Figure 5 : Lithologic log, Lackland Air Force Base #1. From Zeisloft and Foley (1984).
Figure 6 : Thermal log, Lackland Air Force Base #1. From Zeisloft and Foley (1984).
Table 1 : Lackland AFB #1 representative water analysis (Sample 3 in Zeisloft and Foley (1984). For sample and analytic details, consult that source.)
The Balcones Fault Zone of Central Texas (figure 1) has been recognized as a geothermal area since the 19th century. In the past, the Hot Wells Hotel in San Antonio, the Driskill Hotel in Austin, spas in Marlin, and numerous other sites have used geothermal waters, mainly for bathing. Thermal waters of the Balcones system are currently being used to heat a hospital in Marlin and for aquaculture and agriculture in Corsicana (Blackett and others, 1986). There is even a hot well on the grounds of the State Capitol in Austin.
Through recent programs, funded largely by the U.S. Departments of Energy and Defense, new data have been developed about the thermal regime of the Balcones system. These data provide insight into subsurface hydrodynamic conditions, are important for understanding exploration for geothermal resources, and provide knowledge about constraints on the occurrence of petroleum resources. This report summarizes several previous studies that have not been widely distributed (for complete list, see Woodruff and Gever, 1983), and presents geological, hydrologic, and thermal results from a 1,225 m geothermal well at Lackland Air Force Base in west-central Bexar County.
The U.S. Geological Survey has defined the low-temperature geothermal resource base as those waters that are more than 10oC above the mean annual temperature at a site, but less than 90oC (Reed, 1983). Waters above 90o are classified as moderate- and high-temperature geothermal (Muffler, 1979). For a well to be considered a resource, Reed (1983) stipulates that it must also have a thermal gradient of at least 25oC/km.
The studies reported herein were initiated as part of a program sponsored by the U.S. Department of Energy to assess low-temperature geothermal resources throughout the nation. The studies along the Balcones Fault Zone are summarized in Woodruff and Gever (1983).
The geology of Bexar County records the history of the Ouachita structural belt, and the influence of a hinge line that developed along the belt on sedimentation, tectonics, and hydrology (Woodruff and Foley, 1985). The Ouachita belt is a foundered Paleozoic orogen. The hinge line separates the stable craton from the downwarping Gulf Coast Basin. The faults of the Balcones Escarpment area (figure 1) developed along the hinge zone. Hydrologic characteristics of aquifers are influenced by these faults (Abbott, 1977).
The temperature at any point in the earth's crust is the product of many factors. These include, but are not limited to, the flux of heat from the mantle, the generation of heat in the crust, proximity to igneous activity, the thermal conductivity of the rocks at the site, local and regional geological and topograhic contrasts, and secondary factors such as fluid movement, especially along fault zones. In the Balcones Fault Zone area, few data are available for either the flux of heat from the mantle or the generation of heat in the crust. Until such data are developed, we assume that mantle flux and crustal generation are regionally uniform, although some differences may exist between the Ouachita fold belt and the Gulf Coast basin. The refraction of heat at the boundary between these two geologic provinces has not been assessed as part of this study. No local sources of abnormal heat, such as young volcanic centers, exist in the Balcones Fault Zone area. Extrapolation of data from other areas suggests that the effective overall thermal conductivity of the rock column changes from site to site, depending upon the degree of compaction and grain size of the sediments. Finer grained sediments will have lower thermal conductivities, and hence higher temperatures. This may have some impact on temperatures in areas with thick shale sequences. These temperature changes are likely to occur over a broad area, rather than being at the local scale at which thermal anomalies along the Balcones apparently occur.
Secondary factors, such as regional and local fluid flow patterns, are probably the dominant control of subsurface temperature. This has been extensively modeled (Smith and Chapman, 1983; Woodbury and Smith, 1985), and described in Tertiary Gulf Coast sediments by Bodner and others (1985). Surficial fluids in recharge zones often generate local cool areas, as the downflowing waters sweep heat out of an area. Upwelling fluids can generate positive thermal anomalies, if they rise quickly enough from depth. Thermal highs, possibly indicating fluid upwelling along growth faults, have been noted in the Wilcox Fault Zone southeast of Bexar County (Bodner and others, 1985).
Thermal gradient data from Bexar County and the surrounding area are depicted in figure 2. Positive anomalies along the Balcones system are local, discontinuous features. Hydrodynamic coupling of fluid and thermal flow, including water movement along faults, can explain the discontinuous pattern of anomalies. Permeable zones with upwelling fluids may be warmer, and zones with downwelling fluids or that are impermeable may be cooler. The localized nature of the anomaly pattern may be caused by areally restricted permeability and fluid flow contrasts, rather than regional changes in the thermal regime of the crust. Abbott (1977) documents extensive movement of water along faults in the Edwards Limestone; such movement could greatly impact local thermal patterns.
GEOTHERMAL RESOURCES AND EXPLORATION IN BEXAR COUNTY
The attractiveness of geothermal resources as an alternative energy resource has led to several projects to define their extent along the Balcones Fault Zone. The discussions below of the Bexar County area are primarily based on data from Woodruff and others (1982a) and Zeisloft and Foley (1983, 1984). The regional concepts are expanded from those in Woodruff and Foley (1985).
Exploration for geothermal resources in Bexar County was driven by the target concept of seeking zones of upwelling basin brines, where vertical permeability and well performance would be increased by the presence of faults. Data from both water and oil wells were sought to define structural and stratigraphic characteristics of likely reservoir rocks.
The limitations of the regional data base for geothermal interpretations of the subsurface have been described by Woodruff and others (1982b). Two largely exclusive sources of information exist. Shallow aquifers, such as the Edwards and associated units, have much data from water-well drilling and testing, and long-term production records. These data include water temperature, water quality, and productivity characteristics of the aquifers. Deeper aquifers, such as the bad-water zone of the Edwards Limestone and the Hosston Formation, have few data. Data from the deeper units are largely from geophysical logs of oil wells and typically do not include water quality and productivity records. As a result of these limitations, our analysis of geothermal characteristics is based on abundant stratigraphic and structural data, with few reliable geothermal data points.
Fault locations were largely identified from existing geologic maps or interpreted from well data. No seismic data were available for use in this investigation; such data might greatly improve future interpretations of geothermal systems.
Geothermal resources in the Cretaceous Edwards Limestone are restricted to areas south and east of the "bad-water line" (Figure 1). This line is located where the aquifer changes from yielding large quantities of cool, culinary-quality water, to yielding irregular amounts of warm but saline fluids. The change in aquifer properties occurs over a short distance, and does not correspond to either fault traces or the strike of the Edwards Limestone or to original depositional environments of the limestone (Abbott, 1974). The occurrence of the line changes in depth across Bexar County, from approximately 150 m in the northeastern portion to greater than 600 m in the southwestern portion. These characteristics led Woodruff and Abbott (1979) to conclude that the "bad-water line" delimits the downdip portion of the Edwards Limestone that is influenced by surface discharge, marking the boundary of the surface hydrologic system with a deeper system dominated by deep basin brines. The line converges near modern springs, suggesting that they represent potentiometric base levels.
The oblique trend of the "bad-water line" with structural trends in the Edwards Limestone is important for the occurrence of geothermal resources. In areas where springs cause a deflection of the "bad-water line" to shallow depths, anomalously warm temperatures are not likely to exist. At greater depths, in areas without such deflections, higher temperatures may exist in the Edwards Limestone. This is particularity likely to be true where faults allow upwelling of deeper basin brines.
Chemical analyses of waters from the Edwards Formation define three geochemical regimes: fresh, saline, and transitional waters. The fresh waters typically have total dissolved solids contents of 350 ppm or less; this contrasts with values of up to 5,000 and from 350 to 3,500 for the saline and transitional zones respectively. Temperatures are less than 30oC in the fresh zone, and often more than 40oC in the saline zone. Major chemical species are also different. Fresh waters are dominately Ca-Mg-HCO 3 and saline waters are Na-Cl-SO 4. Details of the origin of chemical constituents in the Edwards Limestone waters are not resolved (Land and Prezbindowski, 1985; Stoessell and Moore, 1983, 1985). Current models agree, however, that updip movement of fluids from the Gulf Basin is an important process. Transition-zone waters are usually intermediate in both temperature and water chemistry. The contact between fresh and saline waters in many areas is abrupt, and extends to depth.
The boundary between fresh and saline waters in the transition zone has migrated with time, probably as a response to drought and recovery periods. During drought, artesian pressure and water levels drop, and saline fluids move updip. In recovery, increased pressure from fresh waters push saline fluids to greater depth (Garza, 1962).
Temperatures in the Edwards Limestone (figure 3) increase both laterally and with depth. The lateral increase is most marked at the change from fresh to saline water. The increase of temperature with depth is typically gradual.
The transition zone is geothermally important. It provides areas where relatively fresh fluids are apparently heated by upwelling saline solutions. Temperatures of 40oC and dissolved solids contents of less than 1,000 ppm may locally be obtained.
The Hosston Formation is the lowermost Cretaceous unit in Central Texas (Bebout and others, 1981). It is deposited either on Ouachita metamorphic rocks or on sediments deposited in grabens in the metamorphic rocks. Flawn and others (1961) document nearly 800 m of unmetamorphosed sediments in a well immediately south of Lackland Air Force Base. The geothermal well drilled on the base, which is discussed below, encountered sandy red beds beneath the Hosston.
The Hosston was deposited in both fluvial and marine environments in Bexar County. In the northwestern part of the county, where the Ouachita rocks are relatively shallow, the Hosston is composed of dip-oriented river and delta deposits (Woodruff and others, 1982b). Beneath the southeastern portions of the county, marine, strike-oriented beds dominate. The Hosston also includes dolomites in some areas (Bebout and others, 1981). Formation and sand thicknesses increase from north to south across Bexar County (figure 4). The elevation of the Hosston also changes dramatically across the county, from at or near sea level in the north to more than 1.6 km below sea level in the south.
Woodruff and others (1982b) noted the potential for geothermal resources in the Hosston Formation, and Woodruff and others (1982a) depicted the area of Bexar County that they felt was most likely to have such resources. Sorey and others (1983), using different criteria than Woodruff and others (1982a, 1982b) to define geothermal resources, identified five counties along the Balcones Fault Zone that had resources in the Hosston Formation and equivalent units. Bexar County was not among these.
The choice of the Hosston Formation as a target unit for detailed geothermal exploration at Lackland Air Force Base was based upon the potential production characteristics of the rocks, which were known geothermal producers in the cities of Marlin and Corsicana, and upon the stratigraphic position of the Hosston as the basal and therefore warmest sand overlying basement rocks.
Woodruff and others (1982b) suggested that the different environments probably have major impacts on hydrodynamic and geothermal properties of the formation. Dip-oriented sands allow recharge waters to circulate down. This may allow ground water to remain relatively fresh and cool at moderate depths. Strike-oriented sands, however, provide no continuity for downward circulating waters, and may contain residual brackish fluids. These conditions suggest that, in general, fresher and slightly cooler fluids will be found in the dip-oriented sands, and warmer, more brackish waters will occur in the strike-orierited units. The Hosston, in a manner analogous to the overlying Edwards Formation, should have a mixing zone between the two units.
Lackland Air Force Base
The U.S. Air Force funded a program to site and drill a geothermal test well at Lackland Air Force Base (Zeisloft and Foley, 1983, 1984; Foley and others, 1984). Lackland AFB is located in west-central Bexar County; the base overlies several faults of the Balcones Fault Zone. The Hosston Formation was chosen as the target, owing to its known geothermal productivity in north-central Texas, and its position as the deepest and presumably warmest unit. Stratigraphic, hydrologic, and thermal data were obtained from the test well drilled in 1983. Normal regional stratigraphy was encountered, with a red bed sequence (Triassic graben fill?) occurring beneath the Hosston Formation. Preliminary hydrologic data suggest moderate productivity may be expected, but water temperatures beneath the base are not anomalously warm (42 oC).
Stratigraphy of the well is illustrated in figure 5. No apparent faults were noted in either the cuttings or well log data and no major zones of lost circulation were present. The basal red-bed sequence is presumed to be equivalent to similar units encountered in several other wells in the area (Flawn and others, 1961).
Sidewall cores were collected as part of the lithologic analysis program. Data from these cores do not indicate the presence of commercial quantities of oil or gas. No major shows were encountered, despite drilling through zones that are either productive or the focus of exploration activity nearby. Porosity ranges from 14 to 24 percent in sand-rich beds; permeability ranges from less than 4 to nearly 80 millidarcys.
Hydrologic testing consisted of short-term air lifts. No long-term tests have been made on the well. The data were analyzed by Dr. David Allman of EG&G Idaho, Inc., under Air Force funding. Dr. Allman's unpublished interpretations indicate that the Hosston Formation has a specific capacity of 2.32 gpm/ft. He calculated a transmissivity of 3800 gpd/ft, assuming a storage coefficient of 1x10-4.
Water samples were collected during the air lift testing. Results of the analyses are presented in table 1. These samples might be slightly contaminated with drilling fluid, but the data are very similar to analyses reported by MacPherson (1982) for thermal fluids in Hosston-equivalent units in north-central Texas. These waters are probably from the transition zone between presumed fresher waters to the north and more saline waters to the south. They have moderate total dissolved solids and relatively high sulphate.
Geothermometer calculations to estimate possible deep reservoir temperatures suggest that the fluids may have equilibrated with rocks at about 60 oC (Zeisloft and Foley, 1984). It must be noted, however, that geothermometers are probably unreliable in this temperature range.
Dr. David Blackwell of Southern Methodist University obtained a high-precision thermal log of the well approximatly three months after drilling. The well had been shut in between air lift testing and the thermal logging. The results of the thermal logs are presented in figure 6. The maximum temperature encountered was slightly more than 42 oC. This compares closely with the maximum temperature measured during the air lift test of approximately 41 oC. The thermal gradient of the well is relatively low, at approximately 15 oC/km.
Results of the drilling at Lackland suggest that the Hosston Formation beneath the base is probably in the hydrodynamic mixing zone. Relatively cool temperatures probably occur in the vicinity of the well, and are related to downwelling fluids. The moderate total dissolved solids of the waters, however, suggests that some influence from upward-moving deep basin brines is also present.
Regional groundwater flow is an important geologic process in the generation of geothermal systems (Sorey and others, 1983), the creation of ore deposits (Bethke, 1986), and petroleum migration and trapping (Jones, 1984). All three processes are probably taking place currently along the Balcones Fault Zone area. Bodner and others (1985) noted these processes in Gulf Coast sediments south of Bexar County.
The association of high thermal gradients and warm waters with saline fluids typical of oil fields suggests a correlation of geothermal resources with upwelling Gulf Basin brines. The discontinuous pattern of thermal anomalies suggests that local controls, such as hydrodynamic movement, may be an important factor in controlling the location of anomalies. The Balcones Fault Zone provides an area of increased vertical permeability, which may allow the deeper fluids to reach the surface.
Future exploration for geothermal resources along the Balcones area will need to include the possibility that faults can also provide channels for downmoving waters, as may be the case near Lackland Air Force Base. These cooler fluids can create zones where geothermal resources are not likely to exist at depth. Determination of zones with down- or up-moving fluids is complicated by an often unreliable data base and the apparently relatively rapid areal changes in thermal conditions. It is possible that the radius of influence of a thermal data point along the Balcones may be less that two or three kilometers; this needs to be studied in more detail.
Basin hydrodynamics have long been recognized as an important contributing factor to the generation of Mississippi Valley type stratiform ore deposits. A recent synthesis of the role of hydrodynamics in ore formation is presented by Bethke (1986). The generation of ore deposits in Texas is currently poorly documented, but high lead concentrations were documented by Prezbindowski (1981) in the bad-water zone of the Edwards Limestone in the inner Gulf Coast.
The association of oil fields with thermal highs and the probable role of water as a driving mechanism in both movement of oil and generation of thermal highs were first noted in Texas by Plummer and Sargent (1931). This phenomenon has since been documented in many other areas (e.g., Meyer and McGee, 1985; Zielinski and Bruchhausen, 1983). A relationship of thermal highs with petroleum resource accumulations may exist along the Balcones Fault Zone (Woodruff and Foley, 1985), and could provide a valuable tool in petroleum exploration.
This work has been partially supported by U. S. Department of Energy contracts ET-78-S-05-5864 and DE-AS07-79ID12057. Foley, while at the Earth Science Laboratory/University of Utah Research Institute, was funded under contracts DE-AC07-76ID01570 and DE-AC07-80ID12079. The Earth Science Laboratory permitted us to use figures from several of their publications. We would also like to acknowledge the support and cooperation of the U. S. Air Force, and thank them for their permission to publish the results of the drilling program at Lackland Air Force Base. The discussion of Bexar County and the Edwards Limestone has largely been adapted from Woodruff and others (1982b). The review and comments of Stephen Benham are greatly appreciated. The views and opinions expressed in this paper are those of the authors, and may not represent the views of the U. S. Government.
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