Heat Flow and Geothermal Potential in the South-Central United States

Petru T. Negraru 1 David D. Blackwell 1 Kamil Erkan 0 0 School of Earth Sciences, The Ohio State University , 125 S. Oval Mall, 275 Mendenhall Laboratory , Columbus OH 43210, USA 1 Southern Methodist University , 750395, Dallas TX 75206, USA Geothermal exploration is typically limited to high-grade hydrothermal reservoirs that are usually found in the western United States, yet large areas with subsurface temperatures above 150 C at economic drilling depths can be found east of the Rocky Mountains. The object of this paper is to present new heat flow data and to evaluate the geothermal potential of Texas and adjacent areas. The new data show that, west of the Ouachita Thrust Belt, the heat flow values are lower than east of the fault zone. Basement heat flow values for the Palo Duro and Fort Worth Basins are below 50 mW/m2 while, in the frontal zone of the belt, they can exceed 60 mW/m2. Further east, along the Balcones fault system the heat flow is in general higher than 55 mW/m2. The eastern most heat flow sites are in Louisiana and they show very high heat flow (over 80 mW/m2), which is associated with the apparently highly radioactive basement of the Sabine uplift. The geothermal resource in this area is large and diverse, and can be divided in high grade (temperature above 150 C) convective systems, conductive based enhanced geothermal systems and geothermal/geopressured systems. One of the most attractive areas east of the cordillera extends from eastern Texas across Louisiana and Arkansas to western Mississippi. Here temperatures reach exploitation range at depths below 4 km, and tapping such a resource from shut in hydrocarbon fields is relatively easy. The initial costs of the development can be greatly reduced if existing hydrocarbon infrastructure is used, and therefore using shut-in hydrocarbon fields for geothermal purposes should not be neglected. INTRODUCTION Driven by ever-increasing dependence on imported oil, energy security will be one of the most important problems facing the United States in the next century. In addition, global warming emphasizes that renewable energy sources must become a larger portion of our energy supply. Among them geothermal energy plays an important role. Geothermal exploitation is usually associated with resources located in the western United States, where high temperatures may be encountered over large regional areas (Blackwell, Negraru, and Richards, 2007), but in general only high-grade (in excess of 150 C) geothermal resources are exploited. Yet areas east of Rocky Mountains possess an important geothermal potential. This paper presents new heat flow data in Texas and discusses the available heat flow information in south-central United States east of the Rocky Mountains, as well as the implications for the geothermal resource of the Texas and adjoining areas. The geothermal Palo Duro Basin Amarillo Uplift iver Up lift NIC O T C E T resources in the north central third of the country are addressed in a companion study (W. D. Gosnold, personal communication, 2007). Our assessments overlap in Oklahoma and Kansas, but the approach described below is somewhat different. Figure 1 shows the locations of the wells for which new heat flow values are reported in this paper. Most of them are located in north-central Texas, in an area extending from the Fort Worth Basin in the west, across the Ouachita Tectonic front, including the Dallas area, up to the Balcones Fault Zone in the east. Two wells are located in the Palo Duro Basin, and two others in northwestern Louisiana, a few miles east from the Texas border. Gradient information for one well in central Texas is also available (located within the Lackland Air Force Base). The current geologic setting is dominated by the Paleozoic Ouachita system, which has an approximate northsouth orientation and splits the study area in two main parts. To the west Paleozoic sedimentary rocks with a thin cover of Mesozoic sediments overly undeformed basement rocks of Precambrian age belonging to the Texas Craton. To the east, Paleozoic Ouachita rocks are overlain by Cretaceous sediments. Within the Ouachita system two tectonic provinces are recognized (Flawn and others, 1961). The first is a frontal zone bordering the Texas Craton where deformation was by flexure, and the second is an interior zone where shear deformation predominated (reactivated by the Balcones Fault zone). The nature of the basement beneath the Ouachita rocks is unknown. In spite of the extensive drilling in the area, very few reliable heat flow data are available. Only in two relatively similar geologic areas have heat flow values been published. To the southwest of the Fort Worth Basin, a heat flow of 46 mW/m2 was measured for the Midland Basin (Herrin and Clark, 1956) while southeast of the Ouachita front, in the Gulf Coast, Blackwell and others (1999) determined heat flow values of 52 and 55 mW/m2 in two wells. They also used high resolution temperature-geophysical log correlations to obtain information about the thermal conductivity values of particular formations. A similar approach was employed for a number of wells in this study. Heat flow values as low as 39 mW/m2 were reported north of our area, in southwest Oklahoma, associated with low heat production rocks (mafic complex) of the Wichita Uplift (Carter, 1993; Carter and others, 1998; Gallardo and Blackwell, 1999). This complex might affect the heat flow regime of the most northern wells in our study area. The new data presented here are based on temperature logs that are characterized by a number of high resolution well-logs in the Fort Worth Basin (the first accurate heat flow values for the Fort Worth Basin), Palo Duro Basin and East Texas Basin. In addition, eight mostly high resolution welllogs were measured in the Dallas area, around the frontal zone of the Ouachita, and eight low resolution well-logs were made in the Mexia and Talco Fault zones. The purpose of this work is to understand the general temperature regime in the southwest and to assess the geothermal potential of the area. HEAT FLOW DETERMINATIONS Dallas Area The Mobil New Exploration Ventures Farmers Branch #1 (MNEV) was drilled on the Mobil Dallas Research campus in the spring of 1991. The hole was cored top to bottom because the purpose of the well was to evaluate drilling and testing techniques for hydrocarbon exploration using core-drilling equipment and small diameter wells. Extensive openhole logs were run on the well and the core samples were cataloged, videotaped, and gamma ray logged. Because of the extensive information available from the well and the fact that the well remained accessible for a long time, there were opportunities to carry out experiments and evaluations and develop techniques not possible in the typical exploration situation. For example, the well was thermally logged seven times following drilling as it approached thermal equilibrium allowing the d (...truncated)