Cutaway view showing the internal structure of the earth.  Note that the small circular image of the Earth is drawn to scale, whereas the wedge is exaggerated to show the interior structure.  Image adapted from USGS.  Temperature data from Stacey, 1977.

This image represents a temperature model at a mid-oceanic ridge in the upper 10% of the Earth.  The center of the 3D image shows a heat plumb upwelling from within the mantle at a depth of 660 km.  Two subduction zones are shown where the two yellow plumbs extend downward into the hottest region on either side of the central heat plumb.  Image from Ruedas et. al., 2001, 2002.

The areas highlighted  in red constitute active volcanic regions.


Active mud pots associated with geothermal activity at south end of the Salton Sea in California.


The 1976 USGS/AAPG North American Geothermal Study.  This image gives the BHT readings for wells scattered throughout Texas.  The dark gray region represents an area where the contoured isotherm is 150^oC (302^oF).  Blue = 66^o - 81^o C.  Green = 82^o - 142^o C.  Red = 143^o C and higher.

A summary map of Texas geothermal resources as defined by the Texas Renewable Energy Resource Assessment of 1995.  The location and boundaries of these geothermal areas are approximate.

Geothermal energy is naturally occurring heat that is stored within the earth from the time of the earth's creation during gravitational accretion, and from naturally occurring radioactive decay found deep within the earth's mantle and core.  It represents the thermal energy that is contained within rock and fluid that fills fractures and pores within the rock.  Geothermal energy is generally associated with active volcanoes and geysers in tectonically active regions world wide, or in regions where volcanic activity has been sufficiently recent geologically speaking that heat plumbs are still present at depths close to the earth's surface.  Texas has generally not been considered as a state with the potential for major geothermal development (Valenza, 1995).

Some of the earliest work involving Texas geothermal energy investigations date back to the late 1960's and into the 1970's.  Two approaches that these studies used were surface spring temperature and near surface heat flow measurements, and deeper subsurface investigations using wells drilled by the oil and gas industry or by the U.S. Department of Energy.  

Few Texas aquifers have been specifically measured to assess their thermal properties (Valenza, 1995).  Many of the hydrothermal wells that do exist were used for balneology purposes rather than for other geothermal purposes.  The greatest number of these springs and shallow wells are found within Central Texas along the Balcones/Ouachita structural trend, and within the Trans Pecos region of far West Texas.  

The greatest amount of data available for thermal assessment actually comes from the many wells drilled by the oil and gas industry.  The earliest large survey of this type was conducted by the U.S. Geological Survey and the American Association of Petroleum Geologists, with data and analysis being provided by 23 oil and gas companies and universities.  This 1976 North American survey extended from Canada south into Central America.  Maps that displayed bottom hole temperature (BHT) readings and calculated subsurface geothermal gradient were generated as part of the study, as well as a digital dataset.  

More recently, the Texas Renewable Energy Resource Assessment of 1995 generated a summary map of possible geothermal resource areas along with likely uses (Valenza, 1995).  This map showed three types of geothermal areas within the state: hydrothermal; geopressured; and hot dry rock.  The entire state has the potential for ground source (geothermal) heat pumps, and many areas have sufficiently hot waters that can be tapped for various direct use industries.  Electrical energy production was given a low potential in the state due to the lack of shallow heat plumbs that can be tapped in similar manner to other areas around the world.  However, Erdlac and Swift (2004) contend that there is a huge potential for electrical power generation as well.  This can be accomplished by the conversion of deep, depleted gas wells from the oil and gas industry into geothermal extraction wells for electrical energy production.  This approach was originally given the name "Deep Permeable Strata Geothermal Energy" (DPSGE) because of the deep reservoirs within deep sedimentary basins, like the Permian Basin, that lie within the proper temperature range for extracting hot water for power generation.  

References

Erdlac, R.J., Jr., and Swift, D.B., 2004, Deep permeable strata geothermal Energy (DPSGE): Tapping giant heat reservoirs within deep sedimentary basins – an example from Permian Basin carbonate strata, in Shook, G.M., Technical Program Chairman, Geothermal energy – the reliable renewable: Geothermal Resources Council Transactions, Vol. 28, p. 327-331. 

Ruedas, T., Schmeling, H., and Marquart, G., 2001, Dynamical models of melt migration and crust formation in a plumb beneath a MOR: AGU Fall Meeting, San Francisco, poster session and 2002, German Geophysical Society (DGG) Annual Meeting, Hannover, poster session.

Stacey, F.D., 1977, Physics of the earth, John Wiley & Sons, New York, 414 p.

Valenza, J., 1995, Geothermal energy, in Faidley, R., ed., Texas Renewable Energy Resources Assessment: Survey, Overview and Recommendations: Virtus Energy Research Associates, p. 113-126.

Visitors since 7/28/05