Role of Seismic Reflection Profiles in Delineating Basin-Centered Geothermal Reservoirs

摘要

Seismic reflection techniques have not proved very useful in delineating hydrothermal reservoirs in the Great Basin due mainly to the challenging geologic setting of interlayered fan and playa deposits adjacent to steeply down-faulted bedrock. However, basin-centered geothermal reservoirs in the Great Basin offer a much simpler sedimentary setting with sub-horizontal reflectors analogous to most large oil and gas producing basins where seismic reflection imaging is a powerful exploration technique. A review of 1980s-vintage, public-domain COCORP seismic surveys in western Utah and eastern Nevada allows construction of balanced structural cross sections. Important constraints were stratigraphic information derived from numerous nearby oil exploration wells, and outcrop geology exposed in the ranges. Examples are shown for two regions - Black Rock Desert, Utah, and North Steptoe Valley, Nevada - where geothermal reservoirs appear to be located at about 3 km depth. Subsequent evaluation of selected industry seismic reflection data collected over the last 40 years from western Utah and Nevada indicates variable quality, but a potentially valuable data resource for more detailed structural and stratigraphic analysis of basin-centered Paleozoic reservoirs. In some basins there are multiple seismic lines available to be licensed for these studies, making 3D structural modeling of the basins and their associated faults possible at a fraction of the cost of new seismic data acquisition. The Black Rock Desert, Marys River Basin, and North Steptoe Basin are identified as obvious geothermal reservoir targets where industry seismic reflection data will be helpful. A summary map is presented for the Great Basin showing the location of past industry seismic reflection lines and an overlay of major basins derived from gravity surveys. When combined with thermal gradient and heat flow measurements allowing the prediction of the temperature at 3 - 4 km depth, these three geophysical techniques (seismic reflection, gravity, and heat flow) are a powerful tool for prioritizing basin-centered reservoirs most suitable for future power development.