UGS Uses Geophysics to Explore for New Geothermal Resources
By Richard G. Allis
A year ago we reported that the UGS had begun an investigation of the geothermal potential beneath the Black Rock Desert south of Delta (September 2011 Survey Notes). This region has experienced episodes of volcanism over the last few million years, the most recent dated at 600 years ago, indicating the possibility of unusually high temperatures deeper within the crust. In the 1970s and early 1980s several companies explored for geothermal energy and for oil and gas, drilling shallow and deep wells, but they all abandoned their exploration efforts. However, an oil exploration well near Pavant Butte found temperatures of over 200°C at more than 3000 m depth (400°F below about 10,000 feet). Although these results point to potential geothermal reservoirs below about 3 km depth, the geothermal exploration industry was then looking for shallower targets, so further investigation in the region was neglected for the next 30 years. Last year the UGS began reassessing the potential of this area using federal funding allocated to promote geothermal development. The results look very interesting and indicate a major geothermal resource.
A geothermal power development requires at least two critical characteristics for a reservoir: adequate temperature (ideally at least 200°C) and rocks with good permeability (so the hot water flows easily through the reservoir between injection and production wells). Because the likely reservoirs beneath Black Rock Desert will be between 3 and 4 km depth, we are using geophysical techniques to detect conditions at these depths. In addition to drilling several wells for temperature gradient measurements, we are applying gravity, magnetotelluric, and reprocessed seismic reflection technologies. Gravity measurements enable the thickness of the unconsolidated sediments filling the basin beneath the desert to be calculated. Magnetotelluric measurements allow the electrical resistivity at depth to be mapped. This can be very useful because geothermal reservoirs are often associated with low resistivity due to the presence of high temperature, saline pore fluids, and clay minerals. Seismic reflection techniques are commonly used by the oil exploration industry to image the underlying basin structure and stratigraphy. Here we had a Cocorp seismic reflection line that had been recorded in the 1980s reprocessed and reinterpreted based on formation tops from abandoned oil exploration wells (such as the Pavant Butte well).
The available temperature information at the moment suggests the highest temperatures are around Pavant Butte and Clear Lake where near-surface temperature gradients are between 60 and 100°C/km (33 to 55°F/1000 feet). The highest temperatures appear to exist in the central Black Rock Desert where the unconsolidated Quaternary and Tertiary sediments are the thickest (e.g., Arco Pavant Butte well). This is to be expected because of the thermal insulating properties of these sediments. Six additional thermal gradient wells are currently being drilled, so later this summer we will have a better idea of the extent of the high temperature area.
Low Density Sediments
The same property (porosity) that causes the sediments to be thermal insulators also causes them to have a relatively low density. This means that thick sediments cause a gravity low anomaly, which can easily be mapped with gravity measurements (white contours on map). During 2011, 168 new measurements were made to improve resolution of the gravity low beneath the Black Rock Desert. Modeling of the 30 mgal low gravity anomaly (relative to the gravity over the bedrock of Cricket Mountains) that extends northwards from the Twin Peaks area in the southern Black Rock Desert towards Delta shows it is due to about 3 km of sediments filling an elongate, north-trending basin.
The magnetotelluric measurements indicate very low resistivity at 1–3 km depth along the axis of the basin (red color, 1–3 ohm-meters). These are surprisingly low values, and a preliminary interpretation is that they are due to hot saline water with clay-rich sediments. Towards the south end of the profile, near Hatton hot springs, higher resistivity (green colors) that is associated with more resistive bedrock beneath the sediments is being detected below about 2 km depth.
The reinterpreted seismic reflection line reveals complicated stacks of bedrock units beneath the Cricket Mountains as a result of Late Cretaceous Sevier shortening, and a major detachment beneath the Black Rock Desert that forms the base of the unconsolidated sediments. The shape of the bedrock-sediment interface is very similar to that inferred from the gravity modeling. An important feature of the seismic reflection results is the variety of bedrock units beneath the Black Rock Desert. These present targets for finding some high permeability, and therefore geothermal reservoirs, at 3–4 km depth beneath the Black Rock Desert.
The UGS is also reviewing permeability characteristics of likely bedrock units beneath the desert based on outcrop observations and their well log properties when they have been encountered in deep oil exploration wells. Later this year, the project will be integrating these new geophysical findings with other geological characteristics of the basin. In addition to the six UGS staff contributing to the project, the UGS is working with other team members, many of whom are at the University of Utah. The Black Rock Desert study is part of a much larger project investigating the geothermal power potential of sedimentary basins in the U.S. Other components involve economic modeling of the resource potential and reservoir simulation of development scenarios. We hope that the new results discussed here will confirm a major new geothermal resource south of Delta, adding to the existing geothermal and wind developments in Millard and Beaver Counties.
Survey Notes, v. 44 no. 3, September 2012