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Geothermal energy is a renewable, widespread, baseload energy source that has the potential to supply a large portion of the world’s energy needs. High development risk, specifically high drilling costs combined with uncertainty in subsurface flow properties, have hampered growth of geothermal energy development. One option to reduce subsurface risk is to target locations where reservoir data are already available, including sedimentary basins where there has been extensive hydrocarbon production. Sedimentary basins are widespread and often have suitable geothermal gradients. This chapter presents a low-cost methodology that can be used at the pre-drilling exploration phase of a low-temperature geothermal projectmore » to predict the location of high-productivity and low-risk reservoirs, applied to a case study of the Appalachian Basin in the eastern United States. Because only averaged reservoir data were available for analysis, the technique uses a Monte Carlo simulation of the Reservoir Flow Capacity (RFC) and Reservoir Productivity Index (RPI) for over 1800 individual reservoirs, using liquid water or supercritical carbon dioxide (sCO₂) as the reservoir fluid. RFC is used as an initial screening tool to identify high-priority geologic formations and reservoirs. Here our results indicate that 99% of the reservoirs in the basin are of insufficient quality for geothermal heat production with water as the assumed fluid and without using stimulation technologies associated with Enhanced Geothermal Systems (EGS). RPI is also used as an alternative metric to identify high-productivity reservoirs. When comparing the RPI results to natural gas production volumes from several selected reservoirs, the predicted RPI proves to be a good estimate of flow potential for porous reservoirs, but overestimates flow by a factor of three for fractured or vuggy reservoirs. A greater uncertainty ought to be utilized when applying the RPI equation to non-porous media, therefore we recommend utilizing the RPI metric when higher resolution data are available for analysis. This methodology can be utilized in other basins that have experienced hydrocarbon exploration and production, to highlight low-risk reservoirs or geologic formations for further geothermal development.« less

The rock alteration and rind formation in analog environments like Antarctica may provide clues to rock alteration and therefore paleoclimates on Mars. Clastic sedimentary rocks derived from basaltic sources have been studied in situ by martian rovers and are likely abundant on the surface of Mars. Moreover, how such rock types undergo alteration when exposed to different environmental conditions is poorly understood compared with alteration of intact basaltic flows. Here we characterize alteration in the chemically immature Carapace Sandstone from Antarctica, a terrestrial analog for martian sedimentary rocks. We employ a variety of measurements similar to those used on previousmore » and current Mars missions. Laboratory techniques included bulk chemistry, powder X-ray diffraction (XRD), hyperspectral imaging and X-ray absorption spectroscopy. Through these methods we find that primary basaltic material in the Carapace Sandstone is pervasively altered to hydrated clay minerals and palagonite as a result of water–rock interaction. A thick orange rind is forming in current Antarctic conditions, superimposing this previous aqueous alteration signature. The rind exhibits a higher reflectance at visible-near infrared wavelengths than the rock interior, with an enhanced ferric absorption edge likely due to an increase in Fe³⁺ of existing phases or the formation of minor iron (oxy)hydroxides. This alteration sequence in the Carapace Sandstone results from decreased water–rock interaction over time, and weathering in a cold, dry environment, mimicking a similar transition early in martian history. This transition may be recorded in sedimentary rocks on Mars through a similar superimposition mechanism, capturing past climate changes at the hand sample scale. These results also suggest that basalt-derived sediments could have sourced significant volumes of hydrated minerals on early Mars due to their greater permeability compared with intact igneous rocks.« less