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together observed and inferred information to identify geothermal targets and predict reservoir capacity. Such models are often combined with geostatistical and classical technologies such as those employed for reservoir characterization. Hydrothermal conceptual models combine


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observed and inferred information to illustrate reservoir fluid and rock properties and often include data captured through cation and gas geochemistry. They also take into account MT resistivity interpreted in the context of basic geology and hydrology and through mapping of surficial hydrothermal alteration.8 The most important element of a hydrother-


100 100 200 300 Temperature calculated from chemical geothermometer, °C


> Subsurface temperature predictions. Temperatures measured in wells drilled into hydrothermal systems are compared with temperatures calculated from geothermometers before drilling. The dashed line indicates the location where points would plot if measured and calculated values agreed perfectly. Points above the line indicate calculated temperatures that were underestimated. (Adapted from Duffield and Sass, reference 9.)


mal conceptual model is a predicted natural- state isotherm pattern—solid lines drawn to indicate temperature and depth across a subsur- face section. Though difficult to arrive at during the exploration stage, case histories indicate it can be done based on interpretation of the geothermometry—a technique that allows the determination of subsurface temperature using a combination of methods including the chemistry of hot-springs fluids and distribution of hydro- thermal alteration minerals at the surface. Patterns of geophysical anomalies and resistivi- ties and a general knowledge of the local geology, hydrology and faulting or structural history may also be used. Hot water circulating in the Earth’s crust may


determined through magnetotelluric (MT) mea- surements, can provide an indication of geother- mal prospectivity.6


MT has become a standard


method for mapping the caprock geometry con- straining geothermal reservoirs. If wells have been drilled in an area, many of the


parameters measured indirectly from the surface can be obtained directly from well log data. These logs can highlight regions of porosity, saline fluid saturation and temperature variations, which may indicate the presence of hydrothermal reservoirs. Since these resources may be found in frac-


tured, tectonically stressed areas, their presence is often marked by microseismic events that also serve as a guide to drilling into the fractured rocks once other favorable geothermal condi- tions are established. By recording a relatively large number of these events over weeks or months and calculating their epicenters, seis- mologists can determine the location and orien- tation of fractures. Seismic reflection and seismic refraction sur-


veys have been used only sparingly in geothermal exploration. Although obtaining refraction pro- files requires a considerable effort at depths of 5 to 10 km [16,400 to 33,000 ft], standard seismic reflection surveys often yield useful results in


8 AUT09–RVF–04


these areas. During geothermal exploration, gravity surveys are used to define lateral density variations associated with a magmatic heat source in volcanic-hosted systems or with fault blocks buried beneath sedimentary cover in sys- tems of deep circulation. But their main value is in defining changes in groundwater level and in monitoring of subsidence and injection, which are directly related to the resource’s ability to recharge itself. By correlating the surveys and weather, it may be possible to define the relation- ship between data from a gravity survey and the precipitation that produces changes in shallow groundwater levels. When corrected for this effect, gravity changes show how much of the water mass discharged to the atmosphere is replaced by natural inflow.7


The Concept The most common approaches to geothermal exploration include anomaly hunting, anomaly stacking and conceptual modeling. Mathematical velocity models are routinely used to predict the depth to a formation of interest, and physical models can be used to simulate rock layers. Conceptual models are hypothetical, bringing


dissolve some of the rock through which it flows. The amounts and proportions of these solutes in the water are a direct function of temperature. If the water rises quickly from the geothermal res- ervoir to the surface, its chemical composition does not change significantly and it retains an imprint of the subsurface temperature. Subsurface temperatures calculated from hot- springs chemistry have been confirmed by direct measurements made at the base of holes drilled into hydrothermal systems.9 Geothermometry uses ionic and stable isotope


ratios in the water to determine the maximum subsurface temperature (above left). Geochemical and isotopic geothermometers developed over the past two decades assume that two species or com- pounds coexist within the geothermal reservoir and that temperature is the main control on their ratio.10


They also assume that no change in


that ratio has occurred during the water’s rise to the surface. Gas ratio geothermometers can also be used


to determine subsurface reservoir conditions. By integrating these geochemical data with infor- mation from temperature-gradient wells and structural mapping, engineers can build concep- tual models that display fluid-flow patterns


Oilfield Review


Subsurface temperature measured in well, °C


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