Exploration of a Geothermal Play Utilizing Subsurface Gases, and Time Domain Electromagnetics

Initial exploration of a geothermal play utilizing subsurface gases and time domain electromagnetics was completed on an island in the Lesser Antillies. The objectives of this exploration was to confirm a geothermal heat source and estimate its area, thickness and depth; determine if permeable zones exist in the form of faults and/or fractures; define the area and location for Magnetic Telluric exploration stations to further define the geothermal source and determine locations for geothermal exploration drilling Two specific types of exploration were undertaken: 1) Subsurface gases and groundwater sampling and analysis to determine the type, concentration, area and ratios of magmatic gases and groundwater which allow for calculation of geothemometers and estimate of the geothermal source depth and volume. 2) Time Domain Electro Magnetic study was completed in the area of three fault traces to determine if these traces were faults and whether they were vertical faults or had a significant dip direction. Two of these fault areas were confirmed and provide excellent targets for geothermal exploration wells. The results of the exploration identified two fault zones with little or no dip. All these features are located within the potential sector collapse area of a volcano and within the geothermal source as identified by the high temperature geothermometer anomalies and previous works that determined the area of the focus of the 1988 earthquakes. The results of the exploration also provided two potential locations for geothermal exploration slim holes and 46 magnetic telluric stations. Two slim hole drilling sites were located on the south western side of the volcano in fault zones. Location for fifty-eight magnetic telluric stations were identified on a grid of 250 to 500-meter centers. The magnetic telluric work provided a broad perspective on the geothermal area, thickness and depth and identification of the radial dikes within the geothermal resource.