Geothermal Resource Prospecting in the Qualibou Caldera, Saint Lucia By Frank Dale Morgan Geo-Caraibes Technical Team & Massachusetts Institute of Technology March 2006

Outline History  Summary Brief Review of Four MIT Studies Conclusion Recommendations

History Serious geothermal resource exploration began in the mid-70s by the British Geological Survey After dozens of investigations and nine drilled holes, no real geothermal development has resulted to date.

Belle Plaine Morne Bonin Fond St. Jacques Terre Blanche Diamond Cresslands Ravine Claire Soufrière Gros Piton Petit Piton Anse Des Pitons Soufrière Bay Kilometers Belfond Rabot Historic Drill Site Recommendations UNRFNRE USAID – DOE & USGS USAID – LANL Gov’t of St. Lucia GENZL & Aquater GENZL British Drilling Programme Etangs Sulphur Springs Time Line 1951British start formal investigation (Willmore) 1964Tomblin performs detailed geology survey 1974Institute of Geological Sciences (IGS) U.K. resistivity survey Seven wells drilled by IGS (Wells 1-7) 1976Aspinall et al. perform seismic monitoring 1982Aquater (Italy): Magnetotellurics, gravity, well data evaluation Los Alamos (USA): Geology, geochemistry, geophysics USAID/UN: Drill two deep wells (SL1 & SL2) 1992Geothermal Energy New Zealand: Gravity, resistivity, audio magnetotelluric resistivity 1998-Pesent M.I.T.: Reinterpretation of British resistivity data, self potential geophysics, decision analysis 2004-PresentUnited Network of the Eastern Caribbean is commissioned to develop geothermal resources.

Geology

Petit Piton Gros Piton Soufriere Bay Geothermal Wells & Hot Springs

GENZL, 1992 USAID (Aquater), 1987

Summary Very complex geology and hydrogeology The chemistry of the geothermal waters beneath the Sulphur Springs is quite severe. The geothermal reservoir cap rocks are weak. Exploration is best optimized by exploring for less hot – but less corrosive – waters away from the Sulphur Springs.

Synopsis of Four MIT Studies Reinterpretation of the British Line 9 resistivity data. 3D rendering of 2D resistivity inversions. Self Potential surveying. Geological/Geophysical data integration using a decision analysis method.

Reinterpretation of the British Line 9 Resistivity Data. Data collected in 1974 by Lee and Greenwood, British Geological Survey

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Depth (m) Sulphur Springs (1974) SL Depth (m) Sulphur Springs (2002) SL Log Resistivity (ohm-m)

SL-2 SL Sulphur Springs 150 o C 250 o C 30 ohm-m 30 to 1000 ohm-m 1000 ohm-m

3D Rendering of 2D Resistivity Inversions. All the British resistivity lines were inverted into 2D cross-sections and then rendered into 3D.

3D Resistivity Tomogram Southeast -to- Northwest

3D Resistivity Tomogram Northeast -to- Southwest

Self Potential Surveying Small, millivolt electrical potentials were mapped, processed and interpreted throughout the Qualibou Caldera.

Diamond. Fond St. Jacques Main Sulphur Springs Belfond Cressland Malgretoute 3 SP Survey Lines

Distance Self Potential Fond St. Jacques Cresslands Diamond Sulphur Springs Rabot Belfond Malgretoute Individual SP Lines

Correlation of observed SP anomalies and fault locations. N

Sulphur Springs Interpolated SP Data mV

Geological/Geophysical Data Integration Using a Decision Analysis Method. Eleven disparate data sets were jointly analyzed by a decision analysis methodology to determine possible drilling locations.

Discretized Caldera Re-interpreted British Resistivity 700m BGS 0 km3 km6 km Target RegionExample Data Map Morgan et al., (2002) Method Eleven datasets were used: Seismics, self potentials, fault structure, deep resistivity, shallow resistivity, geology, topography, wells/springs/geochemistry, shallow AMT, deep AMT, and residual gravity.

Group Result X1X1 X2X2 X5X5 X3X3 X4X4 X6X6 Cresslands Etangs Malgretoute Sulphur Springs Diamond

Conclusion Geothermal resource exploration in Saint Lucia is a very difficult endeavor because of: Geological complexity Highly corrosive geothermal waters.

Recommendations Explore for drilling locations away from the Sulphur Springs to balance improved water chemistry with lower temperatures. The probability of success will increase substantially by doing 3D resistivity/self- potential geophysics to accurately locate drilling positions and to avoid the faux pas of the past.