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.
Historic Drill Site Recommendations Soufrière Bay Anse Des Pitons Gros Piton Petit Piton Soufrière Cresslands Diamond Terre Blanche Sulphur Springs Rabot Belfond Etangs Ravine Claire Fond St. Jacques Morne Bonin Belle Plaine UNRFNRE USAID DOE & USGS USAID LANL Gov t of St. Lucia GENZL & Aquater British Drilling Programme GENZL Kilometers 0 0.5 1 2 3 4 5 6 Time Line 1951 British start formal investigation (Willmore) 1964 Tomblin performs detailed geology survey 1974 Institute of Geological Sciences (IGS) U.K. resistivity survey 1975-76 Seven wells drilled by IGS (Wells 1-7) 1976 Aspinall et al. perform seismic monitoring 1982 Aquater (Italy): Magnetotellurics, gravity, well data evaluation. 1983-84 Los Alamos (USA): Geology, geochemistry, geophysics 1987-88 USAID/UN: Drill two deep wells (SL1 & SL2) 1992 Geothermal Energy New Zealand: Gravity, resistivity, audio magnetotelluric resistivity 1998-Pesent 2004-Present M.I.T.: Reinterpretation of British resistivity data, self potential geophysics, decision analysis United Network of the Eastern Caribbean is commissioned to develop geothermal resources.
Geology
Geothermal Wells & Hot Springs Soufriere Bay Petit Piton Gros Piton
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) 0 200 400 600 40 15 Sulphur Springs (1974) 7 6 1 3 4 5 SL-2 30 2 50 100 5 30 200 500 5 800 0 Sulphur Springs (2002) 7 6 1 3 4 5 SL-2 Depth (m) 200 400 600 800 0.5 1 1.5 2 2.5 3 3.5 4 Log Resistivity (ohm-m)
2 Sulphur Springs 7 6 1 3 4 5 SL-2 SL-1 1000 ohm-m 150 o C 30 ohm-m 250 o 30 to 1000 ohm-m C
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.
Fond St. Jacques SP Survey Lines 3 Cressland Diamond. Malgretoute Sulphur Springs Belfond Main
Individual SP Lines 200 100 Self Potential 0-100 -200-300 Fond St. Jacques Cresslands Diamond Sulphur Springs Rabot Belfond Malgretoute 0 2000 4000 6000 8000 10000 12000 Distance
N Correlation of observed SP anomalies and fault locations.
Interpolated SP Data mv Sulphur Springs
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.
Method Re-interpreted British Resistivity 700m BGS 0 km 3 km 6 km Morgan et al., (2002) Target Region Discretized Caldera Example Data Map 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 X 4 Cresslands Diamond Malgretoute X 6 X 3 X Sulphur 1 Springs X 2 X 5 Etangs
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/selfpotential geophysics to accurately locate drilling positions and to avoid the faux pas of the past.