Geothermal Resource Prospecting in the Qualibou Caldera, Saint Lucia


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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
  • 0
  • 2
  • 1
  • 0.5
  • 6
  • 5
  • 4
  • 3
  • 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
  • British start formal investigation (Willmore)
  • 1964 Tomblin performs detailed geology survey
  • 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 M.I.T.: Reinterpretation of British resistivity data, self potential geophysics, decision analysis
  • 2004-Present United Network of the Eastern Caribbean is commissioned to develop geothermal resources.
  • Geology
  • Petit Piton
  • Gros Piton
  • Soufriere Bay
  • 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
  • 9
  • 5
  • 200
  • 500
  • 50
  • 2
  • 5
  • 15
  • 200
  • 400
  • 600
  • 800
  • 0
  • Depth (m)
  • 30
  • 30
  • 40
  • Sulphur Springs (1974)
  • 1
  • 3
  • 4
  • 5
  • SL
  • -
  • 2
  • SL
  • -
  • 2
  • 7
  • 6
  • 100
  • 200
  • 400
  • 600
  • 800
  • 0
  • Depth (m)
  • Sulphur Springs (2002)
  • 1
  • 3
  • 4
  • 5
  • SL
  • -
  • 2
  • SL
  • -
  • 2
  • 7
  • 6
  • Log Resistivity (ohm
  • -
  • m)
  • 0.5
  • 1
  • 1.5
  • 2
  • 2.5
  • 3
  • 3.5
  • 4
  • 5
  • 1
  • 3
  • 4
  • SL-2
  • SL-1
  • 7
  • 2
  • 6
  • Sulphur Springs
  • 150oC
  • 250oC
  • 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
  • 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
  • 0
  • 2000
  • 4000
  • 6000
  • 8000
  • 10000
  • 12000
  • -300
  • -200
  • -100
  • 0
  • 100
  • 200
  • 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 km
  • 3 km
  • 6 km
  • Target Region
  • 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
  • X1
  • X2
  • X5
  • X3
  • X4
  • X6
  • 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.



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