Summary of geothermal settingin Geo-Caraïbes target islands

Dr Simon R YoungConsultant to OAS

Presentation made at the Guadeloupe workshop, 27 April 2004

St Lucia - overview• Only one broad target area - Soufrière valley, or

Qualibou Depression• Surface manifestations include hot springs and

fumaroles and diverse warm springs• Now identify major volcanic collapse and debris

avalanche scar filled with more recent deposits (pyroclastics and lava domes) as main feature

• Area is topographically complex. SulphurSprings subject of significant investment for improvement of ‘eco-tourist’ facilities

Soufrière town and Qualibou Depression from the North

Sulphur Springs

3-D rendition of swath bathymetry showing major debris avalanchedeposit offshore of Soufrière. From Deplus et al (2001)

St Lucia - exploration history• ‘50s & ‘60s - General interest and some research• ‘70s - UK-funded project - geology, geophys. &

geochem. and drilling of 7 holes to ~700m depth• Early ‘80s - Exploration projects by RGI (US),

Aquater (Italy) and LANL (US)• Late ‘80s - Two deep holes (~2km) drilled with UN

money; SL1 at Belfond (hot, no perm.), SL2 at Sul. Spr. (hot, acid steam with high non-condensables)

• Early 90s - GENZL review project• Early 00s - MIT run new SP and re-interpret UK

res. lines with state-of-the-art inversion techniques

British well #3 at Sulphur Springs

Contrasting views of the geology of the Qualibou Depression; at leftfrom Tomblin (1964) and at right from Wohletz et al (1986)

Deep well SL-2

MIT re-interpretation of British resistivity line #9 (from Morgan et al.(2004)

St Lucia - resource character• Much exploration has proven a resource, but reveals

challenging chemistry (acid) and permeability issues• Sul. Spr. area has been main focus, but environmental

and acidity problems would be expensive to overcome• New geophysics supports previous interest in Belfond

area not too far from SL1• Lack of deep geophysics makes detailed evaluation

tough but data is consistent with a resource of many 10s of MW

• Main challenge is overcoming complex geology to find permeability and better resource chemistry

MIT Self-potential map for Qualibou. From Vichabianet al. (2004)

Summary of recommended sites or areas for drilling from the various studies undertaken on St Lucia

Dominica - overview• Most volcanically active island in eastern Caribbean• Four main target areas defined - Boiling Lake, Wotten

Waven, Soufrière and Galion, all in south of island• Surface manifestations are numerous in all of these

target areas, and are especially diverse in WW• Geology is complex: Galion is on side of most recent

(~500yr) lava dome; Soufrière valley is a collapse scar with surface emanations around the scarp faults

• WW is associated with ?recent phreatic activity and close to Micotrin, source of major Roseau Tuff eruption; and Boiling Lake is global-scale thermal feature

Map of Dominica showing the volcanoes and surface thermal emanations

Morne Patates volcano from Upper Sulphur Springs

Boiling Lake

Dominica - exploration history• Some UK work in ‘70s as part of regional studies• Main BRGM (French) studies in early ‘80s included

geophysics, geochem. and some geology• Recent geological studies by Alan Smith (UPRM),

SRU (as part of hazards study after ‘98-’99 eqs) and French IPGP group - none geothermal-specific

• SRU has collected useful earthquake data from recent volcano-seismic crises

• Consensus is that both WW and Soufrière have potential - but exploration to date suggests WW is better initial target

Summary of BRGM gravity and resistivity data for WottenWaven prospect

Backscatter image detailing one of the debris avalanche deposits off Dominica. From Deplus et al (2001)

Summary of 1998-99 earthquake epicentres and 1974 swarm (orange). Data from Seismic Research Unit.

Dominica - resource character• Existing data for WW is very encouraging, but need

more geophysics and geology to produce a comprehensive geothermal reservoir model

• Indications suggest a substantial heat source (possibly shared with Boiling Lake) and fault-related permeability around River Blanc area. Geophysics suggests good general reservoir characteristics

• Previous resource estimates of many 10s to several 100s MW are not yet substantiated but are consistent with our existing knowledge

• WW is favourable terrain for exploitation

Map of the Wotten Waven prospect

Boiling pool at Wotten Waven

St Kitts-Nevis overview• Both islands have potentially active volcanoes, though

Nevis considered ‘less dormant’• Mt Liamuiga in northern St Kitts probably has long-lived

magma source (=heat) region beneath, but surface geothermal manifestations are limited to the crater, Brimstone Hill area and wells around Basseterre (+/-offshore vents)

• Nevis Peak dominates Nevis and certainly has a magma storage region 5-10km beneath at 700-800ºC

• Geothermal emanations are diverse, main ones at Farms/Bath Estate & Cades Bay/Estate, other indicators in well water from Belmont area and Saddle Hill. Main surface emanations show structural control

Basseterre, St Kitts from offshore to the south

Farms Estate solfatara on Nevis, with Nevis Peak behind

St Kitts-Nevis exploration• Relatively little work done on either island.

Surface emanations and associated geology are mapped and sampled but no geophysics

• On basis of surface indicators and with some comparative information from the region, Farms/Bath Estate on Nevis is best prospect

• Charlestown fault is major, active structure controlling the emanations. Seismic swarms indicate ‘active’ hydrothermal field and geochemistry is consistent with reservoir temps of 200-250ºC (taking M’rat as a model)

Redevelopment of The Baths with Charlestown Fault scarp behind

St Kitts-Nevis resource character• Main challenge is to collect sufficient information

(mainly through geophysics) to adequately define the resource

• Excellent knowledge of the hydrothermal and volcanic system of neighbouring Montserrat will be of great benefit to modelling Nevis

• Resource estimates of several 10s of MW are consistent with current knowledge

• Farms Estate area adjacent to the soufrière is an ideal location for exploitation if subsurface resources are located there - flat, industrial and close to current generating facilities

Comparison of surface thermal features and structure on Montserrat(red/orange) and Nevis (blue)

Farms Estate thermal features and industrial area