Evaluation of the St. Lucia geothermal resource: geologic, geophysical, and hydrogeochemical investigations Page: 8 of 98
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Analysis and interpretation of hydrogeochemical data from the
Qualibou caldera indicate that a main geothermal reservoir underlies
the Sulphur Springs area and it consists of three two-phase layers:
(1) an upper steam condensate zone; (2) an intermediate vapor zone,
which may be restricted to the Sulphur Springs area only; and (3) a
lower brine zone. Temperatures as high as 2120C were measured at a
depth of 600 m during previous shallow drilling at Sulphur Springs.
Additional evidence indicates that temperatures of the brine layer
may exceed 250 0C.
Three other outlying thermal springs discharge in Qualibou
caldera, including two along the northern caldera collapse zone, but
their geochemistry does not indicate they overlie high-temperature
reservoirs. Rather, they appear to be waters derived from a steam
condensate layer in the vicinity of Sulphur Springs. If this is the
case, it supports the theory that the high-temperature brine upflows
in the area of Belfond-Sulphur Springs and flows laterally northwards
toward the caldera wall.
Each of the scientific investigations yielded individual yet
remarkably similar recommendations for the location of the explora-
tory geothermal welts. The recommendations made by each field team
are covered in greater detail in the appropriate sections of this
report. The selected well site locations are a synthesis- of the
results of the geologic, geophysical, and hydrogeochemical investi-
gations. Based on all available data, the recommended locations for
exploratory drilling in the Qualibou caldera are as follows:
(1) Craters of Beyond - Caldera-related faulting and recent
phreatomagmatic voTcanism indicate fracture permeability, and
low resistivity suggests that geothermal brines occur at a depth
of less than 1 km.
(2) Valley of Sulphur Springs - The presence of hot springs and
fumaroles, the flu chemical compositions, and the low
formation resistivity all indicate a geothermal brine reservoir
near a depth of 2 km with the possibility of a hot dry steam
field above the brine reservoir.
(3) Etang s - The southern caldera fault and a very low shallow
resiistvity suggest a reservoir of geothermal brine at a depth
as shallow as 1 km.
(4) Diamond Spring, Belle Plaine, and the Valley o the Miny River
are also recommended for further exploration to lineate the
size of the Qualibou geothermal reservoir. Initial exploratory
drilling at these locations, however, is not recommended.
In summary, the Qualibou caldera has excellent geothermal
potential and exploratory drilling should result in the discovery of
a high-temperature brine reservoir. Geothermal brines (and perhaps
dry steam) should be found at a depth of 1-2 km under the central and
southern caldera area and in abundance where permeable formations and
faults allow greater fluid movements.
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Ander, M.; Goff, F.; Hanold, B.; Heiken, G.; Vuataz, F. & Wohletz, K. Evaluation of the St. Lucia geothermal resource: geologic, geophysical, and hydrogeochemical investigations, report, August 1, 1984; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1111322/m1/8/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.