Fluid geochemistry and soil gas fluxes (CO2-CH4-H2S) at a promissory Hot Dry Rock Geothermal System: The Acoculco caldera, Mexico
Por:
Peiffer L., Bernard-Romero R., Mazot A., Taran Y.A., Guevara M., Santoyo E.
Publicada:
1 sep 2014
Resumen:
The Acoculco caldera has been recognized by the Mexican Federal
Electricity Company (CFE) as a Hot Dry Rock Geothermal System (HDR) and
could be a potential candidate for developing an Enhanced Geothermal
System (EGS). Apart from hydrothermally altered rocks, geothermal
manifestations within the Acoculco caldera are scarce. Close to ambient
temperature bubbling springs and soil degassing are reported inside the
caldera while a few springs discharge warm water on the periphery of the
caldera. In this study, we infer the origin of fluids and we
characterize for the first time the soil degassing dynamic. Chemical and
isotopic (delta O-18-delta D) analyses of spring waters indicate a
meteoric origin and the dissolution of CO2 and H2S gases, while gas
chemical and isotopic compositions (N-2/He, He-3/He-4, C-13, N-15)
reveal a magmatic contribution with both MORB- and arc-type signatures
which could be explained by an extension regime created by local and
regional fault systems. Gas geothermometry results are in agreement with
temperature measured during well drilling (260 degrees C 300 degrees C).
Absence of well-developed water reservoir at depth impedes
re-equilibration of gases upon surface. A multi-gas flux survey
including CO2, CH4 and H2S measurements was performed within the
caldera. Using the graphical statistical analysis (GSA) approach, CO2
flux measurements were classified in two populations. Population A,
representing 95% of measured fluxes is characterized by low values
(mean: 18 g m(-2) day(-1)) while the remaining 5% fluxes belonging to
Population B are much higher (mean: 5543 g m(-2) day(-1)). This low
degassing rate probably reflects the low permeability of the system, a
consequence of the intense hydrothermal alteration observed in the upper
800 m of volcanic rocks. An attempt to interpret the origin and
transport mechanism of these fluxes is proposed by means of flux ratios
as well as by numerical modeling. Measurements with CO2/CH4 and CO2/H2S
flux ratios similar to mass ratios of sampled gases were considered as
reflecting advective transport. A numerical model of CO2 migration in
the subsoil system under fully water and gas saturated conditions was
performed using the TOUGH2 code in order to reproduce
semi-quantitatively field measurements. The main results show that high
flux values produced by advective geothermal degassing can be very
localized and that low and heterogeneous permeability conditions can
induce low advective CO2 flux values. Therefore, in this case the
populations discriminated by the GSA method should not be interpreted in
terms of origin and/or transport mechanism but rather in terms of
permeability conditions. (C) 2014 Elsevier B.V. All rights reserved.
Filiaciones:
Peiffer L.:
Univ Nacl Autonoma Mexico, Inst Energias Renovables, Temixco 62580, Morelos, Mexico
Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Privada Xochicalco s/n, Centro, 62580 Temixco, Morelos, Mexico
Bernard-Romero R.:
Univ Nacl Autonoma Mexico, Inst Geofis, Mexico City 04510, DF, Mexico
Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México D.F., Mexico
Mazot A.:
Department of Volcanology, GNS Science, Private Bag 2000, Taupo 3352, New Zealand
Taran Y.A.:
Univ Nacl Autonoma Mexico, Inst Geofis, Mexico City 04510, DF, Mexico
Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México D.F., Mexico
Guevara M.:
Univ Nacl Autonoma Mexico, Inst Energias Renovables, Temixco 62580, Morelos, Mexico
Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Privada Xochicalco s/n, Centro, 62580 Temixco, Morelos, Mexico
Santoyo E.:
Univ Nacl Autonoma Mexico, Inst Energias Renovables, Temixco 62580, Morelos, Mexico
Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Privada Xochicalco s/n, Centro, 62580 Temixco, Morelos, Mexico
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