No abstract

<p>In Lesser Antilles arc several volcanic islands present high potential for the high enthalpy geothermal production. Saint-Kitts Island is one of them, where numerous surface evidences of active hydrothermal system such as fumaroles, boiling water are present. All of these activities are structurally controlled. Field analysis highlights four main families of faults or fractures with NE-SW, NW-SE, N-S, and E-W trends respectively.</p><p>The interaction of fluids (rain and sea waters) with host rock leads to meteoric and hydrothermal alterations of rocks according to depth of infiltration, usually consisting in their argillization. The characterization of these alterations allows a better understanding of the hydrothermal system of St Kitts and to estimate the geothermal potential of the resource in this island.</p><p>In the Frigate Bay area, in the southern part of the island, the rocks present an intensive structural and petrological transformations related to hydrothermal fluid flows. From study of mineralogical transformations with microscopic, SEM, EPMA, Raman and XRD analysis, we identify clays minerals (kaolinite, smectite), sulphates (jarosite, alunite, gypsum), quartz, opal, calcite and chlorite. These paragenesis are consistent with fluids above 100°C and allow to constrain the spatio-temporal activity of the hydrothermal system and the geothermal reservoir. Petrophysical properties, on a selected set of representative petro-structural facies, show large ranges of variation, porosity from 2 to 40%, permeability from 10<sup>-3</sup> to 3 D; grain density between 2.84 and 2.31 g.cm<sup>-3</sup>, thermal conductivity is relatively low, 0.5 to 2 W.m<sup>-1</sup>.K<sup>-1</sup>. Samples alteration results in increasing of porosity and decrease in density. In turn the porosity increasing causes a decrease in the thermal conductivity.</p><p>These investigations allows us to interpret this site as part of a hydrothermal paleosystem and consider as an analogue of the deep northern part of the island under a current hydrothermal activity.</p>

Abstract. Lake Abhe is sitting on the Gob Aad graben within the tectonic Afar triangle in the Republic of Djibouti. It is known for its exposures of massive hydrothermal chimneys on the lake’s eastern shore. The many hydrothermal surface manifestations on this side of the lake, including steam vents, hot springs and carbonate chimney structures, reflect the geothermal field of this area. This study describes the structural settings of the Lake Abhe Geothermal Field (LAGF), using multiscale structural lineament distribution mapping. It also investigates the hydrothermal surface manifestations distribution in order to specify structural controls on local fluid flow and discuss its evolution. Structural features of the LAGF area are dominated by ESE-trending extensional faults that form a series of narrow elongated horst, graben and half-graben structures. Fault interaction and accommodation zones, as well as fault intersections, relay ramps and possible breaching faults are also recognized and may represent interesting structural features in terms of fluid flow pathways. Hydrothermal chimneys and hot springs distribution over the LAGF area is controlled by the main structural trends, and show signs of higher hydrothermal activity located at intersecting structural traces. Field observations, in conjunction with satellite images analysis, suggest a progressive lateral evolution of the LAGF hydrothermal fluid outflows over time. Therefore, this study provides new insights on the local tectonically driven fluid flow of the LAGF, that may support further exploration of this remarkable site and may promote its geothermal development.

Summary The petrophysical properties of 41 volcanic samples from La Soufrière volcanoe (Guadeloupe Island, Eastern Caribbean, France) are investigated. We first measure the complex conductivity spectra of these rock samples at 4 salinities (NaCl) at laboratory conditions (∼20° C). For each rock sample, we determine the (intrinsic) formation factor, the surface conductivity, and the Cole Cole normalized chargeability (Ghorbani et al., 2007). We also measure the compressional wave velocity (dry and saturated), the shear wave velocity in saturated conditions, the (dry and saturated) thermal conductivity, the dry specific heat capacity, and the permeability of the rock samples as well as their cation exchange capacity (CEC) and connected porosity. The formation factor versus porosity obeys Archie's law with a cementation exponent of 2.16 ± 0.10. The surface conductivity and the normalized chargeability are proportional to each other and to the CEC divided by the tortuosity of the material (product of the formation factor by the connected porosity) as predicted by the dynamic Stern layer model. Permeability can be predicted from the normalized chargeability and the formation factor inside one order of magnitude. The thermal conductivity and the seismic properties can be evaluated from the connected porosity of the core samples formation factors. A non-linear relationship is established between the shear wave velocity and the compressional wave velocity for the present dataset and other data from the literature. Finally, we show on a specific example, how to convert an induced polarization survey on a strato-volcano into a seismic velocity model (P- and S-waves velocity distributions). We perform a specific application to Papandayan Volcano, a strato-volcano located in Java Island (Indonesia). This work paves the way to the joint inversion problem of seismic and induced polarization surveys for volcanic unrest monitoring.

<p>Nuclear magnetic resonance (NMR) is being used since 1990 in the petroleum industry. NMR is a powerful tool for petrophysical properties estimation (porosity, permeability, pore size distribution, and irreducible saturation). Despite its large success in the conventional carbonate and sandstone reservoirs, some tight sandstones, volcanic and metamorphic rocks, contain a high amount of paramagnetic and clay minerals, which can complicate the interpretation of NMR results. These complications are due to the inhomogeneities of the internal magnetic field generated by the magnetic susceptibility contrast between the pore-fluid and the matrix. The magnitude of the internal gradients depends on the strength of the background magnetic field, magnetic susceptibility contrast, and pore size.</p><p>Many studies are focused on the investigation of the effect of clay and paramagnetic minerals on the internal gradient and their implications on the NMR-derived petrophysical properties mainly of the high magnetic susceptibility sandstones. The primary goal of this analysis is to investigate the magnitude of the internal magnetic gradient of volcanic rocks with different alteration grad and its relationship with the rock properties (magnetic susceptibility, iron, and manganese content, pore type, and pore size).</p><p>The data were collected using the Minispec q10®, with Larmor frequency of 10 MHz, on the water-saturated samples with magnetic susceptibility between 26.8 10<sup>-3</sup> and -0.4 10<sup>-3</sup> SI. The average effective internal gradient was calculated from the slope of the mean log relaxation rate (T2gm<sup>-1</sup>) versus the squared echo time (TE<sup>2</sup>). The preliminary results show that samples presented a multi-distribution of T2 peaks corresponding to the different pore types observed for these samples (micro, meso, and macropores). The average effective internal magnetic field gradient calculated from the slope of T2gm<sup>-1</sup> vs TE2 ranges from 0 to 43.16 T.m<sup>-1</sup>. The average effective internal gradient increases with the increase of magnetic susceptibility and decreases as the T2gm increase, suggesting that the pore size also impact internal gradient magnitudes. However, No clear relation exists between iron content and average effective internal gradient.</p>