Etude de l’analogue de r´ ´ eservoir de Terre-de-Haut, Les Saintes

5.2.1 Enjeux

Cette section est consacr´ee `a l’´etude de l’ˆıle de Terre-de-Haut et en particulier de la partie marine de l’ˆıle qui n’a pas pu ˆetre ´etudi´ee lors des relev´es g´eologiques de terrain. Nous avons choisi de pr´esenter ces travaux sous la forme de l’article qui a ´et´e soumis `a Journal of Volcanology & Geothermal Research en 2019. De ce fait, certains ´el´ements d’introduction sont redondants avec les parties pr´ec´edentes de ce manuscrit. L’´etude des transform´ees de carte s’inscrit en continuit´e de la section pr´ec´edente. D’autre part, une technique d’inversion spectrale est propos´ee, appliqu´ee `a des cas synth´etiques puis sur les donn´ees acquises aux Saintes.

5.2.2 High-resolution magnetic mapping using a speed boat: Contri-bution to the geological understanding of Terre-de-Haut, Les Saintes, Lesser Antilles

Authors list: Mercier de L´epinay J.¹, Munschy M.¹, G´eraud Y.², Diraison M.², Navelot V.²

1 Universit´e de Strasbourg, Institut de Physique du Globe de Strasbourg, UMR 7516 CNRS, 5 rue Ren´e Descartes, Strasbourg 67084, France

2 Universit´e de Lorraine, UMR 7359 GeoRessources, Ecole Nationale Sup´erieure de G´eologie, 2 rue du Doyen Marcel Roubault, Vandoeuvre-l`es-Nancy 54500, France

Keywords: magnetic mapping, magnetic method, marine acquisition, spectral inver-sion, map transforms

5.2.2.1 Abstract

Volcanic settings are key environments for geothermal exploitation due to their high heat flows combined with the presence of favorable conduits for fluid circulation. How-ever, seismic prospection in such contexts is demanding. Imaging of reservoir geometry is often uncertain. Moreover, volcanic systems usually have complex three-dimensional geometries and high heterogeneity in their tectonic and lithologic structures as well as poor outcropping conditions, which makes imaging these geothermal reservoirs difficult. High-resolution non-seismic prospecting can be decisive in such environments. In the Les Saintes archipelago, south of Guadeloupe (Lesser Antilles), an outcropping paleo-geothermal system has been identified in Terre-de-Haut Island. Susceptibility

measure-ments suggest that the highly altered materials of the paleo-geothermal reservoir have weak magnetization compared with other volcanic materials on the island: the context is propitious for magnetic exploration. Geophysical surveys offshore Les Saintes show little evidence of deep structures and the correlation with the onshore structures is not clear. To address both the issues of the continuation of deep structures and of the offshore paleo-reservoir, a high-resolution magnetic survey is conducted in the Terre-de-Haut coastal area. To achieve the desired resolution of at least 50 m, an innovative acquisition method is designed using a lightweight three-component magnetometer aboard a small speedboat. The latter’s magnetic effect is compensated with an original process, which allows good data reliability. The obtained dataset is interpreted with potential field map transforms, that display the main structures around Terre-de-Haut. Finally, a spectral inversion method is proposed, derived fromGunn (1975), that gives a repartition of the magnetization intensities. This spectral inversion is illustrated by synthetic cases that match the parameters of the survey and is then applied to the acquired dataset from Terre-de-Haut. The results confirm previous interpretations and validate the offshore continuation of the weakly magnetized body in the central part of Terre-de-Haut, which is interpreted to be a paleo-geothermal reservoir.

5.2.2.2 Introduction

Volcanic fields are one of the main targets for high enthalpy geothermal production Moeck (2014). Prospecting of geothermal plays in a volcanic context is difficult due to the heterogeneity of the volcanic formations and the complex organization of the plumbing network. Seismic prospecting is expensive and of poor quality in this con-text. Development of joint inversion methods for non-seismic data is a main axis, with magnetic, gravimetric or magnetotelluric data. For that, recognition, of the different lithology signatures for each technique is of first importance. Magnetic data can be obtained from airborne, marine or field surveys and are currently used to characterize underground formations (Nabighian et al.,2005;Coyle et al.,2014) and high-resolution magnetic analyses of recent volcanic architectures are increasing in number. Geothermal prospects have been investigated in several islands of the Caribbean archipelago (Guade-loupe, Martinique, Dominique, Saint Christopher and Nevis), and a specific workflow is needed to properly interpret geophysical data and develop new conceptual geological models in andesitic volcanic plays. Terre-de-Haut Island (les Saintes archipelago) is con-sidered as a possible analogue of the geothermal field of Basse-Terre Island (Guadeloupe) and magnetic mapping has been performed to characterize its petrographic and struc-tural elements. Such elements are presented and discussed to develop a methodological approach in the prospect of conceptual reservoir modelling.

In this study, the marine extension of an outcropping paleo-hydrothermal system is investigated using high-resolution magnetic acquisition and processing. Geological and

magnetic susceptibility studies reveal that such data can help the understanding of the hydrothermally altered material that underlies the Les Saintes reef plateau. Because the area to be surveyed is shallow and near the coasts, a specially designed survey method is developed to acquire high-resolution marine data. Different specific corrections are applied before the interpretation to highlight lithotypes and structural features used for fluid circulation. Magnetization is then computed in the spectral domain on a flat surface, and the methodology is assessed via a synthetic case. These results are then compared with the workflows of classic interpretations.

5.2.2.3 Regional setting and geothermal system

Figure 5.25 – (a) Geodynamic setting of the Lesser Antilles arc (Pindell et Kennan, 2009 ; Corsini et al.,2011). Les Saintes archipelago is squared with black and Terre-de-Haut is its most easterly island. (b) Tectonic structures in Guadeloupe archipelago (Feuillet et al., 2002;Bazin et al.,2010;Mathieu et al., 2013). MBS = Montserrat-Bouillante-Les Saintes.

The convergent boundary between Caribbean and North American plates has been responsible for the formation of two distinct volcanic arcs through time (Bouysse et al.,

1986 ; DeMets et al., 2000) (Figure 1-a). Guadeloupe is located at the meeting point of these two arcs. Whereas the eastern islands Grande-Terre and Marie-Galante can be associated with the ancient and extinct arc, the western island Basse-Terre and the Les Saintes archipelago are parts of the recent and active arcs (Bouysse et Westercamp,1990

; M¨unch et al., 2014,2013). The important heat flow due to the geological activity in Basse-Terre creates an optimal context for geothermal exploitation (Bouchot et al.,2010 ;Lopez et al.,2010a). In 1985 a geothermal plant was set up in Bouillante (Figure 1-b) for electricity production: it was able to provide 7% of the island’s electricity in 2015 with a capacity of approximately 16 MWe (Sanjuan et Traineau,2008). However despite the geoscience studies that have been conducted in Basse-Terre (Lopez et al., 2010a ; Thinon et al.,2010 ; M¨unch et al., 2013 ;Deparis et al.,2014 ;Legendre et al., 2014; Gailler et al.,2014), knowledge of the Bouillante geothermal system remains limited due to the complexity of the structural and geological background and to the outcropping conditions (Calcagno et al.,2012).

In the Les Saintes archipelago, located 15 km to the south of Basse-Terre (Figure 1-b), a geothermal paleo-system was identified by (Verati et al.,2016) in Terre-de-Haut Island. For this reason, the study of Terre-de-Haut can aid understanding of hydro-thermal systems in volcanic settings. Moreover, this area could be a paleo-analogue of the geothermal system in Basse-Terre (Verati et al.,2016).

The study of such an analogue is essential to better understand the geothermal resource of Guadeloupe. The geothermal reservoir that is exploited in Bouillante remains active today and has been thoroughly studied. However, the onset of hydrothermal activity is still uncertain. Recent studies have determined that the geothermal system began its activity 0.25 Ma ago (Verati et al.,2014). The latest interpretations in Terre-de-Haut state that the paleo-analogue in Les Saintes has been functioning for 0.4 My in the Pliocene (from 2.4 to 2 Ma) and is currently inactive (Verati et al.,2016).

Terre-de-Haut geology

The Les Saintes archipelago is surrounded by an isolated carbonate reef plateau (Leclerc et al.,2014) and includes two main islands, Terre-de-Haut and Terre-de-Bas and several smaller islets. They are the emerged remnants of volcanic edifices (Jacques et al., 1984 ;Jacques et Maury,1988). Zami et al. (2014) identified three volcanic phases on Terre-de-Haut, all of them dated between 2.00 and 2.98 ± 0.04 My. The age uncertainties are large since they are inferred from a reduced number of samples. Offshore, the carbonated terraces that form the seafloor are reef structures that are considered as non-magnetic even in the absence of magnetization or non-magnetic susceptibility measurements. These platforms are 250 m thick and were emplaced during the Upper-Pleistocene to the Holocene (figure 5.26).

Large scale offshore geophysical studies (Feuillet et al., 2002 ; Gailler et al.,2014 ; Leclerc et al.,2014,2016 ;Barnoud et al.,2017) have shown a significant NW-SE fault network. Leclerc et al. (2014, 2016) noted several bathymetric steps off the shore of Terre-de-Haut at shallow depths (0 to 40 m) from a high precision bathymetric study. These authors also delineated all the visible faults in the Les Saintes plateau with a main