Proceedings World Geothermal Congress 2020

Reykjavik, Iceland, April 26 – May 2, 2020

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Fluids and Heat Transfer Properties and Associated Petrophysical Properties for Different

Structural and Petrographic Facies of Volcanic Formations, Contribution to Building a High

Temperature Geothermal Reservoir Model

Yves Geraud, Vivien Navelot and Marc Diraison

GeoRessources, Université de Lorraine, ENSG, 2 rue du doyen Roubault, Vandoeuvre les Nancy

yves.geraud@univ-lorraine.fr

Keywords: volcanic rocks, porosity, permeability, thermal conductivity, resistivity, geological model

ABSTRACT

Volcanic edifices are recognized to represent a preferential target for geothermal play location. Geothermal reservoirs in this context

are clearly heterogeneous and composed by lithological elements and structural patterns. Circulation of hot hydrothermal fluids

induces strong transformation of the mineralogical content and of the structure and texture of the materials. These transformations

control sharply the petrophysical properties and especially the transfer ones, for fluids and heat.

Caribbean archipelagos are an important target for development of high enthalpy geothermal resources. In the objective of prospecting

action in Guadeloupe islands (Basse-Terre and Les Saintes islands), the GEOTREF program is set up with the support of the French

government.

To contribute to a characterization of these reservoirs, a large set of lithology is sampled from fresh and hydrothermalized facies.

Fresh materials have mainly calc-alkalin composition with lava and ignimbritic-debris flow textures and are principally sampled on

Basse-Terre Island while the hydrothermal facies of these both textures are sampled on Les Saintes Island. By this way, we assume

that Les Saintes system is an exhumed analogue for a potential active geothermal system on Basse-Terre Island like Bouillante system

with its active power plant.

For fresh facies, porosity ranges between 0 to 30 % and 10 to 65% for lava and ignimbritic-debris flow materials respectively, density

ranges between 2.7 and 2 g.cm-3 and from 1.8 and 0.5 g.cm-3, permeability ranges between 10-19m2 and 10-12m2, and from 10-14 to 10-

11m2, thermal conductivity ranges from 2 and 1.3 W.m-1.K-1 and from 1.4 and 0.4 W.m-1.K-1. A large variability of these properties is

observed within the same lava flow.

Hydrothermalized facies, as well for lava and ignimbritic-debris flow protoliths, show intermediate values between the ranges of the

both fresh facies. Hydrothermal alteration could be divided in two main groups of processes, dissolution and secondary minerals

crystallization. First, dissolution is the main process affecting lava and increases the porosity and the permeability, decreases density,

thermal conductivity and propagation speed of the acoustic waves. Second, crystallization of secondary minerals is the main process

affecting the porous network of ignimbritic-debris flow textures, it decreases weakly porosity and permeability and increases its

density and its thermal conductivity.

In relation with the local structural pattern, the organization of the different facies, fresh and hydrothermallized, are localized in a

conceptual geological model, their petrophysical properties are used to propose a first shape of the network usable for potential heat

and fluid exchanges.

1. INTRODUCTION

Volcanic systems are preferred targets for prospecting for high temperature geothermal energy because they are associated with large

heat flows. These systems are abundant in subduction contexts and resources are known to exist in Caribbean arc systems and several

sites are being explored. In Guadeloupe, only the boiling site is currently being exploited, an exclusive prospecting license is being

developed. As part of this permit, an innovative research program has been set up, the Geotref program, financed as part of the French

government's investment program for the future. The Geotref program aims to propose new concepts for prospecting and modelling

of high-temperature geothermal resources. Several presentations are offered as part of the WGC2020.

To improve the location of reservoirs and resource estimation, it is necessary to acquire new data on both structural and facial

organization, but also on the petrophysical characterization of the different facies. The latter will also contribute to the interpretation

of subsurface geophysical data.

The approach used includes the study of a surface analogue and the characterization of different facies; and the comparison of these

data with those acquired at the study site with structural geology and subsurface geophysics elements. In the case of the area surveyed

on the island of Basse-Terre, a analogue one has been recognized on the island of Terre-de-Haut in the Saintes archipelago, located

south of Basse Terre island (figure 1). The hydrothermalized zone of Terre-de-Haut was the subject of a structural analysis, the

sampling of a selection of facies representative of the variability of these facies of petrographic, structural or hydrothermal origin.

These data will be compared with those acquired on site, to interpret the geophysical data and build a geological model of the reservoir

that is in place.

Géraud, Navelot and Diraison

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2. GEOLOGICAL CONTEXT

The Guadeloupe archipelago is located at the level of the subduction of the North American plate under the Caribbean plate. This

subduction is done at a slow rate of 2cm/year (DeMets et al 2000, Symithe et al 2015), it has produced two volcanic systems, the first

in the East which operated from Eocene to Miocene, the second in the West includes the active systems of La Soufrière and Les

Saintes whose activity began at the end of the Tertiary period.

Structurally, the study area is located at the intersection of four major regional structures: the grabens of Marie Galante, Bertrand-

Falmouth and Les Saintes, and the sinister transtensive zone of Montserrat-Bouillante (figure 1). Thus, four directional families of

structures are described regionally, N10-N160, N50-N70, N90-N110, N120-N140.

The volcanic activity of the Basse-Terre of Guadeloupa has led to the installation of a series of 6 composite buildings generally

aligned in a NNW-SSE direction. It is characterized by effusive activity that produces large volumes in the form of domes, lava flows

and pyroclastics, the sequence is calc-alkaline. The system of the Saintes archipelago is of the same composition and the beginning

of activity is synchronous with that of the Lower Earth (Jacques et al., 1984; Zami et al., 2014, Samper et al. 2007). The central part

of the island of land above the archipelago of the saints has been affected by intense hydrothermal activity that has totally transformed

the rock Verati et al (2016).

The hydrothermal alteration sequence consists of 3 steps. The first is associated with the crystallization of chlorite and epidote at the

expense of plagioclase feldspar and pyroxenes. The second step is associated with the precipitation of chlorite, serpentine, and oxide

at the expense of pyroxenes, with the precipitation of chalcedony, pyrite, quartz, smectite chlorite and goetite in the crack networks.

The third association is composed of clay, smectite, kaolinite and illite. High concentrations of gypsum, pyrite and native sulphur are

observed in the most altered areas.

Figure 1: location of the studied areas, Basse-Terre island and Terre-de-Haut island, Les Saintes archipalego.

3. MATERIAL AND METHODS

For the both areas, the structural networks are defined at the regional and local scales. Petrophysical properties are also measured on

a set of selected samples representative of the different petrology and alteration facies.

Structural analysis was conducted at two scales, the regional scale with detection of lineaments on satellite images and at the outcrop

scale with measurements of at least 100 plans. The objectives are the recognition of corridors at the regional scale and the definition

of the local fracture network.

A large set of petrophysical properties are measured. Magnetic susceptibility is measured on outcrops with a GFSM-20 Instruments

model handheld magnetic susceptibility or on sample with a MFK1-A Kappabridge. The SM-20 has 10−6 SI (International system

of units) sensitivity in the range of 0–999 · 10−3 SI. Apparent density and porosity are measured by triple weighing with water as

saturation fluid on sample with volumes between 10 and 50 cm3. Grain density was measured using a Micromeritics®Accupyc II

1340 gas pycnometer. Porosity was also measured by mercury injection with a Micromeritics® model AutoPore IV 9500 v1.09. This

model investigates pore throat diameters from 360 μm to 5.7 nm. Matrix permeability is performed with a nitrogen permeameter

under low confining pressure (20 bars). The permeability measurements did not exhibit Klikenberg effect, indeed it was not taken

into account. This covers a wide range of permeability (10−11 m2 to 10−19 m2). Permeability measurements were performed on 25

mm diameter plugs. Thermal conductivity λ and diffusivity D were measured using a Thermal Conductivity Scanner from Lippmann

& Rauen GbR. The device has an accuracy of 3% in the measuring range from 0.2 to 25 W·m−1·K−1. The procedure is described in

detail by Popov et al. (2016, 1999). Temperature measurement is every 1 mm step. The measurement of compressional wave velocity

(P-wave) was performed on dry samples using the ultrasonic velocity method with the Proceeq® brand PunditLab model. The

instrument is composed of two 54 kHz piezoelectric P transducers (a transmitter and a receiver) and a pulse generator.

Géraud, Navelot and Diraison

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4. RESULTS

The different elements useable for fluid flow are analyzed, first of all the fault and fracture pattern at the regional and local scales

and the petrophysical properties of the different petrographic and structural facies (Navelot ,2018; Navelot et al., 2018).

4.1 Structural patterns

At the regional scale, the structural pattern of the south-western part of Basse-Terre Island is mainly controlled by the four fractures

families, with the N-S and the E-W as major sets while the two others are minor sets. The outcrops data sets confirm these directions

and highlight the controls of the close major structure on the local distribution (figure 2).

Figure 2: structural data for the Basse-Terre island, fault structures obtained from geological maps and lineament analysis,

and stereogram of the outcrop analysis.

At the regional scale the structural pattern of the Terre-de-Haut island is mainly controlled by N-S and E-W structures, the N40 and

the N120 direction seem to be secondary structure sets. Outcrop data confirms the four directional families. The presence of a close

macrostructure controls the distribution of fractures locally. The highly hydrothermalized area is located at the intersection between

the two main structure orientations (figure 3).

Figure 3: structural data for the Terre-de-Haut island, fault structures obtained from geological maps and lineament analysis,

and stereogram of the outcrop analysis.

For the both islands, the structural patterns are quite similar, with the same directional families of fault and fracture.

4.2 Petrographic characteristics

From the chemical analysis, the materials of the both areas belong to the medium to low K-types.

From petrographic analysis, lavas flows are composed by 30% of phenocrysts (80% plagioclases, 15% pyroxens and 5% of iron

oxides), the mesostase is composed by the same set of minerals without glass. Fresh debris flows and pyroclastics consist of the same

minerals’ assemblage as the lava flows. The highly hydro thermalized facies contain large amount of illite, chlorite, smectite replacing

Géraud, Navelot and Diraison

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the primary minerals in the matrix and gypsum, pyrite and calcite in fractures. Intermediate facies present variable contents of

secondary clays. Hand sample views of the different facies are presented on figure 4.

Figure 4: Hand sample views of the different facies observed on the both islands.

4.3 Petrophysical properties

These properties were determined on more than 200 samples, data are summarized on the tables 1 and 2, Navelot (2018),

Navelot et al (2018).

Table 1: Summary of magnetic susceptibility measurements on outcrops and blocks with handheld susceptibility meter and

using AMS.

The magnetic susceptibility for the fresh facies is high (from 11.6 to 6.6 S.I.) and decreases over deux orders of magnitude for the

hydro thermalized facies (from 0,14 to 0,06 S.I.). These characteristics should be used to assess the magnetic maps of the sector (see

Mercier de Lepinay et al., 2020).

Petrophysical properties are discussed through the figure 5. All the properties range over large intervals. the relationship between the

porosity and the bulk density illustrates the sharp dependence between these parameters (figure 5a). The solid phase density is

relatively homogeneous varies less and it is calculated as the ordinate at origin than the bulk density showing that the solid phase

composition is relatively constant (figure 5b). The relationships between the porosity and the permeability, the thermal conductivity

and the Vp (figure 5c, and d) are decomposed in two parts. The first one is for the lava flows, the second one is for the pyroclastics

and debris flows facies. Globally, the fresh lava flow and the fresh pyroclastic facies act as end members for the different petrophysical

properties, and for example, porosity is low for lava flow and is high for pyroclastic materials. Hydrothermal alteration homogenizes

the properties toward intermediate values.

1 cm

Alt

erat

ion

Lava flows and dykes Debris flows + pyroclastics

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Figure 5: petrophysical characteristics of the samples

5. GEOLOGICAL MODELS

The data acquired on the island of Terre-de-Haut make it possible to propose a geological model of the organization of facies and

structures (figure 6). The exhume d hydrothermal zone is located at the intersection of major structures E-W and N-S.

This intersection acts as a conduit for the rapid migration of deep fluids. These fluid flows lead to a complete transformation of the

rock, including the deposition of gypsum, pyrite and Sulphur in the area where the hottest fluids have circulated. These hydrothermal

transformations make it possible to maintain important transfer properties. They are also made of materials with low magnetic

susceptibility. Laterally, hydrothermal fluids could have spread by following fault zones that show markers of hydrothermal

transformations. The magnetic mapping presented by Mercier de Lepinay (2019) and Mercier de Lepinay et al (2020) makes it

possible to propose an extension of this area at sea. This model will serve as a basis for the interpretation of the Basse-Terre data.

2.0

2.2

2.4

2.6

2.8

3.0

0.5 1.0 1.5 2.0 2.5 3.0

So

lid

ph

ase

den

sity

Bulk density

1E-04

1E-03

1E-02

1E-01

1E+00

1E+01

1E+02

1E+03

1E+04

1E+05

1E+06

1E+07

0 20 40 60 80

Perm

ea

bil

ity

[m

D]

Mercury porosity [%]

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

0 20 40 60 80

Th

erm

al

co

nd

ucti

vit

y [

W.m

-1.K

-1]

Mercury porosity [%]

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

0 2000 4000 6000

Th

erm

al

con

du

ctiv

ity

[W

.m-1

.K-1

]

P-wave velocity [m/s]

d

a

b

c

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Table 2: Summary of petrophysical properties measurements on blocks.

Figure 6: Geological model for Terre-de-Haut island.

REFERENCES

DeMets, C., Jansma, P.E., Mattioli, G.S., Dixon, T.H., Farina, F., Bilham, R., Calais, E., Mann, P.: GPS geodetic constraints on

Caribbean-North America Plate Motion. Geophysical Research Letters, 27, (2000), 437–440.

Jacques, D., Maury, R.C., Bellon, H. :Geologie et geochronologie 40K-40Ar des iles des Saintes (Guadeloupe). Comptes-rendus des

seances de l'Academie des sciences. Serie 2, Mecanique-physique, chimie, sciences de l'univers, sciences de la terre, 299, (1984),

721–726.

Mercier de Lepinay J. : Acquisitions et interpretations magnetiques pour l’exploration geothermique en Guadeloupe, Petites Antilles,

PhD thesis, Univ. Strasbourg, (2019), 304 p.

Géraud, Navelot and Diraison

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Mercier de Lepinay et al.: A Geothermal Reservoir Model in Volcanic Environment: Input of Structural, Petrophysical and Magnetic

Data (2020), WGC2020, (2020).

Navelot, V.: Caracterisations structurale et petrophysique d'un systeme geothermique en contexte volcanique d'arc de subduction,

Exemple de l'archipel de Guadeloupe, PhD thesis, Univ. Lorraine, (2018), France, 440 p.

Navelot, V., Géraud, Y., Favier, A., Miraison, M., Corsini, M., Lardeaux, J-M., Verati, C., Mercier de Lépinay, J., Legendre, L.,

Beauchamps, G. : Petrophysical properties of volcanic rocks and impacts of hydrothermal alteration in the Guadeloupe

Archipelago (West Indies), Journal of volcanology and geothermal research, 306, (2018), 1-21.

Popov, Y.A., Pribnow, D.F.C., Sass, J.H., Williams, C.F., Burkhardt, H., 1999. Characterization of rock thermal conductivity by

high-resolution optical scanning, Geothermics, 28, (1999), 253–276.

Popov, Y.A., Beardsmore, G., Clauser, C., Roy, S., 2016. ISRM suggested methods for determining thermal properties of rocks From

laboratory tests at atmospheric pressure, Rock Mechanics and Rock Engineering, 49, (2016), 10, 4179-4207.

Samper, A., Quidelleur, X., Lahitte, P., Mollex, D.: Timing of effusive volcanism and collapse events within an oceanic arc island:

Basse-Terre, Guadeloupe archipelago (Lesser Antilles Arc). Earth Planetary Sciences Letters, 258, (2007), 175–191.

Symithe, S., Calais, E., de Chabalier, J.B., Robertson, R., Higgins, M., 2015. Current block motions and strain accumulation on active

faults in the Caribbean. Journal of Geophysical Research, Solid Earth, 120, (2015), 2014JB011779.

Verati, C., Mazabraud, Y., Lardeaux, J.-M., Corsini, M., Schneider, D., Voitus, E., Zami, F., 2016. Tectonic evolution of Les Saintes

Archipelago (Guadeloupe, French West In- dies); relation with the Lesser Antilles arc system. Bulletin de la Société Geologique

de France, 187, (2016), 3–10.

Zami, F., Quidelleur, X., Ricci, J., Lebrun, J.-F., Samper, A.: Initial sub-aerial volcanic activity along the central Lesser Antilles inner

arc: new K–Ar ages from Les Saintes volcanoes, Journal of volcanology and geothermal research, 287, (2014), 12–21.