Petrophysical properties of volcanic rocks and impacts of hydrothermal alteration in the Guadeloupe Archipelago (West Indies)

Abstract

The results of the GEOTREF project presented here aim to develop high enthalpy geothermal energy in fractured reservoirs. They focus on active volcanic islands, especially the Basse-Terre Island (Guadeloupe Archipelago). As part of this project we measured several petrophysical properties to better understand how fluids flow in the hydrothermal system, provide a set of parameters to modelers and build datasets for future interpretation of well logs. The thermo-physical rock properties of bulk and grain density, porosity, permeability, compressional wave velocity, thermal conductivity and diffusivity, specific heat capacity of outcropping volcanic rocks from the Guadeloupe Archipelago (Lesser Antilles) were measured and a dataset was built. According to field observations, samples were grouped into three lithological classes and then separated by their degree of alteration according to macroscopic aspects and optical microscopy results. Geochemical data were obtained, and magnetic minerals identified by combining anisotropy of magnetic susceptibility and variation of magnetic susceptibility with temperature.The lithotypes investigated were lava flows, debris flows and pyroclastics (gathering ash, pumice and scoria deposits). For lavas and debris flows, three degrees of alteration were defined; fresh, slightly to moderately altered and highly hydrothermalized. Only fresh pyroclastics were studied because the altered state was not outcropping.Typically, samples are composed of andesite and belong to tholeiitic and calc-alkaline series. Lava porosity consists of high-density cooling joints and vesicles of different shapes, whereas pores in debris flows and pyroclastics are located both between and within particles. Median porosities and permeabilities are clearly distinct for lavas (4% and 6 · 10−18 m2), debris flows (33% and 4 · 10−14 m2) and pyroclastics (34% and 8 · 10−13 m2). The porosity difference implies a thermal conductivity twice to three times higher for lavas (1.68 W·m−1·K−1). Specific heat capacity of all lithothypes is similar (0.75–0.97 kJ·kg−1·K−1).Advancing hydrothermal alteration removes magnetic minerals and transforms initial rocks. The original plagioclases and pyroxenes in neoformed chlorite, white micas, quartz, epidote and clay minerals are replaced. This modifies the shape of the pore network. Furthermore, hydrothermal alteration results in a reduction of pore throat diameter. It tends to decrease petrophysical differences between lavas and debris flows, increasing the porosity and permeability of effusive rocks, while the opposite effects are observed for debris flows.