Quantitative evolution of the petrophysical properties of andesites affected by argillic alteration in the hydrothermal system of Petite Anse-Diamant, Martinique

The evolution of the petrophysical properties of rocks induced by hydrothermal alteration is often considered qualitatively by sorting the hydrothermal alterations encountered from low to high intensity, depending on the identified mineral paragenesis. In this paper, we studied the evolution of three different petrophysical properties (connected porosity, permeability and electrical conductivity) as a function of the argillization degree of andesites affected by argillic alteration identified in the caprock formation of the Petite Anse-Diamant hydrothermal system in Martinique. These petrophysical measurements were supplemented by quantitative mineral evaluation via scanning electron microscopy (i.e., the Quantitative Evaluation of Minerals by Scanning electron microscopy or the QEMSCAN® method), connected porosity mapping using 14C-PMMA method and measurements of methylene blue (MB) values and structural water proportions (H2O+). In the series of rocks investigated, an increasing trend of clay mineral abundances (montmorillonite and kaolinite) from 5.68 to 37.56% was revealed by the QEMSCAN® method. Based on the quantitative results, the structural water proportions were used as a proxy of argillic alteration progression. Comparison between the mineral maps provided by the QEMSCAN® system and connected porosity mapping observed in autoradiographs using 14C-PMMA method revealed a good correlation with clay mineral phases, dissolution vugs and fractures. The connected porosities evaluated with the triple weight method range from 3.74 to 35.35% and show a good correlation (R2 = 0.8835) with the H2O+ values, revealing that porosity development is mainly due to the replacement of primary phases by clay minerals. In contrast, the crystallization of silica, carbonates and, to a lesser extent, iron oxides tends to clog connected pores, inducing a local decrease in connected porosity, as revealed in autoradiographs. The Darcian permeability ranges from 7.66∙10−20 to 5.36∙10−17 m2 and shows a moderate correlation with the connected porosity. The bulk electrical conductivity measured as a function of the conductivity of the saturating solution reveals the significant contribution of surface conduction arising from montmorillonite. At a pore fluid salinity of 2 wt% (the salinity of the Eaux Ferrees thermal spring), the bulk electrical conductivity shows a relatively good correlation with the H2O+ values (R2 = 0.8591) and an even better correlation with the MB values (R2 = 0.9579). The contribution of montmorillonite to the bulk electrical conductivity was estimated with the use of the isoconductivity point, showing an increase in the BM values and a strong correlation (R2 = 0.9806).