The impact of diapirism on geothermal reservoir properties of the Estopanyà and Boix synclines (South-Central Pyrenees)

This study investigates petrological and thermophysical properties of rocks from the Estopanyà and Boix synclines (salt basins) to evaluate their potential as analogues for geothermal reservoir. A total of 45 samples were collected, including 26 carbonates, 16 arenites, and 3 altered carbonates (chalky limestones and calcitized dolomites). These samples were classified into eight distinct rock types based on 106 thin sections. Thermophysical measurements revealed mineral densities ranging from 2.64 to 2.72 g cm⁻³ and variable connected porosity (0.50–17.63%), permeability (< 0.001 to 15.30 mD, equivalent to < 10⁻¹⁸ to 10⁻¹⁴ m²), P-wave velocities (1.8–6.6 km s⁻¹ in dry and 2.7–6.3 km s⁻¹ in water-saturated samples), thermal conductivity (2.1–4.7 W m⁻¹ K⁻¹), and specific heat capacity (724–860 J kg⁻¹ K⁻¹). Correlations between thermophysical properties suggest that connected porosity predominantly influences permeability, P-wave velocity, and specific heat. In contrast, thermal conductivity is more dependent on rock composition. Key diagenetic processes such as dissolution, cementation, brecciation, and dolomitization significantly alter rock texture and composition, impacting critical thermophysical properties (e.g., thermal conductivity, permeability, and porosity), essential for geothermal reservoir potential. These alterations are particularly pronounced near the Estopanyà salt wall, indicating that fluid flow along diapir margins intensifies rock alteration. Away from the diapir margin, these effects diminish, underscoring the localized influence of salt diapirism. Results indicate that natural fluid convection likely occurred in two sedimentary units within the Estopanyà and Boix synclines. The first unit, composed of diapir-margin breccias, probably had high permeability in the past, as suggested by its present-day intense cementation. Similarly, the ongoing dedolomitization of these breccias also hints for a past dolomitization in them, which should have enhanced the thermal conductivity of this unit in the past, making it a favorable geothermal target prior to cementation and dedolomitization. The second unit consists of arenites from the Tremp Group, which exhibit sufficient permeability for fluid storage but lack the necessary permeability for natural fluid convection, in the absence of open fractures. These surface data underscore the value of outcrop analogues, demonstrating how petrological insights can reveal past geological processes that influence the thermophysical properties and reservoir potential of salt basins.