Intrasalt carbonates in the Estopanyà Salt Wall (South-Central Pyrenees): reservoir quality and implications for geothermal exploration in salt diapirs

Abstract

Stringers are disrupted rock bodies commonly enclosed within salt diapirs. Traditionally, they have been regarded as a risk factor in the energy exploitation of salt structures due to their heterogeneity and general sub-seismic scale, which hinder their detailed identification. Nevertheless, rock units in stringers may exhibit sufficient permeability and temperature to act as potential reservoirs, for instance, in geothermal energy. Despite this possibility, studies regarding the reservoir potential of intrasalt stringers remain scarce.The well-exposed Estopanyà Salt Wall in the South-Central Pyrenees contains abundant Muschelkalk carbonate stringers, providing an excellent opportunity to study the geological controls on their reservoir properties. Four lithofacies within two stratigraphic intervals were identified. Depositional lithofacies (DLF-1 and DLF-2) preserve original rock textures, whereas alteration lithofacies (ALF-1 and ALF-2) result from intense brecciation, cementation, and dolomitization-dedolomitization affecting the depositional facies. Thermophysical analyses of 30 samples reveal limited variation in mineral density (2.61–3.00 g cm⁻³), bulk density (1.92–2.95 g cm⁻³), and thermal conductivity (2.37–3.48 W m⁻¹ K⁻¹), with the latter being primary controlled by rock mineralogy. In contrast, connected porosity (0.33–29.20 %), permeability (<0.001–19.85 mD; <10⁻¹⁸–10⁻¹⁴ m²), P-wave velocity (3.7–6.4 km s⁻¹ dry; 3.7–6.5 km s⁻¹ saturated conditions), and specific heat capacity (640–804 J kg⁻¹ K⁻¹) show greater variability. Property cross-plots indicate that porosity percentage and distribution are the main factors controlling the observed thermophysical behavior, which is ultimately linked to rock texture.The area and scarce thickness of porous and permeable intervals (estimated at reservoir volumes of 0.14–4.6 km³), likely preclude the limited reservoir use of the studied stringers. However, petrographic evidence from them highlights interesting processes such as brecciation and dolomitization that may have enhanced the permeability and thermal conductivity in the past, while subsequent cementation and dedolomitization reduced reservoir quality. Accordingly, although current outcrop conditions show diminished reservoir potential, higher porosity-permeability values may be preserved in the subsurface.Overall, this study establishes a conceptual model for the geological controls, limitations, and potential of intrasalt carbonates as unconventional reservoirs, providing comprehensive petrological and thermophysical data that can guide their exploration for geothermal production in salt diapir settings.