The potential of salt-embedded basins for energy transition. The Estopanyà and Boix synclines (South-Central Pyrenees) as outcrop analogues of a geothermal reservoir

Abstract

Salt-embedded basins and their sedimentary successions may play an important role in energy transition as geothermal reservoirs, due to the high thermal conductivity of some evaporites (halite) and the comparatively higher porosity and permeability of basin-filling sedimentary successions. However, outcrop analogue studies on the reservoir potential of salt-embedded basins are scarce. This contribution discusses field (stratigraphy and structural data), petrological and thermophysical data acquired in the Estopanyà and Boix synclines (salt-embedded basins) to evaluate them as geothermal reservoir analogues. Carbonates, arenites, and altered rocks (chalks and calcitized dolomites) were collected and classified into eight rock types according to the description of 106 thin sections. Petrophysical measurements indicate grouped density values and variable connected porosity, permeability, and P-wave velocities. Thermal conductivity is well clustered, whereas specific heat capacity is higher for arenites than for carbonates. Thermophysical correlations reveal that porosity is the key property modifying permeability, P-wave velocity, and specific heat capacity, whereas thermal conductivity is mainly controlled by rock composition. Depositional textures and diagenesis play an important role on rock porosity in Estopanyà. In this sense, intense dissolution, cementation, brecciation, and dolomitization are observed next to the Estopanyà salt wall, suggesting that diapir-related diagenesis likely control the pore-space geometry and connectivity of the flanking sedimentary rocks. The alteration progressively decrease away from the diapir, being the thermophysical properties related to rock depositional textures and non diapir-related diagenesis. Despite the measured low permeability would prevent for fluid convection in the Estopanyà and Boix synclines (i.e., petrothermal systems), two potential geothermal reservoir units are identified due to their higher permeability. The first reservoir unit belongs to the diapir margin breccia units that are intensely cemented in the present-day outcrop. Actual cementation accounts for their past high permeability that likely allowed fluid convection across this unit and along the diapir margin in Estopanyà. In addition, actual calcification indicate a past dolomitization, which likely increased the thermal conductivity and reservoir quality of these breccias. The second reservoir unit are the basin-filling hybrid arenites of the Tremp Group that show moderate-to-high permeability characterising them as a transitional geothermal system with a forced convective heat transfer. The results in Estopanyà would serve as an exploration tool for similar structures worldwide and highlight the importance of considering rock petrology and diagenesis when establishing the controls on reservoir thermophysical properties. Our study presents two new analogue structures and discusses their petrological and thermophysical characteristics, supporting the potential of salt-embedded basins as geothermal reservoirs.