Salt (i.e., halite and other evaporite rocks) is a geological material with physical and mechanical properties of high interest for the oil and gas (O&G) industry (Kirkland and Evans 1981; Hodgson et al., 1992; Jackson and Hudec, 2017). Due to the almost impermeable character of salts, they have been broadly studied for natural gas and petroleum reservoirs as potential seals and hydrocarbon traps (e.g., subsalt traps, Evans et al., 1991; Magri et al., 2008; Urai et al., 2008; Warren, 2016). Moreover, the rheology of halite and other chlorides allows them to migrate from deep to shallower locations, at the time they modify the geothermal gradient due to their high thermal conductivity (i.e., salt-chimney effect, O’Brien and Lerche, 1984; Downs, 2012; Scheck-Wenderoth et al., 2014; Canova et al., 2018; Cedeño et al., 2019; Clauser, 2021; Célini et al., 2024). The low permeability, high thermal conductivity, and plastic rheology of halite are also of high interest in the energy transition to non-fossil energy production (Crotogino et al., 2018; Kukla et al., 2019; Stephenson et al., 2019; Tester et al., 2021; Raymond et al., 2022; Duffy et al., 2023; Hao et al., 2023) (Fig. 1).
In this context, salt diapirs and walls and their flanking sedimentary successions are being proposed for an increasing amount of carbon-neutral and de-carbonization purposes, like the emplacement of salt caverns for subsoil hydrogen storage (Lankof and Tarkowski, 2020; Lankof et al., 2022; Muhammed et al., 2022; Allsop et al., 2023) Carbon Capture and Storage (CCS) (Evans and Holloway, 2009), or geothermal energy production (Huenges, 2016; Daniilidis and Herber, 2017; Duffy et al., 2023). Contrary to the non-porous, low permeable, and highly thermal conductive halite, salt-flanking sedimentary rocks show significant compositional and petrophysical variability depending on factors such as lithology, diagenesis, and fracturing (Sass and Götz, 2012; Moeck, 2014; Rowan et al., 2020a; Mitjanas et al., 2024). Thus, albeit evaporites present no-to-little natural reservoir capacity, sedimentary rocks adjacent to salt diapirs can act as geological storage sites if suitable geometry and minimum thresholds of reservoir porosity and permeability are accomplished (Heinemann et al., 2012, 2021). Therefore, salt structures and their associated sedimentary basins may constitute proper analogues of sedimentary reservoirs sealed by evaporites, which often include high thermal conductive salts such as halite (over 6 Wm− 1K− 1 at 20 ºC, Clauser and Huenges, 1995). This scenario puts salt-embedded basins and minibasins in the focus of geothermal exploration as potential targets (Raymond et al., 2022; Duffy et al., 2023; Marín et al., 2023) (Fig. 1).
<<Fig. 1, 17 cm, colour>>
Diapirs are usually complex and difficult to model due to the occurrence of significant heterogeneities affecting both, the salt structure itself (i.e., the presence of non-evaporitic stringers or changes in the salt-body composition) (Landrø et al., 2011; Van Gent et al., 2011; Burliga, 2014; Raith et al., 2016; Strozyk, 2017; Rowan et al., 2019, 2020b; Cyran, 2021; Pichat, 2022; Cofrade et al., 2023a; Marín et al., 2023) and the adjacent sedimentary successions (e.g., stratigraphic unconformities, strata pinch outs, folding and fracturing, compositional variations near the diapir, sedimentary facies variation and rapid lateral shifts) (Callot et al., 2016; Ribes et al., 2016, 2018; Saura et al., 2016; Teixell et al., 2017, 2024; Cofrade et al., 2023b; Kalifi et al., 2023; Rowan and Giles, 2023; Lartigau et al., 2023; Pichel et al., 2024; Ramirez-Perez et al., 2024). In addition, diapir-influenced diagenesis may also affect the flanking rocks by processes like brecciation and dolomitization, which are common features in worldwide diapir exposures (Posey et al., 1987; Posey and Kyle, 1988; Hudson et al., 2017; Kernen et al., 2019; Moragas et al., 2019; Cruset et al., 2023).
Common approaches for reducing the risk of energy storage and production in diapirs include borehole data acquisition and multiscale geophysical surveys such as seismic or gravimetric methods (Strozyk et al., 2014; Dooley et al., 2015; Jackson et al., 2015; Casas et al., 2016; Santolaria et al., 2016, 2020; Roelofse et al., 2019; Soto et al., 2022; Minougou et al., 2023). Borehole data may provide detailed and continuous information about the sedimentary succession while permitting a punctual study of the reservoir conditions (i.e., temperature, pressure, and water-saturation), fracture patterns, petrophysical properties (i.e., porosity, permeability, and thermal conductivity), and radiogenic production (e.g., Gamma-ray). However, due to the limited dimensions of well-logs (from millimetres to a few meters, Müller et al., 2010; Moore et al., 2011), the representativeness of analysed data may depend on the dimensions of the studied salt-embedded basin and usually assume a high economic cost to cover the complete explored area. Contrarily, geophysical methods such as seismic or gravimetry are useful for exploring meter-to-kilometre-scale areas allowing a good assessment of the structure and broad stratigraphy of salt-embedded basins (Davison et al., 2013). The strong acoustic contrast between the evaporites and other sedimentary rocks allows for a well-definition of the geometry of salt structures and their adjacent basins by seismic methods, allowing the calculation of the stratigraphic thicknesses and the location of the most favourable exploitation points. The resolution of seismic and gravimetry is in the order of 10-to-30 m and 5-to-50 m respectively, depending on the survey, which means that much of the structural, stratigraphic, and sedimentological heterogeneities (sub-seismic scale) are poorly recorded (Jones and Davison, 2014). This scale of heterogeneity is expected to be common in salt-embedded basins owing to rapid sedimentary facies changes outward from the diapirs and occurrence of halokinetic geometries such as halokinetic sequences reducing the stratigraphic thickness in the vicinity of salt structures (Hudec and Jackson, 2007; Andrie et al., 2012; Giles and Rowan, 2012; Jackson and Hudec, 2017; Rowan and Giles, 2023)
This study aims to analyse the potential of salt-embedded sedimentary basins as geothermal reservoirs using the Estopanyà and Boix synclines (South-Central Pyrenees) as field examples. The continuous exposure of the sedimentary succession allows to carry out a complete petrophysical and petrological analysis of the sedimentary units deposited synchronously to salt inflation and subsequent extrusion of the Estopanyà salt wall during the Alpine orogeny (Ramirez-Perez et al., 2024). Specifically, this study aims to 1) establish how rock texture and composition conditioned the petrophysical and petrothermal properties; 2) establish which factors conditioned the most the petrophysical and petrothermal properties of sedimentary rocks; 3) discuss the stratigraphic and structural controls on the reservoir capacity of the Estopanyà and Boix synclines, and 4) determine the suitability of the Estopanyà and Boix salt-walled basins as outcrop analogues of geothermal reservoirs and the overall potential of salt-embedded basins for this purpose.