The Role of Facies and Composition in Evaluating Geothermal Reservoirs in Fold-And-Thrust Belts (Southern Pyrenees)

The Oliana anticline (10 km wide by 15 km long) is a well-exposed structure located in the northern margin of the Ebro foreland basin, at the footwall of the Southern Pyrenean orogenic wedge. The upper Eocene to Oligocene sedimentary succession of the Oliana anticline brings the opportunity to develop outcrop analogue studies in the proximal margin of a foreland basin and discuss its potential as a geothermal reservoir analogue. The main objective of this contribution is to analyze which processes modified rock thermal conductivity and permeability in the Oliana anticline, which are key factors for the evaluation of geothermal reservoirs. Five sedimentary facies (conglomerates, hybrid arenites, lithic arenites, carbonates and evaporites) were identified in the anticline from the description of 63 thin sections. The mineral density (2334–2767 kg/m³), bulk density (2107–2710 kg/m³), porosity (0.42–22.14%), permeability (0.001–393 mD, equivalent to 10^–19-10^–13 m²) and acoustic velocity (2236–6322 ms⁻¹) of 40 samples and the thermal conductivity (1.846–3.232 Wm⁻¹ K⁻¹) of 35 samples were measured. Acoustic velocities decrease with porosity, and no clear correlation was detected between the other variables. Abnormal petrophysical measures coincide with uncommon porosities. The main processes modifying porosity in Oliana are fracturing, cementation and dissolution. Thermal conductivity is associated with rock composition, where carbonates are less conductive than detrital rocks. The most proximal facies (conglomerates) occurring in the northern limb of the fold, below the main thrust planes, are intensely fractured and calcite-cemented and show low porosities. However, open fractures and stylolites led to uncommon high permeability values. Distal facies (sandstones) at the southern limb of the anticline are less fractured and cemented and display isolated vugs leading to high porosity and low permeabilities. Results suggest that thrust evolution influenced thermal conductivity because the clasts forming the detrital rocks derived from the erosion of different uplifted structural units. Thus, the tectonosedimentary units show differences in quartz and dolostone content, modifying their thermal conductivity. We conclude that thrusting had a strong impact on facies distribution, fracturing and cementation, which are in turn the main processes modifying the thermal conductivity and permeability of the studied folded sedimentary succession.