Characterization of a naturally fractured ignimbrite reservoir: Subsurface model validated from worldwide analogue outcrops and production data

Abstract

Ignimbrites are deposits found in sedimentary basins globally, commonly linked with evolved volcanism. Understanding fractured ignimbrites in sedimentary basins is crucial for various economic applications, including hydrocarbon extraction, CO2 storage, fractured aquifers, and geothermal resources. Fractured ignimbrite reservoirs typically consist of welded deposits with high transmissibility due to the fracture network, but with low matrix permeability. In this study, we demonstrate how a large-scale rhyolite eruption progressed through varying gas-particle ratios and magma discharge rates, which along with the influence of paleotopography, controlled the type and rate of deposition of pyroclastic density currents (PDCs). These factors determined the distribution and thickness of the ignimbrite unit, as well as the post-emplacement processes, such as cooling rate, welding and devitrification, which influenced the occurrence of discrete discontinuities defining the hydraulic system of the Naturally Fractured Reservoir (NFR). Fracture network is closely related to the cooling stage, composed of hybrid, fluid-assisted fractures and extensional fractures associated with fumarolic system and tensional thermo-elastic geomechanics, respectively. The subsurface model was validated through analysis of reservoir dynamic response and comparative studies of worldwide outcrop analogues, focusing on the determining factors of NFRs. In this case, tectonic fractures resulting from the structural evolution of basins may also influence reservoir conditions, albeit to a lesser extent, affecting fracture porosity and permeability of the NFR. However, these tectonic fractures played a significant role in fluid migration and reservoir charging. We conclude that an ignimbrite geobody, with a low porosity and permeability matrix (Øm ∼1 %; Km ∼0.1 mD), constitutes a NFR, where the fracture network provides a low porosity (Øf ∼0.08%) and a high anisotropic permeability of the reservoir (Kf max ∼ 2100 mD and, Kf Av ∼1500 mD). The obtained results have important implications for the study of fluids transmissibility and storage of ignimbrite reservoirs.