Petrologic analyses of corehole Mat-25 provide a detailed look at the influence of lithology, depth of burial, temperature and hydrothermal fluid chemistry on matrix porosity and permeability in a geothermal system. Average porosity values decline from about 10% at the top of the reservoir to 2.5% at the bottom. A similar trend of declining porosity with depth is observed in spot cores from other wells. The trend of declining porosity with depth is ascribed mainly to increasing overburden stresses and chemical reaction with hydrothermal fluids. Local variations in porosity are a complex function of sample-to-sample variations in the original texture, phenocryst abundance, and amount of glass in the primary rocks, and their exposure to tectonic and hydrothermal processes. Matrix permeability mimics porosity, but is very low within the reservoir section (< 10 microdarcy), and even lower above and below the reservoir (< 1 microdarcy) in Mat-25. Image analysis indicates that pores > 1 mum form mainly by dissolution and partial replacement of plagioclase phenocrysts in the andesitic reservoir rocks, with calcic plagioclase being most susceptible. This relationship between hydrothermal alteration and pore formation may link these larger pores to fracture permeability. The size of the pores can be enhanced by further dissolution of the plagioclase and other minerals or sealed by illite, which is supersaturated in the present-day reservoir fluid. (C) 2003 CNR. Published by Elsevier Ltd. All rights reserved.