Hydrologic and geochemical properties were measured on metagraywacke plug samples of matrix rock from shallow and deep cores from The Geysers geothermal field. Gas permeability ranged from 2x10(-22) to 4x10(-20) m(2) and Klinkenberg slip factor ranged from 1 to 20 MPa. Porosities ranged from 1.9 to 2.2%. Capillary-pressure curves were measured and van Genuchten fitting parameters were used to estimate relative-permeability curves. Shallow samples were weakly altered, but deeper ones were intensely altered and mineralized to chlorite, secondary potassium feldspar and various calc-silicate minerals. Deeper samples were more permeable. and this difference is related to chlorite-potassium feldspar alteration and related porosity enhancement. Published by Elsevier Science Ltd on behalf of CNR.