Hydrologic characterization of reservoir metagraywacke from shallow and deep levels of The Geysers vapor-dominated geothermal system, California, USA

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 2×10−22 to 4×10−20 m2 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.