Dissolved gases in the DOSECC Cajon Pass well: First year results

Fluid sampled from granitic rock near the 2 km depth in the DOSECC Cajon Pass well contained He, H₂, CH₄, C₂H₆, and C₂H₄ in concentrations much greater than in air-saturated water. The dissolved helium, which ranged in concentration from 0.6 to 0.8 µmole/kg-fluid, was almost entirely radiogenic (³He/⁴He = 0.06 Ra). The δ¹³C-CH₄ value (−36 per mil) and (C₂H₆ + C₂H₄)/CH₄ ratios (0.02 - 0.05) indicate that the dissolved methane (50-55 µmoles/kg-fluid) was produced by thermogenic breakdown of organic matter. Measured pCO₂ values were very low, about 10⁻⁵ atm., and the carbon isotopes (δ¹³C = −18.9 per mil) point to an organic source such as plant root respiration for the dissolved carbonate species. No evidence of mantle volatiles was found despite proximity of the well to the San Andreas fault.