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

Abstract

Fluid sampled from granitic rock near the 2 km depth in the DOSECC Cajon Pass well contained He, H 2 , CH 4 , C 2 H 6 , and C 2 H 4 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/ 4 He = 0.06 Ra). The δ 13 C-CH 4 value (−36 per mil) and (C 2 H 6 + C 2 H 4 )/CH 4 ratios (0.02 - 0.05) indicate that the dissolved methane (50-55 µmoles/kg-fluid) was produced by thermogenic breakdown of organic matter. Measured pCO 2 values were very low, about 10 −5 atm., and the carbon isotopes (δ 13 C = −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.