Real-time mud gas logging during drilling of the SAFOD Pilot Hole in Parkfield, CA

The real-time mud gas monitoring during drilling of the 2.2 km deep SAFOD Pilot Hole has proved successful. A nearly complete depth profile was obtained for methane, radon, helium, and, with limitations, for carbon dioxide. Within the sedimentary section (0-768 m) mud gas yielded the highest CH4, CO2, and Rn-222 concentrations, whereas He was comparably low. Four major gas-rich zones were identified. Methane is a mixture of microbial and thermogenic origin. Mud gas concentrations of CH4 and Rn are much lower in the underlying granitic rocks and the average background level of He is higher. In addition to several gas-rich zones, two shear zones inferred from geophysical measurements are also indicated by increased mud gas concentrations. Gas results permit speculation that one major shear zone is hydraulically connected to the sedimentary section above. Up to 9% of the helium is of mantle origin, which is probably a lower limit.