Characterization of gas from seismogenic depths of the San Andreas fault at SAFOD

Models on fault zone permeability and the behavior of faults as barriers or conduits for fluid flow are generally based on geophysical data, laboratory experiments on fault zone rocks, and surface observations from exhumed fault zones. Here, we present a different approach to obtain such information from seismogenic depths of the San Andreas Fault (SAF) by analyzing concentration, composition and spatial distribution of gas extracted from returning drill-mud during drilling of the SAFOD wells (San Andreas Fault Observatory at Depth) and from some drill core samples. SAFOD is a component of U.S. research initiative EarthScope, and supported by the ICDP (International Continental Drilling Program). The SAFOD wells traverse 768m of Tertiary and Quaternary sediments on the Pacific Plate, underlain by Mesozoic granites. The straight SAFOD Pilot Hole was drilled down to 2168m hole depth, whereas the main hole (MH) was deviated to intersect the SAF between approx. 3100–3450m bore hole depth and to penetrate the North American Plate. Below approx. 1900m depth, the MH drilled only sedimentary strata. In 2007, three side tracks (Holes D, E, and G) were drilled to obtain drill core samples from the active moving part of the SAF at seismogenic depths.
The most abundant formation-derived gases in drill-mud of the SAFOD-MH were CH₄ (up to 10vol.-%), H₂ (up to 6vol.-%), and CO₂ (up to 4vol.-%). Carbon isotope data imply an organic origin of hydrocarbons and CO₂ for the entire well. Two gas-rich zones at the margins of the SAF differ significantly in the composition of these gases, which most likely enter the bore hole through bedding-plane fractures in the upper zone (approx. 2700–2900m) and probably also below 3550m. High concentration of hydrogen in these zones might be explained by mechanochemical gas genesis.
The SAF is generally low in gas, with two interstratified more gas-rich sections in 3150— 3200m and 3300-3340 m depth. Separation of two individual hydrogeologic systems by a low-permeable fault core is also indicated by the helium isotopic composition, which is 0.4-0.6Ra on the Pacific Plate and 0.8-0.9Ra on the North American Plate. However, the overall contribution of mantle-derived helium is relatively low. Little gas accumulation in the well during downtime as well as low 222Rn activities furthermore suggest a low-permeable fault. Gas from drill core samples reveal a good correlation of CH4 distribution at depth and C1/(C2+C3) ratios with drill-mud gas analysis from the corresponding SAFOD-III Hole G. Absolute gas concentration in SAFOD drill core samples are in a range typical for sedimentary strata (max. 64mg/g).
We conclude that the center of the SAF is generally low-permeable, with some gas-rich lenses interstratified, but surrounded by more permeable country rock. The gases mostly derive from local sources, whereas the contribution of possible over-pressurized gases from greater depths is low in particular on the Pacific Plate.