Evaluating the San Andreas Fault Zone Permeability Structure by Using On-Line mud gas Monitoring Data
To archieve a better understanding of the permeability structure at seismogenic depths of the San Andreas Fault, we have evaluated data from drill mud gas from the SAFOD Main Hole (San Andreas Fault Observatory at Depth). Two gas-rich zones at the margins of the fault core differ significantly in the composition of CH4, H2 and CO2, which are the most abundant non-atmospheric gases in the entire hole. The gases enter the bore hole through bedding-plane fractures in the upper zone (approx. 2700 2900 m) and probably from below 3550 m. Separation of two individual hydrogeologic systems by a low-permeable fault core is also indicated by the helium isotopic composition, which is 0.4-0.6 Ra in the upper zone and 0.8-0.9 Ra in the lower one. However, the overall contribution of mantle-derived helium is relatively low. The carbon and hydrogen isotopic composition display an organic gas source of hydrocarbons and CO2. High concentration of hydrogen in the fractured zones at the margins of the fault core are consistent with the concept of hydrogen formation by interaction of water with fresh mineral surfaces generated by tectonic activities. The fault core, localized between approx. 3100 - 3450 m depth, is generally low in gas, in particular in hydrogen. Within the fault core, two sections with higher gas content, but distinct gas composition were identified in 3150 3200 m and 3310 - 3340 m depths. We conclude that the SAF consists of permeable strata at the fault zone margins and a generally low- permeable fault core. In the fault core, separate gas-rich lenses are interstratified. The gas inventory of the San Andreas Fault Zone is dominated by in-situ produced gases, the contribution of gases migrated from greater depths is probably low.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- 8100 TECTONOPHYSICS;
- 8111 Continental tectonics: strike-slip and transform;
- 8194 Instruments and techniques