Results of Elemental, Stable Isotope, Organic Matter, and Fission-track Analyses of SAFOD Drill-hole Cuttings and Core Material

Geochemical analyses of cuttings and core material from the SAFOD drill hole provide important information on the subsurface geology and the fluid-rock interactions that have occurred in and adjacent to the fault zone. We have obtained an integrated geochemical data set from the main drill hole that includes elemental data for 120 cutting and core samples, stable isotope data for 100 vein and breccia samples, organic maturity data for 30 samples, and zircon fission-track data for 6 samples. In the elemental data set, there are 6 breaks/transitions in the abundance of individual elements: at 3197 m (potentially corresponding to the western fault-bounded margin of the SAF damage zone); at 1926 m (a granite-sandstone fault contact); at 3319 m (perhaps the most important fault contact within the core of the SAF zone); at 2570 (core of clay-rich shear zone?); at 3481 (near faulted contact between folded and non-folded sediments); and at 3078m (near clay-rich fault zone that was encountered at end of phase 1 coring). In the stable isotope data, the carbon isotope values of the carbonate veins and cement vary significantly from +8 to negative 20 per mil; the oxygen isotope values vary from +26 to +12 per mil. The data can be separated into two groups, both of which are present on either side of the inferred SAF. The group with low carbon isotope values are consistent with the carbon having been derived from, or exchanged with, either 1) biogenically derived methane, 2) thermogenic methane at temperature greater than 180C rather than at their current down hole temperature of approximately 120C, or 3) dissolved carbon in groundwater that had been produced in the root zone of C3 plants. The latter hypothesis would be consistent with the carbon isotope data of soil gas in the Parkfield area (Lewicki et al., 2002). In contrast, the other group of veins and breccias are in oxygen isotope equilibrium with the silicate-buffered formation water obtained from the drill hole and close to carbon isotope equilibrium with the thermogenic methane sampled in the drill hole. The total organic content (TOC) of the cuttings are below 1 percent by weight. The kerogen is considered to be "gas prone" on the basis of the TOC and Rock-Eval pyrolysis, hydrogen index, and oxygen index values. The vitrinite reflectance of 23 samples increase with depth from 0.70 at 3230 m to 0.95 at 3960 m. This increase in reflectance values is consistent with increasing temperature and organic maturation with depth. Although the data is limited, there is no significant offset in organic maturity across the inferred SAF zone. Six zircon fission-track dates range from 64 to 70 Ma, which are interpreted as cooling ages of the sediments' source terranes. There is no discernable difference in dates across the inferred SAF zone. Zircons from approximately 15 other samples are in the process of being analyzed at this time.