HR: 15:25h
AN: T23E-08    [Abstracts]
TI: Chemical and Isotopic Composition of Water and Gases From the SAFOD Wells: Implications to the Dynamics of the San Andreas Fault at Parkfield, California
AU: * Thordsen, J J
EM: jthordsn@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Evans, W C
EM: wcevans@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Kharaka, Y K
EM: bmkennedy@lbl.gov
AF: US Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Kennedy, B M
EM: bmkennedy@lbl.gov
AF: Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720 United States
AU: van Soest, M
EM: mcvansoest@lbl.gov
AF: Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720 United States
AB: To investigate the source of fluids within the San Andreas fault zone, we obtained downhole fluid samples from both the SAFOD pilot well (open hole at vertical depth of ~2.2 km) and the adjacent SAFOD main well, from open holes at depths of 1443-1470 m and 2540-2557 m. Each fluid sampling opportunity followed coring runs, which provided open holes at these depths, enabling formation fluid to enter the wells. Prior to coring, the drilling fluids were tagged with fluorescein and Rhodamine WT tracer dyes to allow for calculation of the contamination effects. We used an evacuated Kuster sampler and positive-displacement Westport samplers, that both allow for accurate determination of the dissolved gas concentrations. Chemical data and water-level measurements in the SAFOD pilot well and the shallower zone of SAFOD main well indicated that no significant amount of formation water was produced. Significant amounts of formation water, however, were produced from the deeper open hole of the SAFOD main well. The water level in the well rose ~60 m from completion of coring (October 1, 2004) to the first fluid sampling (April 13, 2005), when three samples were obtained, and rose ~12 m more by June 8, 2005, when an additional 4 samples were collected. Chemical data show that these samples are a mixture of formation water (75-80%) and the dye-tagged `KCl' drilling solution. High pH values (9.5-10.5) and high Ca concentrations indicate contamination from the cement used for casing the well. Mixing proportions and geochemical modeling, utilizing the tracer dyes and conservative solutes, are used to calculate the compositions of formation water. Results show a Na-Ca-Cl type water with a salinity of ~20,000 mg/L TDS, very low Mg (0.1 mg/L) and carbonate alkalinity (<1 mg/L), but moderate SO₄ and very high DOC and organic acid anions. This chemical composition is typical of formation water from sedimentary rocks, such as oil field waters from California. The deepest samples from SAFOD main well are extremely gas-rich, with calculated in-situ gas pressures exceeding 50 bar. The gas composition is also consistent with that obtained from sedimentary rocks, about 80% CH₄, 5% higher hydrocarbons, and 10% N₂. Low O₂ (<0.02%) and high N₂/Ar ratios (~300) reveal a strong non-atmospheric N₂ component. Helium was enriched by a factor of ~1500 over the level in air-saturated water, and ³He/⁴He ratios yielded R/Ra values of 0.33 to 0.36, indicating a small mantle He contribution to the fluids. Although the absence of CO₂ (<0.002%) may be an artifact of high pH, the relatively low R/Ra values compared to the nearby Varian-Phillips well (CO₂ ~40% of total gas, R/Ra=1.5) and other wells in the Parkfield area indicate a relatively low flux of CO₂ and other mantle volatiles, and a relatively minor contribution of these fluids to the San Andreas system at the SAFOD location and depth.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8010 Fractures and faults
DE: 8150 Plate boundary: general (3040)
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: Fall Meeting 2005