HR: 0800h
AN: H11B-0489    [Abstracts]
TI: He diffusion in SAFOD core samples
AU: * Ali, S
EM: shahlaa@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AU: Stute, M
EM: martins@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AU: Torgersen, T
EM: thomas.torgersen@uconn.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AU: Torgersen, T
EM: thomas.torgersen@uconn.edu
AF: University of Connecticut, 1080 Shennecossett Road, Groton, CT 06340-6097, United States
AU: Winckler, G
EM: winckler@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AU: Schlosser, P
EM: schlosser@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AB: Fluid flow in rocks is determined by porosity and fractures in the host matrix. In an effort to quantify fluid flow in the San Andreas Fault (SAF) and since direct fracture fluid sampling of the fault zone was not available, we have adapted a method to extract rare gases from matrix fluids of whole rocks by diffusion. Helium was measured on matrix fluids from the SAFOD main hole at ~3060m (clay-rich shale; American Plate), 3432m (clay-rich siltstone; very near active fault trace), and 3990m (siltstone; Pacific Plate) measured depth in September 2004, July 2005, and August 2005, respectively. Samples were typically collected as 2.54cm diameter subcores drilled into the ends of the cores, or from the core catcher and drillcore fragments within <2hr after core recovery. The samples were placed into ultra high vacuum stainless steel containers, flushed with ultra high purity nitrogen and immediately evacuated. Helium was measured at 4-6 times over a period of up to 1000 days in two sets of samples; one set (collected in 2004) was stored at room temperature, while the second set (collected in 2005) was kept in an oven at 120C. Our measurements indicate a 4He diffusion coefficient of 2-3x10^-8 cm²/s at room temperature. Diffusion coefficients measured in the solid phase of sands and clays vary from 1.2x10^-18 cm²/s at 21C to 3x10^-15 cm²/s at 150C. (Solomon et al, 1996). Using the molecular diffusion coefficient of helium in water (7x10^-5 cm²/s; 25C) for a measured matrix porosity (4%) and assumed tortuosity (approx. 5) produces an effective diffusion coefficient of 1x10^-8 cm²/s, consistent with our results. Thus, our method effectively isolates the matrix pore fluid 4He from the 4He contained in the rock matrix.
DE: 1000 GEOCHEMISTRY
DE: 1065 Major and trace element geochemistry
DE: 1094 Instruments and techniques
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
SC: Hydrology [H]
MN: 2007 Fall Meeting