Helium Isotope Measurements on Matrix Fluids From the SAFOD Drillcore
Abstract
Matrix fluids from the SAFOD main hole at ~3060m have been isolated from subcores, core catcher and core fragments for noble gas analysis. Because SAFOD cores are under (hydrostatic to lithostatic) compressive stress, matrix fluids within this material can be released over time by a series of microcracks, diffusive transport, etc. Preliminary results indicate that this release occurs on a time scale of months for a transport scale of 1-3 cm. By placing subcores, core catcher bits, etc. in high vacuum containers as soon as possible after becoming available at the surface (on site), flushing with ultra-pure nitrogen and evacuating, the noble gases from matrix fluids have been measured in several SAFOD samples. 3He/4He ratios from these samples (3He/4He = 4.9-5.5 e-7) are identical to the 3He/4He ratios recovered from fluid samples captured during an open hole flow test (Kennedy, et al. pers. comm.) and indicate a mantle derived He component of ~4%. Assuming 36Ar is at saturation, open hole flow tests suggest 4He concentrations of 4-6 e-5 cc4He/g-water, which is largely in agreement with matrix fluid concentrations calculated from the measured mass of volatile fluids (evaporated water) and 4He amounts (4-7 e-5 cc4He/g-water). These data are preliminary, because at this time water and He have not been quantitatively released from the rocks. The results suggest that matrix fluids collected from core material is representative of fracture-filling fluids and represents a viable means for fluid collection when flow tests are restricted by limited permeability and/or drilling schedules. Our preliminary results indicate that our method might be suitable for constraining the long-term dynamics of fluids in the San Andreas Fault.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFM.T21A0453S
- Keywords:
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- 1829 Groundwater hydrology;
- 7223 Earthquake interaction;
- forecasting;
- and prediction (1217;
- 1242);
- 8155 Plate motions: general (3040)