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| Titel |
The small-scale permeability structure of the San Andreas Fault at SAFOD deduced from drill mud gas data |
| VerfasserIn |
T. Wiersberg, J. Erzinger |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250025129
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| Zusammenfassung |
| The San Andreas Fault Observatory at Depth (SAFOD) is a component of the U.S. research
initiative EarthScope and supported by the ICDP (International Continental Drilling
Program). It consists of two wells, from which the SAFOD main hole (MH) traverses the San
Andreas Fault (SAF) between approx. 3100–3450m bore hole depth. In 2007, three
side tracks were drilled out of the MH, including the holes D (3137m-3262m), E
(2952m-3182m) and G (3125m-3356m). These holes are almost parallel to the MH and
penetrate the SAF nearly orthogonal. The holes D and G intersect two active moving
tracks of the SAF, identified by casing deformation in the MH at 3194m and 3301m
depth. During all drilling phases of SAFOD, gas was continuously extracted from
drill mud and analyzed in real-time for its composition. Off-line gas samples were
investigated in the laboratory for selected isotopes. MH and all sidetracks show enhanced
concentrations of hydrocarbons between approx. 3150-3190m and also below 3310m (if
reached), but distinctly lower contents of hydrocarbons in the section enclosed by
both active moving tracks at 3194m and 3301m (inner section, IS). Moreover the
molcular, but not the isotopic composition in the IS is clearly different from above and
below, suggesting hampered fluid flow orthogonal to the fault direction. Helium
isotopes also imply that the permeability at and around 3194m and 3301m is low.
Wiersberg and Erzinger (2007) report relatively constant Ra values on the Pacific
Plate (0.354±0.021 Ra at 3051m) but increasing contribution of mantle-derived
helium below 3196m on the North American Plate with maximum air-corrected Ra
values at 3903m depth (0.938±0.097 Ra). This observation suggests that the SAF
is not the principal conduit for mantle-derived fluids, however, only one sample
from their study derives from the IS (0.46±0.26 Ra at 3196m). Ra values of -¤0.85
were found in two samples from hole D in 3203m and 3262m depth, indicating
somewhat enhanced flow of mantle-derived fluids in the IS. In contrast, very low
Ra values were observed outside of this interval at 3147m (0.26±0.12 Ra) and
3312m (0.22±0.12 Ra). The considerable variations in Ra values on short spatial
scale imply that the SAF consists of strata with different permeability parallel to
fault, from which low-permeable rocks hamper the fluid flow orthogonal to the
fault (Wiersberg and Erzinger, 2008). It is, however, not clear weather the elevated
Ra values found in 2007 in the IS reflect temporal variation in the fluid flow, i.e.
recharging of the fault with fluids from the mantle after e.g. the earthquake from
09/28/2004.
Wiersberg T. and Erzinger J. (2007) A helium isotope cross-section study through the San
Andreas Fault at seismogenic depths, G-cubed 8, No.1, doi: 10.1029/2006GC001388
Wiersberg T. and Erzinger J. (2008) On the origin and spatial distribution of gas at
seismogenic depths of the San Andreas Fault from drill mud gas analysis, Applied
Geochemistry 23, 1675-1690, doi:10.1016/j.apgeochem.2008.01.012 |
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