HR: 0800h
AN: T21A-0438    [Abstracts]
TI: Preliminary Observations of Stress and Fluid Pressure in and Near the San Andreas Fault at Depth in the SAFOD Boreholes
AU: * Zoback, M D
EM: zoback@pangea.stanford.edu
AF: Stanford University, Dept. of Geophysics, Stanford, CA 94305 United States
AU: Hickman, S H
EM: hickman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS977, Menlo Park, CA 94025 United States
AB: A variety of observations in the SAFOD pilot hole, drilled in 2002, and the first two Phases of the main SAFOD borehole allow us to place preliminary constraints on the orientations and magnitudes of principal stresses as well as pore pressure at depth. It will be possible to improve these preliminary results once detailed data on the shape of the main hole acquired during Phase 2 (i.e., through the San Andreas fault zone) are fully processed. In addition, the core holes to be drilled during Phase 3 in 2007 will be used to make measurements of the least principal stress via hydraulic fracturing. Stress orientation has been determined from the orientation of wellbore breakouts in both the vertical pilot hole to a depth of 2.2 km and in the deviated portion of the Phase 1 SAFOD hole down to a vertical depth of 2.5 km. These indicate that the maximum horizontal principal stress rotates with depth, attaining a high angle to the San Andreas Fault at depth and consistent with the hypothesis that there are low shear stresses acting on the fault. A similar conclusion was reached based on the direction of shear velocity anisotropy determined from cross-dipole sonic logs conducted in SAFOD during Phase 1 (see abstract by N. Boness and M.D. Zoback). Three hydraulic fracturing tests were carried out to constrain the magnitude of the least principal stress along the trajectory of the SAFOD borehole, at vertical depths of 1.5 km, 2.5 km, and 3.2 km. The 1.5- and 3.2-km-deep tests, which were conducted at distances of about 1.1 km SW and 0.5 km NE, respectively, from the two seismically active traces of the San Andreas identified at this location, indicate that the least principal stress is the vertical stress. In conjunction with modeling of wellbore failure and estimates of rock strength, these tests further indicate a transitional strike-slip/reverse faulting stress state, which is consistent with our earlier stress analyses from the pilot hole. The test at 2.5 km, carried out within only about 100 m of the closest seismically active fault trace, indicates that the magnitude of the least principal stress is 20 MPa (or more) higher than the calculated overburden stress. While such a result is quite unusual (one principal stress normally corresponds to the lithostat) several theoretical models of the state of stress within the active fault zone predict such high stress magnitudes for the case of a fault with low frictional strength. Observations made to date indicate subhydrostatic fluid pressures both in the pilot hole and at the bottom of SAFOD Phase 1. In contrast, elevated pore pressures are indicated in the deep sedimentary section drilled during Phase 2 by the influx of gas into the well at times when drilling stopped. As the density of the drilling mud in the hole was approximately 40% greater than fresh water, these gas influxes indicate pressures appreciably in excess of hydrostatic pressure. However, it is not yet clear whether these elevated pressures influence the mechanics of faulting or are simply related to hydrocarbon maturation (and other processes) in the low permeability shales encountered at depth. This uncertainty will be addressed when fluid pressure observations are made within the active strands of the San Andreas in 2007 when the Phase 3 core holes are drilled.
DE: 7215 Earthquake source observations (1240)
DE: 7250 Transform faults
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8110 Continental tectonics: general (0905)
SC: Tectonophysics [T]
MN: Fall Meeting 2005