The Stress State of the Northwest Geysers Geothermal Field and Implications for Fault-Controlled Fluid Flow
Abstract
A dataset comprised of first motion polarities and S/P amplitude ratios for 4900 microseismic events recorded in the Northwest Geysers geothermal field during the period of Jan 2005 - May 2012 was inverted to obtain double-couple focal mechanisms. These mechanisms were organized into gridblocks of varying size using a three-dimensional oct-tree gridding algorithm in which discretization is governed by data density. This method retains the spatial distribution of hypocenters but allows separate inversions for contiguous blocks of seismicity. The orientations of the three principal stresses were obtained for each gridblock that contained a minimum of 25 events by finding the best-fit deviatoric stress tensor for all nodal planes within that gridblock. The principal stress orientations in the gridblock were used to determine which of the two nodal planes for each focal mechanism had the highest ratio of resolved shear to normal stresses and thus was more likely to slip in that gridblock's stress field. The faulting regime was determined to be normal/strike-slip, reaffirming that extensional tectonic behavior is present in the Geysers. An SHmax orientation of approximately N20E was consistently obtained across the three-dimensional grid. Presumed fault planes were generally steeply dipping and included a range of northern to eastern strike directions consistent with the faulting regime. The greatly improved resolution of the data allows us to observe induced microseismicity in detail and to examine the orientations of faults providing paths for fluid flow in the reservoir.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.H12F..03B
- Keywords:
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- 7200 SEISMOLOGY;
- 7230 SEISMOLOGY / Seismicity and tectonics