Stress and Fracture Orientation in the Northwest Geysers Geothermal Field

A dataset comprised of 3946 well-located and wellconstrained earthquake focal plane mechanisms recorded in the Northwest Geysers geothermal field during the period of Jan 2005 – May 2012 was used to determine stress orientations and relative magnitudes at fine scale. The stress inversions were organized into gridblocks of varying size using a three-dimensional oct-tree gridding algorithm in which scale of the gridblocks is governed by threedimensional data density. This method retains the spatial distribution of hypocenters but allows separate inversions for contiguous blocks of seismicity at the finest scale warranted by the data. A minimum of 25 focal mechanisms were used to find the best-fitting orientation of the stress tensor in each gridblock. The stress orientations in each gridblock were then used to determine which of the two nodal planes of the focal mechanism had the highest ratio of resolved shear to normal stresses and was thus more likely to be the active fault plane. Not surprisingly, the faulting regime was found to be normal/strike-slip, as is known for the Geysers, and was nearly uniform, even at fine scale. The maximum horizontal principal stress orientation, SHmax, and the vertical principal stress, Sv, are approximately equal in value with an average SHmax of N23E. A very consistent stress field was found within the three-dimensional grid studied. The apparent fault planes (assumed to be the primary conduits for fluid flow) were found to be steeply dipping and include approximately north to east striking strike-slip faults and NE trending normal faults, all consistent with the regional SS/NF faulting regime. The greatly improved resolution of the earthquake data allows us to analyze the seismicity in great detail and to determine the orientations of faults providing pathways for fluid flow in the reservoir. INTRODUCTION The northern California coast ranges are characterized by right-lateral strike slip and reverse faulting (Castillo and Ellsworth, 1993; Provost and Houston, 2001). The Geysers Geothermal Field (GGF), which is nested between several northwesttrending right-lateral strike-slip faults that belong to the San Andreas Fault (SAF) system, is a tectonically unique region exhibiting normal and strike-slip faulting (Oppenheimer, 1986). A N/NE orientation of SHmax has been consistently observed throughout northern California (Provost and Houston, 2001) and in the Geysers Geothermal Field (Oppenheimer, 1986) based on inversion of groups of earthquake focal mechanisms to obtain the orientations of the principal stresses. Previous stress inversions were performed with few high-quality focal mechanisms (Oppenheimer, 1986) and had moderate angular misfit in nodal plane orientation (Provost and Houston, 2001) for Geysers events. Additionally, previous inversions of focal mechanisms for stress have assumed uniformity of stress orientation for large regions. Recent studies (Townend and Zoback, 2001) have revealed the benefit of recursive gridding of the study area when conducting the stress inversion. This method, which uses data density to determine gridblock size, leads to inversion for stress within spatially distinct but contiguous blocks of seismicity. This allows a stress result to be adopted with higher confidence for the local volume from which its focal mechanisms originated. The GGF is a vapor-dominated steam field that is the largest-volume geothermal operation in the world. The reservoir consists of a fractured metagraywacke interval approximately 1 km thick overlaying felsite. A high-temperature reservoir, with temperatures exceeding 350°C, underlies the main reservoir, and is likely shallower in the northern section of the field than in the southern (Stark, 2003). Seismicity in the Geysers is ubiquitous, and is concentrated in regions of high injection and production. The northwest section of the field alone contains more than 65% of the field’s seismicity, and its large, heterogeneously distributed seismic clusters make it perfect for implementation of the recursive gridding scheme in three dimensions. With some clusters so small that they may represent the stimulated volume for just a few wells, the results of a recursively-gridded inversion for stress may illuminate important details about stresses local to the wellbore in the context of a larger tectonic regime.