Dixie Valley Geothermal Field 2.5D Study

We reprocessed 21 miles of 2-D seismic reflection data from the Dixie Valley geothermal field, Churchill County, Nevada. The data had been acquired several years before and processed using conventional petroleum seismic data processing techniques. We proposed to the US Department of Energy (USDOE) that the laterally complex structure associated with geothermal environments are not well suited for application of conventional data processing techniques.

The USDOE funded the application (Grant Number DE-FG07-97ID13465) of SeisOpt® technology to see if seismic can be an effective tool for constraining drill-targets within geothermal anomalies. A successful demonstration would make seismic a practical and economic tool for constraining drill targets and thus lowering the cost of exploring for geothermal resources.

The results of our study revealed a completely different model for the geothermal field, revealing basin-ward structures that had been previously unknown. The basinward structure correlates with 80% of production within the Dixie Valley geothermal field. The models that we derived using SeisOpt® provided a more realistic explanation for the location of geothermal production and injection wells. Later tracer studies performed by Dr. Pete Rose of Energy and Geoscience Institute, Utah, confirmed the SeisOpt® model. The success of the Dixie Valley project provided a springboard for several other geothermal projects using SeisOpt® technology, that continue to this date. Following are some examples of our work from Dixie Valley.

Figure showing geometry of the seismic lines that was reprocessed using SeisOpt® technology. Previous processing using conventional techniques did not reveal sufficient structural detail, leading the USDOE to fund our study. Velocity models along individual seismic lines were interpolated to create a 2.5 D model of the Dixie Valley geothermal field.

2.5 D model of the Dixie Valley geothermal field obtained using SeisOpt® technology showing correlation of velocity model with known basin structure. The model shows the lower velocities corresponding to known basin sediments deepens towards the center of the valley. East-West section at producing well 45-33 correlates with the mapped lateral velocity discontinuity. The interpreted fault (dashed lines), based on the velocity change, intersects the well 45-33 at a depth of about 2.9 km. For reference the trace of the Dixie Valley fault (DVF, dotted lines) obtained from inversion of reflection times is also shown. It is important to note that no other data but the seismic data were used to derive velocities using SeisOpt® technology.

Northeast-southwest section of the 2.5 D model along producing wells 27-33 and 45-33. Velocities derived along individual seismic lines (shown in red and labeled on top) were interpolated to obtain the 2.5D velocity model. Note that this cross-section, which is approximately parallel to the range front, shows high velocities at uniform depth of 1km. Except near the producing wells where there is a consistent deepening of lower velocities. This localized deepening of low velocities defines a synform, or half-graben, basinward of the Dixie Valley fault that had been previously unknown. This structure correlates with about 80% of production within the Dixie Valley geothermal field. It had been previously thought all production within the filed was controlled solely by the Dixie Valley fault. It is important to note that no other data but the seismic data were used to derive velocities using SeisOpt® technology.

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