| Title | Why Basin and Range Systems are Hard to Find: The Moral of the Story is they Get Smaller with Depth! |
|---|---|
| Authors | Blackwell, David D.; Waibel, Albert F.; Richards, Maria |
| Year | 2012 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Basin and Range; model; epithermal; mushroom; Dixie Valley; Sleeper |
| Abstract | A four-dimensional model of the most common geothermal systems in the Basin and Range is presented and discussed. A model founded on empirical data from the known characteristics of active systems at 0 to 2+ km depths expanded by comparison to epithermal mineral deposits to the 0.5 to 4 km depth range. In general the systems have a relatively broad shallow fluid dispersal zone fed by a narrow high-flow-rate stem much smaller than the near surface areas (they are mushroom shaped) The structurally controlled stems are small in area but highly permeable and occur in very localized tensional sites. The subtle configurations of fault, fracture and statigraphic relationships complicate exploration and development efforts. The stems connect (represent a short circuit to) a large, diffuse volume of variably permeable upper crust and harvest fluid in a continuously evolving manner. Consequently, the volume of rock sampled by the flow system (the” reservoir”) cannot be determined by examining shallow/intermediate depth structural settings and use of idealized geological models. These observations explain why the system production capacity estimates in the initial stages of exploration and development are so unreliable and why many of the common geophysical techniques used for exploration and development have often proved to be so ineffective. The most appropriate analogues for active geothermal systems are epithermal mineral systems and the only unique signature of an active geothermal system is its temperature which, by definition, is higher than the surrounding rock. Therefore temperature directed drilling is the most appropriate technique for locating the “stem” and maximizing the production capacity of the geothermal system. |