­­Fault scaling and permeability controls in geothermal systems

Abstract:

Geologic structures play a crucial role in focusing geothermal fluid circulation in the upper crust. In a variety of geothermal provinces worldwide, studies have shown that young or active faults, and especially locations where faults intersect and interact, generate the accentuated fracture permeability required for geothermal circulation. Fault intersection and interaction areas are characterized by concentrated stresses, which leads to the generation of secondary faults and fractures and heightened permeability. In the Great Basin, USA, fault terminations, fault step-overs (or relay ramps) and accommodation zones are among the most common structural settings for geothermal circulation. Fault scaling relationships provide a first-order indication of the ideal extent and geometry of these features as well as the location(s) and extent of concentrated stresses (and therefore a high potential for permeability generation) within these structures. The most favorable locations for fracture permeability, those with optimal stress concentrations, are on the order of ~1-2 km wide for typical 10-20 km fault lengths in the Great Basin. Examination of known, production geothermal systems within each of these structural settings indicates that although thermal anomalies are commonly 5-10 km wide, the production reservoir and corresponding well fields are confined to smaller areas. We suggest that the limited aerial extent of these production zones is controlled by the extent of concentrated stresses and the most accentuated fracture permeability associated the specific fault intersection or interaction areas. Though is it well established that specific structural geometries like fault terminations, step-overs and accommodation zones are promising ‘plays’ for geothermal exploration, our analysis further constrains the scale and extent of the most favorable occurrences of these structures, as well as the most prospective permeability zones within them.