Insights into Extensional Geothermal Systems from Numerical Modeling

Abstract

Numerical models are used to establish basic qualitative relationships between structure, heat input, and permeability distribution, and the resulting extensional geothermal system. Extensional systems rely on deep circulation of ground water (rather than cooling igneous bodies) for heat and extensional tectonics to provide permeable up-flow paths. This report focuses on the characteristics of the Basin and Range province of the United States, but the results apply to extensional settings in general. A series of steady state, twodimensional models are used to evaluate the effect of permeability and structural variations on an idealized, generic Basin and Range geothermal system. An extensional geothermal system only exists in a relatively narrow range of bulk permeability (10-15-10-16 m2). Outside of this window temperatures in the shallow sub-surface decrease rapidly. The presence of a relatively permeable upflow path (provided by geologically recent faulting) is a requirement for system development. Chemical self-sealing of upflow paths does not significantly affect the flow system as long as a central flow path is still available. While topography gives an extra, early ìkickî to convective circulation, it is not a requirement for geothermal system development. A permeable fault in one valley can also induce cross-range flow from adjacent valleys if there are no equally good upflow paths in the adjacent valleys. When bulk permeability is high enough, additional deep circulation cells develop in adjacent valleys diverting heat and fluid from the fault and consequently reducing temperatures in the fault.