| Title | Conceptual Model for Snake River Plain Geothermal Systems Based on Slim-Hole Exploration at Mountain Home, Idaho |
|---|---|
| Authors | Dennis L. NIELSON, John W. SHERVAIS |
| Year | 2014 |
| Conference | Stanford Geothermal Workshop |
| Keywords | basalt, core drilling, burried geothermal system, temperature while drilling |
| Abstract | The Snake River Plain (SRP) is part of the highest heat flow anomaly in the US. It is characterized by thick sequences of young flood basalts that have erupted following the passage of the Yellowstone plume. However, apart from the obvious hydrothermal systems in the Yellowstone caldera, only a few small hydrothermal systems have been discovered, and these are located on the margins of the province. This is consistent with conventional wisdom that suggests basaltic provinces are poor targets for exploration because the low viscosity of basalt results in rapid flow to the surface along narrow conduits rather than forming shallow magma chambers that are large enough to support high-temperature convection. Slim-hole exploration drilling (MH-2) at Mountain Home Air Force Base has intersected fluids with temperature of about 150O C at a depth of 1745 m. The fluids flowed to the surface through 60 mm pipe at a rate of 42 liters/minute. Comparison with results of a 1986 exploration well (MH-1) in the same area suggests that the system has a lateral extent of at least 5 km. No dikes or sill are present in the core from MH-2; however, a prominent gravity high that trends obliquely across SRP may represent either a basement horst block, basalt sill complex or both. Water chemistry and fluid inclusions document high temperatures (140-195°C) and water that is in equilibrium with basalt or andesite. The temperature data are supported by the observed alteration assemblage (chlorite-actinolite-quartz-pyrite-chalcopyrite). We propose that enthalpy is provided by a complex of Layered Mafic Intrusions (LMI) that underlie the axial portion of the SRP, roughly delineated by the gravity high. While individual intrusions have limited heat capacity, the intrusion of multiple LMI sills into the same crustal levels will heat the surrounding country rock and prepare the ground for future intrusions. The occurrence of LMI sill complexes that undergo fractionation and recharge has been clearly documented in the eastern SRP. This model is distinct from existing models for Basin-and-Range geothermal systems because: (a) enthalpy is provided by large basalt intrusions, (b) the position of the system along faults buried below the axis of the graben is distinct from typical B&R systems, which occur along range front faults or their buried precursors. |