Exploration of the Akutan Geothermal Resource Area

Abstract

Gas geochemistry from geothermal fumaroles on Akutan Island, Alaska, indicates that geothermal reservoir temperatures could approach 572 °F (300 °C), and probably consists of a brine liquid overlain by a small steam cap. Fluids produced by core holes show evidence of chemical re-equilibration to lower temperatures, with cation geothermometry providing a range from 392-464 °F (200-240 °C). Geochemistry of hot spring fluids shows evidence of equilibrating at still lower temperatures. These data support a model with a high-temperature upflow system in the vicinity of the fumaroles that transitions to a lower temperature outflow zone that mixes with meteoric water and connects to hot springs 12,000 ft (3600 m) from the fumarole. This model is supported by MT resistivity data. Exploratory drilling targeted the outflow zone with two core holes 9,200 and 12,000 ft (2800 and 3600 m) from the fumarole. The farther core hole encountered expected fluid temperatures of 360 °F (180 oC) at 613 ft (186 m). Static temperature profiles suggest that the 360 °F zone is drawn from a nearby fault zone not located directly below the well. Alteration mineralogy in the two core holes suggests that the rocks were at temperatures greater than 469 °F (250 °C) in the geological past and have cooled to present temperatures. The integrated interpretation of core mineralogy, temperature logs and MT resistivity suggests that the part of the outflow encountered by the wells has insufficient volume and too close a connection to cooler water to support commercial development, although the higher risk of cooling during exploitation as a result of either cold water influx from near-surface aquifers or injection breakthrough might be offset by flexibility in lower cost shallow wells. Targeting the area of the fumarole field with an 8000 ft (2500) m directional well would have the highest probability of encountering commercial production at Akutan. This target is likely to be >430 oF (>220 °C) and could be as hot as 570 oF (300 °C).