Tutuila, American Samoa: A Case History of Geothermal Exploration on a Deep Sea Island

摘要

Geothermal exploration on the island of Tutuila, American Samoa focused on a young extensional volcanic feature, the Holocene Rift Zone, that hosted the most recent eruption on the island, a ～600 year old basalt lava flow. Although a lack of fumaroles or hot springs implied that exploration would be relatively high risk, very high power prices led to an exploration program guided by a preliminary conceptual model including a sub-boiling reservoir with an outflow guided offshore by a clay cap, while a very high flux of meteoric water in the permeable lavas above the cap hid surface evidence of the reservoir. Geology, MT resistivity, CO_2-soil gas, soil chemistry and hydrology surveys were completed to indicate whether this or any resource model was plausible and to target relatively low cost temperature gradient holes. The deep low resistivity zone detected dipping up to the south beneath the coast was consistent with the model supporting a blind geothermal system and supported targeting of two temperature gradient holes continuously cored to ～650 m. The geology, alteration and temperature in these wells demonstrated that no geothermal system existed and that meteoric water apparently penetrated through the low resistivity clay zone, resulting in an unusually low temperature gradient. This failure case history demonstrates a relatively low cost strategy to test plausible but high risk geothermal prospects, such as isolated volcanic islands. This paper also highlights unusual conceptual aspects of exploration on relatively small volcanic islands like Tutuila.