Geothermal resources assessment in Hawaii

Abstract

Abslr8CI-The Hawaii Geothermal Resources Assessment Program was initiated in 1978. The preliminary phase of this effort identified 20 Potential Geothermal Resource Areas (PGRAs) using available geological, geochemical and geophysical data. The second phase of the Assessment Program undertook a series of field studies, utilizing a variety of geothermal exploration techniques, in an effort to confirm the presence of thermal anomalies in the identified PGRAs and, if confirmed, to more compietely characterize them. A total of 15 PGRAs on four of the five major islands in the Hawaiian chain were subject to at least a preliminary field analysis. The remaining five were not considered to have sufficient resource potential to warrant study under the personnel and budget constraints of the program.The results of these studies have allowed us to attempt an estimate of the probabilities of low-to moderate-temperature (50 -125°q and of moderate-to high-temperature (125 -360 0 q geothermal resources in 12 of the survey areas; inadequate data or interpretational difficulties did not allow a valid estimate to be made for the remaining three study sites. Table II presents estimated probabilities for these PGRAs that are based on all currently available data.The results of these studies have also demonstrated that no single surface geothermal exploration technique is capable of providing unequivocal proof of a subsurface thermal anomaly under all field conditions; it is more frequently the case that an estimate of the geothermal potential of a given PGRA must rely on a synthesis of all geological, geophysical and geochemical data available. Experience in the Kilauea East Rift Zone, a Known Geothermal Resource Area, has also demonstrated that none of the currently available surface exploration techniques arC' capable of yielding definitive information regarding the production capabilities of a specific parcel of a geothermal reservoir; the only technique that has proven capable of providing this information has been the drilling and flow testing of deep exploratory wells .The island of Kauai (Figs I and 2) was not studied during the current phase of investigation.Geothermal field studies were not considered to be warranted due to the absence of significant geochemical or geophysical indications of a geothermal resource. The great age of volcanism on this island would further suggest that, should a thermal resource be present, it would be of low temperature.The geothermal field studies conducted on Oahu focused on the caldera complexes of the two volcanic systems which form the island: Waianae volcano and Koolau volcano. The results of these studies and the interpreted probability for a resource are presented below.Lualualei Valley: (Figs I and 3). Geologic mapping located the focus of the late-stage eruptive activity near the back of Lualualei Valley and tentatively identified the Waianae caldera boundaries within the valley. Soil geochemistry studies defined anomalous zones of mercury concentrations and radon emanation that appe...