Geothermal Resources Assessment In Hawaii

Abstract: 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. 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 results of these studies have allowed us to attempt an estimate of the probabilities of low- to moderate-temperature (50-125°C) and of moderate- to high-temperature (125 - 360°C) 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. 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 are 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 was not studied during the current phase of investigation. 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. 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 appeared to be coincident with the caldera boundary faults. Groundwater chemistry and temperature measurements identified a distinctly anomalous well near the back of the valley and several others with slightly anomalous conditions on the caldera boundary faults. Geophysical soundings indicated low subsurface resistivities within the valley that were interpreted to correspond to warm fresh to saline water-saturated basalt. On the basis of the available data, the probability for a low- to moderate-temperature resource (50-125°C) within 3 km of the surface is assessed at 10-20%. The probability for a higher temperature resource is less than 5%. Mokapu Peninsula and Koolau Caldera. Geochemical studies on Mokapu were unable to identify a self-consistent pattern of soil geochemical anomalies or significant groundwater chemical anomalies that would suggest a geothermal resource. Resistivity soundings determined subsurface resistivities that were consistent with cold seawater-saturated sediment. The probability for even a low-temperature geothermal source at depths of 3 km or less beneath Mokapu is considered to be less than 5%. Results of preliminary soil geochemical studies and interpretation of available groundwater data to the south of Mokapu, within the Koolau caldera, suggest that some thermally induced alterations may be present. Interpretation of geophysical data indicates that the temperatures within the ancient Koolau magma chamber are less than 540°C and that the shallow subsurface resistivities show no evidence of thermal effects. On the basis of the rather sparse data currently available, the probability for a low- to moderate-temperature resource associated with the Koolau magma chamber is considered to be 10% or less. Geothermal assessment activities on Maui included an evaluation of the major rift zones and post-erosional volcanic vents on both West Maui volcano and Haleakala volcano. Olowalu and Ukumehame Canyons: extensive geologic mapping characterized the southwest and southeast rift zones of West Maui volcano and interpreted these structures to suggest a migration of the rift zone activity late in the formation of West Maui. Numerous late-stage alkalic and trachitic dikes and plugs were also identified in the survey area. Ground-water geochemical and temperature measurements identified distinctly anomalous water chemistry and temperatures. Resistivity sounding data for the area was interpreted to indicate a thick layer of warm, fresh to saline water beneath the Olowalu and Ukumehame Canyons. The probability of a thermal resource having a temperature greater than or equal to 50°C is estimated to be 50-60%, whereas a temperature greater than or equal to 125°C has an estimated probability of 10% or less. Lahaina-Kaanapali: soil geochemical surveys were unable to identify a self-consistent pattern of soil mercury concentrations or radon emanation rates that would suggest a thermal resource. Geophysical soundings detected subsurface resistivities consistent with cold water-saturated alluvium and basalt. The probability of a thermal resource existing in this area is less than 5%. Honokowai: groundwater chemistry and temperature data for this area were unable to confirm the existence of any thermal impacts and geophysical soundings indicated normal subsurface resistivities. Hence the probability for a resource in this location is believed to be less than 5%. Field surveys on Haleakala were confined to the lower portions of the three major rift zones and yielded the following analyses: Haleakala Northwest Rift: soil geochemical and groundwater chemical studies in this area both indicate potential anomalies. The interpretation of the anomalies with regard to thermal alterations was not, however, unequivocal. Geophysical soundings were unable to identify significantly anomalous subsurface resistivities or self-potential variations. The probability of a low- to moderate-temperature resource is placed at 10-20%, whereas that for a high-temperature resource is less than 5%. Haleakala Southwest Rift: geologic mapping has determined that several flows on this rift are less than 10,000 years of age and that a few are less than 1000 years old. Preliminary geochemical studies were unable to identify unequivocal evidence of thermal effects on the lower rift zone area, whereas geophysical soundings indicated that thermal groundwaters may be present at depths of less than 3 km. The probability for a low- to moderate-temperature resource is estimated to be 30-40%, whereas that for a high-temperature resource is placed at 15-25%. Haleakala East Rift Zone: preliminary geochemical and geophysical surveys were performed in this area. The results of these efforts did not identify significant anomalies; however, difficulties in interpretation and the small amount of data available do not allow an assessment of geothermal potential to be made. The island of Hawaii was found to have the largest number of PGRAs (Fig. 34). The current assessment program performed field surveys in six of the most promising PGRAs on Hawaii, which yielded the following results: Kawaihae: geophysical surveys performed over this area indicate a set of magnetic and resistivity anomalies that suggest that an intrusive body, associated with the Puu Loa cinder cone, may be heating local groundwaters. Groundwater chemistry and temperature anomalies confirm the existence of a heat source in the vicinity; however, the temperatures are not indicated to be very high. The probability of a low- to moderate-temperature resource in the survey area is indicated to be 35 to 45% and a moderate- to high-temperature resource to be 15% or less. Hualalai: Geophysical surveys have identified distinct magnetic, resistivity and self-potential anomalies near the summit of Hualalai, whereas the lower western flank has not shown significant thermal effects. Geochemical data on the lower flanks were similarly unable to identify any obvious thermally induced anomalies.

Author(s): Donald M. Thomas

Published: Geothermics, 1986/01/01

Document Number: Unavailable

DOI: Unavailable

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