Hawai'i statewide geothermal play fairway analysis: Final phase aqueous geochemistry results and work in progress

Hawaii's remote location, lack of fossil fuel resources, and need to address energy generation and pollution in the face of climate change, make it imperative to characterize its indigenous geothermal resources. The Department of Energy funded the Hawai'i Play Fairway project to accomplish this task. Play Fairway analysis is an ideal, cost saving and risk-reducing methodology for subsurface resource exploration. The project is now in its third and final phase. This is the first statewide geothermal assessment since the 1970's, and incorporates numerous advancements in technology and methods developed in the interim. In phase one, we compiled legacy geologic, geochemical, geophysical, structural, volcanic age, and cultural data to create a statistical probability model for blind geothermal resources. Phase two produced new data to enhance this model. The final phase seeks to test the model by investigating some of the identified high probability targets, and locate thermal anomalies. Noble gas, stable isotope, and common ions/metals analysis of well waters from sites on Kaua'i, Lana'i, O'ahu, Maui, and Hawai'i are been undertaken to identify potential blind resource. Many traditional geochemical geothermal exploration techniques have proven to be of no value in Hawai'i, however, Cl/Mg, dissolved SiO₂concentration, sulfate concentration, carbonate concentration, δ¹³C-DIC, δ¹⁸O-H₂O, and ₃He/⁴He are proven for identifying trends at Hawai'i's known geothermal resource locations, and in similar volcanic island settings globally. Six sites have been sampled on Kaua'i, and 68 well sites have been identified on the other islands, though only a subset of these will ultimately be sampled due to issues with land access and operability of well pumps. Preliminary data from the Kaua'i samples showed little direct indication of heat. However, an R/R_A(₃He/⁴He in sample over He/He of the atmosphere) of ∼7 was found in one well, suggesting either dilution of geothermal fluids with a magmatic heat source, or influence from deep structurally controlled fluid circulation. All other Kaua'i wells showed an R/R_Avalue of ∼1. Given that the only sampled island thus far has a positive indicator for mantle heat, and that Kaua'i is one of the farthest main Hawaiian Islands from the hot-spot, it is expected that some of the additional sites will also yield positive results.