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Geothermal Prospectivity and Fluid Geochemistry of The Taranaki and Hikurangi Forearc Hydrocarbon-Bearing Sedimentary Basins, New Zealand
Of the low-enthalpy geothermal systems in New Zealand in sedimentary basins, Taranaki was deemed sixth and the Hikurangi Forearc (HF) seventh of the seven regions in New Zealand with the highest geothermal prospectivity. This submission compares two geothermal systems with very different tectonic settings. Taranaki is the the most westerly expression of subduction related andesitic volcanism in New Zealand where the subduction interface is estimated to be at 250 km depth. Volcanoes are as young as <130ka although petroleum wells with the highest temperatures are located near 1.8Ma volcanic edifices. The Hikurangi Forearc (HF) is located at the forefront of subduction along the Hikurangi Margin east of the North Island. There are two hot springs, no volcanoes and the subduction interface is only 16 km deep. The minimum recoverable heat in Taranaki is 0.54 PJ/a with an estimated temperature of up to 200oC at <4000 m depth. In contrast the recoverable heat in the Hikurangi Forearc is 0.12 PJ/a with temperatures of 110o + 20oC at >4000 m depth. This submission examines fluid geothermometry in more detail as part of a study on the geochemical compositions of oil and gas well discharges and their effects on future geothermal exploitation. High temperatures in parts of Taranaki may be related to the shallowing of the upper mantle and rapid ascent of hot fluids via deep faults, with the latter also found in the HF. Subsurface temperatures at Taranaki show that geothermal energy can be harnessed for both power generation and direct heat use whilst power generation in the HF is limited to two sites. Because of the likelihood of liquid condensation, the presence of >C6 hydrocarbon gases would require a different distribution system for geothermal fluids for power or direct heat use if fluids from petroleum wells are directly harnessed for geothermal energy. However, the use of deep borehole heat exchangers may obviate the problem of hydrocarbon gas condensation and hydrocarbon solidification during geothermal production.