This report presents the results of geothermal temperature gradient hole (TGH) drilling on Tutuila, American Samoa. The TGH drilling was the third of three phases of geothermal resource exploration conducted by Geologica Geothermal Group Inc. (GEOLOGICA) in American Samoa between 2013 and 2016 for the purposes of evaluating the potential of geothermal power generation.
The initial geothermal exploration included evaluation of available geoscientific data and a surface reconnaissance (Phase 1) which focused further exploration on the Holocene Rift Zone (HRZ) of Tutuila. This was followed by geological, geophysical and geochemical investigations, conceptual modeling of a possible geothermal system, and drilling recommendations (Phase 2). Geothermal exploration culminated in a TGH drilling program (Phase 3).
Phase 3 included drilling two TGHs in the HRZ to depths of ~2,200 feet. This depth was targeted to measure the geothermal gradient within the low permeability zone below the influence of cold surface waters. The locations were selected to investigate upflow zones of a possible geothermal system marked by surface faulting and elevated non-biogenic carbon dioxide. TGH-1 was drilled in the western portion of the HRZ in Malaeloa Valley and TGH-3 was drilled in the eastern portion of the HRZ in the village of Ili’ili.
The TGH program included logging borehole geology from cores and cuttings, downhole temperature measurements, laboratory analysis, and relating the borehole geology to the mapped surface geology. These boreholes did not encounter mineralogy or temperatures indicative of an active geothermal system at commercially viable depths. Located in the western portion of the HRZ, TGH-1 drilled through ~100 ft of Taputapu and/or Leone Volcanics deposited as erosional material before encountering >100 ft of Leone Volcanics. Leone carbonate reefs may have been present below this depth but a lost circulation zone resulted in no returns before coring resumed in paleo-deposited Taputapu Volcanics-derived sediments and in-place Taputapu Volcanics. TGH-3 cored ~500 feet of the younger Leone Volcanics and carbonate reef before encountering redeposited Taputapu volcanics and the older in-place Taputapu Volcanics. Alternating sequences of basaltic lava flows and clay-rich volcaniclastic units in both TGHs suggest that horizontal permeability is high within fractured basalt flows while vertical permeability is very low and limited by the clay-rich volcaniclastic units, except where the stratigraphy is cut by high-angle fractures and normal faults associated with the HRZ.
A conceptual model of a possible geothermal system in the HRZ was constructed prior to drilling. It indicated that if a geothermal system existed, it would be below the low-permeability zone identified by geophysics, and the temperature gradients would be high above the geothermal system and within the low-permeability zone. Maximum temperatures measured in the TGHs were ~37°C at the bottom of each of the boreholes. The conductive geothermal gradient is ~20-25°C/km, consistent with the worldwide crustal average. These temperatures indicate that elevated thermal gradients in the low permeability zone suggested by the conceptual model do not exist. Synthesis of available geoscientific data indicates that an exploitable geothermal system does not exist either below or between the TGH boreholes.
In addition to successfully testing the HRZ for the existence of a geothermal system and despite not discovering one, other beneficial geoscientific discoveries germane to the geology of Tutuila have been made in the course of this geothermal resource investigation, including:
- Lithologies found in the two TGHs to depths of ~2,200 ft bgs can be correlated using visual inspection with lithologic units mapped at the surface. Previous drilling confirmed the geologic section to less than ~300 ft bgs.
- Major and minor cation geochemistry previously used to distinguish the Holocene-age Leone Volcanics from the Pleistocene-age Taputapu Volcanics on surface rocks can also be used to distinguish these in situ formations at depth.
- In the course of geologic mapping, youthful-looking craters west of Olotele Crater that were previously mapped as Taputapu Volcanics have been re-mapped as Leone Volcanics.
- A deep, previously unknown, aquifer at a depth of ~850-1,150 ft in TGH-3 may represent a new groundwater exploration target.
- In between the last subaerial eruption of the Taputapu shield volcanics and the initial eruption of Leone rejuvenated volcanics, there was a period of erosion which is marked by Taputapu Volcanics-derived paleosols, and at TGH-3, by subsea conditions.
- Carbonates observed in the TGH-3 core from 270 to 299 ft bgs and from 446 to 516 ft bgs indicated that the initial eruptions of Leone Volcanics at the TGH-3 location were probably deposited subsea.
- Preliminary Carbon-14 dates from TGH-3 core samples indicated that the Leone began to erupt before 10,000 years B.P. and continued at least as recently as ~4,000 years B.P.
- Estimated subsidence rates for Tutuila implied by the depth and ages of carbonates in the TGH-3 core range from -0.74 to 2.43 mm/year for ~7,000 years B.P. to present, consistent with other recent estimates for Upolu Island.