Abstract: In collaboration with the Yukon Geological Survey, Liard First Nation and other project partners, Innovate Geothermal Ltd. performed an analysis of geoscience data in southeastern Yukon as part of an effort to better understand the potential for geothermal energy resources that, if present, could be utilized to help reduce fossil fuel use in off-grid communities. The study area for this project is located near the Town of Watson Lake and straddles the crustal-scale Tintina fault zone. The main aim of this project is to analyze and interpret a variety of pre-existing and newly-acquired geological and geophysical datasets to evaluate whether geothermal reservoirs may be present within the study area. A secondary aim is to propose favourable drilling locations, if warranted, for exploratory wells to collect information on subsurface temperature and permeability. The geoscience work accomplished here includes both 2D map interpretation as well as construction of a 3D geologic model that was guided by geophysical inversion modelling of gravity, magnetic and magnetotelluric survey data. In the study area, the distribution of temperature in the subsurface remains a significant unknown; however, limited evidence suggests subsurface temperatures are modest. Regional-scale, Curie point depth estimates suggest an average geothermal gradient of only ~31°C/km. In contrast, two oil and gas exploration wells from the 1960s located 15–35 km outside the study area give a geothermal gradient range of ~38–50°C/km. Drilling is required to measure actual subsurface temperature gradients in the vicinity of Watson Lake. Evidence for substantial subsurface permeability is generally lacking in the study area. Analysis of the geoscience data did not reveal any specific locations along the Tintina fault that suggest a structural environment favourable for subsurface fracture permeability. Furthermore, the Tintina fault presents little evidence of active tectonism. Active tectonism helps maintain open fractures in the fault zone which could facilitate deep circulation of fluids to form a natural geothermal system. A lack of active tectonism could limit permeability in the fault zone. In addition, geophysical modelling suggests that large portions of the Watson Lake study area are likely underlain by shale-rich bedrock. This type of rock has very low permeability and is not favourable for maintaining open fractures. The only evidence for subsurface permeability found in this study is sand layers in the Cenozoic sedimentary rock of the Tintina trough. These sand layers could be permeable but are interbedded with low permeability layers such as silt and clay; the thickness and lateral extent of the sand layers is unknown. The 3D geologic model developed in this study suggests that the Cenozoic sedimentary rock of the Tintina trough is limited to less than ~1 km thick. This implies that fluids residing in sand layers in this rock unit would have a maximum temperature of between ~38 and 50°C. Geothermal fluids at these temperatures could potentially be utilized to help heat buildings in the community of Upper Liard. Many unknowns regarding the temperature and permeability of the subsurface still exist in the Watson Lake study area. The location of a 1 km deep scientific research well is proposed to help answer many of the remaining questions. However, considering the lack of evidence for bedrock permeability in the study area, other approaches to utilizing the Earth’s heat, such as Borehole Thermal Energy Storage (BTES) systems, could be considered to help the community of Watson Lake reduce dependence on fossil fuels for residential heating.
Authors: Witter, J.B.
NTS Mapsheet(s): 105A02
Citation: Witter, J.B, 2022. Analysis of geoscience data for geothermal exploration along the Tintina fault near Watson Lake, Yukon. Yukon Geological Survey, Open File 2022-8, 50 p. plus digital appendices.
NTS Mapsheet(s): 105A02