Introduction to Exploration

A geothermal project has a number of development phases: preliminary survey/site selection, exploration, test drilling, geothermal field development, power plant design, commissioning and operation. Alongside baseline environmental studies and environmental impact analysis studies are conducted. Various geological, geochemical and geophysical surveys are conducted in the exploration to develop a 3D subsurface model of the reservoir based on conductivity/resistivity imaging, magneto- telluric methods and drill whole geophysics.

Traditional exploration methods are directed to finding the best suitable target locations for steam or fluid production. An initial reconnaissance survey often using airborne or space borne remote sensing in combination with literature study at regional scale results in a selection of a prospective area. This pre-feasibility study explores both the likelihood of the presence of a commercial geothermal reservoir, but also investigates the regional power demand, the regulatory framework, and infrastructure, access to the power grid as well as environmental conditions and legislation.

A hydrogeological survey aims to reconstruct the water circulation system trying to relate surface manifestations of geothermal activity (e.g., hot springs, steam vents, fumaroles, etc) to fault/fracture systems, variation in lithology etc. Depending on the terrain condition and the geology, detailed surface mineralogic mapping may be conducted to get a better understanding of the alteration in relation to the temperature, pressure and chemical conditions. Fluid inclusions in minerals may provide more detailed information on the temperature of geothermal fluids as well as their chemical composition.

Geochemical surveys typically sample water from hot springs, gas from hot pools and steam from fumaroles where the fluid chemistry can be used to develop geothermometers that provide an estimate of the temperature of deep reservoirs.

Multitudes of geophysical techniques are deployed in exploration surveys for geothermal characterization. Gravity and magnetic surveys provide information about subsurface lithology and active seismic surveys as well as (passive) seismic tomography provide information about subsurface structure and identification of warmer and cooler regions. Electrical methods measure resistivity of the shallow subsurface, which is related to the conductivity of the rocks, which in turn is dependent on the composition, the porosity, the fill of pore spaces, and the temperature and salinity of the fluids. Electromagnetic methods in particular magnetotelluric (MT) sounding and the more recently introduced time-lapse MT uses natural variations of the Earth’s electrical and magnetic fields to determine the depth, geometry and geologic characteristics of electrically conductive features including (clay) reservoir caps, fluid-filled reservoirs, melt accumulation in the core, fluid pathways and geothermal reservoir temperatures at depths ranging from 300m down to several kilometres.

Reservoir modelling integrates elements from geological, geochemical and geophysical surveying to refine the geologic model through numerical simulation in order to understand the behaviour of a geothermal reservoir, to find the most suitable and productive reservoir, to estimate reservoir volume and recoverable heat, to identify zones of high permeability, to locate drilling locations, and to forecast future well and reservoir behaviour.