This talk reviews the basics of geothermal energy and our role as geologists to find where it can be economically produced now and where deeper future reserves can be found.

Oil and gas exploration has had a long run since the 1973 Arab oil embargo and the 1978 Iranian Revolution. Scientific and social concerns now bring into question that drive for ever increasing hydrocarbon production. Yes, the decline curve will require consistent reserve replacement for many years, but industry concern is no longer peak oil, rather it is peak demand. One of the driving factors towards peak hydrocarbon demand is climate change.

The scientific consensus is that climate change is real, burning fossil fuels is a primary accelerant; catastrophic social and economic changes are beginning. This is driving a major push to carbon free or at least carbon neutral alternative energy sources. These include nuclear, hydro, solar, wind, biomass, hydrogen, and geothermal. Geothermal is the topic of this paper. Since geothermal can provide 24/7 baseload power and 97% up-time it is a much better renewable energy than wind or solar. It is good for the climate, good for the economy, and good for geologists. The McKelvey diagram, show here, is named after Vincent E. McKelvey (1972), a former Director of the USGS. He came up with this chart to help illustrate concepts and terminology used in resource evaluation. In this diagram the vertical axis represents the degree of feasibility of economic exploitation. The horizontal axis represents the degree of geological certainty. Has resource been discovered and is certainly there; or has it not yet been discovered but is likely to be there? The vertical axis has four divisions with increasingly favorable economics towards the top. The following resource-assessment terms are illustrated on the chart and defined as follows.

1. Resource Base – all the heat in the Earth’s crust beneath a specified area, referenced to local mean annual temperature.

2. Accessible Resource Base – the thermal energy at depths shallow enough to be tapped by drilling using technology available at present or within the foreseeable future

3. Resource – that part of the accessible resource base that is producible given reasonable assumptions about future economic and technology.

4. Reserve – that part of the resource, which is identified and producible with existing technology and under present economic conditions

At present the geothermal energy industry is exploring and developing resources at the top of the McKelvey diagram, in the red and orange.

So, it is important to know where in the world these geothermal resources and reserves exist.