South Australia's Hot Rocks

There are two main components required for a rock within the earth's crust to achieve the high temperatures needed to be a potential hot rock geothermal energy resource.

Intrinsic heat production

The amount of heat produced by a rock is due largely to the concentration of naturally occurring radiogenic minerals. As the concentrations of the radiogenic minerals are very low (commonly less than 50 parts per million or 0.005%) a large volume of the rock needs to be present to generate significantly elevated temperatures. Granites are an example which commonly occur as large masses which have volumes of greater than 100 cubic kilometres, and are thus an ideal geothermal resource target.

Insulation

Without an insulating cover of material, all the heat generated by the heat-producing rocks would be rapidly dissipated. To trap the heat effectively, the blanketing cover must be a good thermal insulator and must also have a minimum thickness of at least three kilometres to maintain suitable temperatures.

South Australia's Hot Rocks

The geology of South Australia is dominated by Proterozoic rocks, which were formed between 570 and 2,500 million years ago. While these rocks are not uncommon worldwide, those in South Australia are remarkable in that they generate significantly more heat than most Proterozoic rock systems elsewhere in the world. Temperature readings, and measurements of heat flow from deep bores define an area of South Australia's crust which is twice as hot as most Proterozoic crust elsewhere in the world. This area of elevated heat flow is known as the South Australian Heat Flow Anomaly or SAHFA. It is likely that the SAHFA exists because the upper part of the South Australia's Proterozoic crust is extremely rich in a type of granite that produces far more heat than other rock types. In areas where these granites outcrop, for example in the northern Eyre Peninsula, the heat they produce dissipates immediately producing no thermal anomaly. However where younger sediments bury them, they can reach temperatures in excess of 250°C at depths of less than 5 kilometres. For this reason, the SAHFA is an exceptional exploration target area for hot rock geothermal energy.

Although temperatures within the SAHFA are elevated compared to typical Proterozoic crust, the temperatures within this area are not uniform. At a smaller scale, temperatures will vary according to the localized geology - some areas may be relatively cool, whilst others are much hotter. Location of target sources is based on the use of two specific geological models known as the thermally anomalous granite (TAG) model and the radiogenic iron oxide (RIO) model. Each has clearly defined geological parameters.