"Sedimentary" is a geological term. It describes the rocks which are deposited as a result of erosion of older rocks. Sedimentary rocks have often been deposited over long periods of time in ancient rivers, coast lines or lake beds.

Quite commonly, sedimentary deposits form large basin type structures, with layers from younger to older deposition periods as we go deeper into the earth's crust. Those basins are often the source of oil and gas where ancient vegetation has decayed to form hydrocarbons.

As a result, many of the world's sedimentary basins have been actively explored for oil and gas, so there is a lot of geological knowledge. That data includes:

  • Temperatures of the rocks and water contained within; and
  • Porosity and permeability, which tells us how any geothermal water will flow from the sediments.

In recent years, the geothermal industry has realised that some sedimentary basins contain attractive geothermal resources - zones which have medium temperatures (120oC to 160oC) and which have good flow rates for geothermal production (80 kg/s to over 200 kg/s per well). Today's geothermal engineers and geologists owe a debt of gratitude to our oil and gas colleagues for these discoveries.

Geothermal reservoirs in sedimentary basins are referred to as the "Hot Sedimentary Aquifier" (HSA) model.

Typical sedimentary basins suitable for geothermal production are:

  • The Otway Basin, Victoria, Australia. Hot Rock Limited is the largest holder of geothermal exploration acreage here.
  • The Great Artesian Basin, Australia. This is one of the world's largest sedimentary basin and the source for Australia's only geothermal power plant to date (at Birdsville) (Figure 1).
    The Paris Basin, France. Numerous wells provide district heating in Paris.
  • The Molasse Basin, Germany. District heating plants and new small power plant development with several new geothermal projects in the development phase.
  • The Salton Sea, in the Imperial Valley, California.

Figure 1 - Australia's first geothermal plant is located at Birdsville, using hot water from the Great Artesian basin.