Coal seam gas is trapped in coal formations, typically 400-1000 metres underground, although it can be found at depths up to 1200 metres.
Gas explorers begin by examining the local geology to see if gassy coals are likely to be present.
If the area looks promising, they then use survey technology, such as seismic surveys, to detect whether the coals are likely to contain gas deposits and how large these deposits are likely to be.
Explorers generate seismic (sound) waves and measure the time taken for the waves to travel from the source, reflect off subsurface features and be detected by receivers at the surface.
The time taken to travel from the source to the receivers can indicate features such as rock density and the likely presence of fluids or gases. This can help build an image of the subsurface.
Seismic is often followed by the drilling of coreholes.
Coreholes between 10cm and 30cm across are used to extract core samples of rock strata. These samples enable measurements of gas content, rock permeability, thickness of the reservoirs and other information. Core holes simply acquire rock samples; they are not wells and they cannot extract gas.
If interpretation of survey results shows it is likely that gas deposits exist in a particular area, the operator might drill an exploration well. But even positive survey results do not guarantee successfully finding commercially viable gas deposits.
During and after the drilling of an exploration well, information is acquired in various ways, including:
These tests give a clearer picture of whether gas is present and if it can be commercially recovered.