Coal seam gas
Coal seam gas (also called coal bed methane) refers to naturally occurring gas in coal seams. The gas is held on coal surfaces by water pressure. In order to produce coal seam gas, water pressure must be reduced.
Coal is a type of sedimentary rock. Dead plant matter, collected in environments such as swamps at the Earth’s surface, is buried, compacted and heated within the Earth’s crust. These processes change the layers of vegetation into coal. As the plant material is buried over millions of years it changes. It first forms brown coal (lignite) but with deeper burial it can convert to black coal (bituminous coal and anthracite).
In Victoria, the coal in the Latrobe Valley and at Anglesea is brown coal (lignite). Geologically these brown coals are more than 200 million years younger than black coals found in Queensland and New South Wales.
The coals in Queensland and New South Wales are black coals. Victoria does have some black coal, but it is limited in its occurrence. Small black coal deposits can be found between Wonthaggi and Korrumburra.
Coal seam gas is formed in two ways – by either biological (biogenic) or thermal (thermogenic) processes. As plant material accumulates and is buried, the organic matter breaks down. Gas is then produced by naturally occurring bacteria.
After a sufficient amount of time (millions of years) if the coal is buried deeper and the temperature increases, more gas is produced. These processes are part of coalification: when organic material deposited at the Earth’s surface changes to coal.