What is conventional and unconventional gas?

Natural gas from conventional reservoirs and unconventional reservoirs is the same. The term unconventional gas does not describe the gas itself, but instead refers to the source rocks that creates the gas and the porosity and permeability of the gas reservoirs.

What is conventional gas?

Conventional gas is obtained from reservoirs that largely consist of porous sandstone formations capped by impermeable rock, with the gas trapped by buoyancy. The gas can move to the surface through the gas wells without the need to pump. (Source: CSIRO)

In the northern Perth Basin, conventional gas is produced from the Waitsia and Beharra Springs gas fields. The recently discovered Waitsia gas field, at depths of 2-3 kilometres, is also predominantly a conventional structure. The Waitsia gas field development proposal involves only conventional gas that flows freely once the wells have been made ready for production.

What is unconventional gas?

Unconventional gas is natural gas trapped in very dense rocks with low permeability that prevents gas flowing into wells in commercial volumes. Unconventional gas generally requires hydraulic fracturing to improve reservoir permeability and extract the gas resource in commercial quantities.

The three most common forms of unconventional gas are:

  • Tight gas
  • Shale gas
  • Coal seam gas

For more information on unconventional gas visit CSIRO.

What’s the difference between shale, tight and coal seam gas?

Shale, tight and coal seam gas are types of natural gas identified by the geology of the underground area where the gas is sourced such as tight sandstone, shale and coal seams.

Most of Australia’s coal seam gas developments are found on the east coast and none are found in Western Australia.

The most significant difference between shale gas and tight gas is:

  • Shale gas is mostly found trapped in layers of sedimentary shale rocks
  • Tight gas is found trapped in sandstone or limestone formations with relatively low permeability.

While coal seam gas is fairly shallow and more easily extracted from the coal seams where it was formed at depths between 300 metres to 1 kilometre, shale gas and tight gas is found at much deeper depths between 2 – 5 kilometres below the surface.

Techniques such as hydraulic fracturing are required to extract the trapped natural gas because of the particular characteristics of the rock where the gas is located.

Mitsui E&P Australia is not pursuing coal seam gas, shale gas or tight gas in the Perth Basin. The proposed Waitsia Gas Project Stage 2 will see the development of conventional gas reserves in the Waitsia field.

Schematic geology of natural gas resources

Source: http://www.eia.gov/

The Perth Basin

The Perth Basin has been the source of some large conventional oil and gas discoveries, which indicate that the area is an active hydrocarbon province. As these existing discoveries have been successfully developed, further sources of oil and gas have been sought to utilise the existing pipeline infrastructure in the region.