Unconventional Petroleum Resources
Unconventional resources are natural resources which require greater than industry-standard levels of technology or investment to exploit. In the case of unconventional hydrocarbon resources, additional technology, energy and capital has to be applied to extract the gas or oil, replacing the natural action of the geological processes of the petroleum system.
Examples of unconventional oil resources include oil shales, oil sands, extra-heavy oil, gas-to-liquids and coal-to-liquids. Oil shale is an example where a thermally immature source rock has not generated and expelled hydrocarbons. Oil or tar sands occur where conventional crude oil has failed to be trapped at depth and has migrated near to the surface and has become degraded by evaporation, biodegradation and water washing to produce a viscous heavy oil residue.
In contrast to conventional gas reservoirs, natural gas can also be found in more difficult to extract unconventional deposits, such as coal beds (coal seam gas), or in shales (shale gas), low quality reservoirs (tight gas), or as gas hydrates. Unconventional gas accumulations reflect the failure or under-performance of the petroleum system.
- Shale gas and coal seam gas are examples where the natural gas is still within the source rock, not having migrated to a porous and permeable reservoir.
- Tight gas occurs within low permeability reservoir rocks, which are rocks with matrix porosities of 10 per cent or less and permeabilities of 0.1 millidarcy (mD) or less, exclusive of fractures. Tight gas can be regionally distributed (for example, basin-centred gas), rather than accumulated in a readily producible reservoir in a discrete structural closure as in a conventional gas field.
- Gas hydrates are naturally occurring ice-like solids (clathrates) in which water molecules trap gas molecules in deep-sea sediments and in and below the permafrost soils of the polar regions.
The recent developments of oil sands in Canada and of shale gas in the United States are examples where rising energy prices and technological development has facilitated the exploitation of unconventional hydrocarbon resources. In the Australian context, coal seam gas is the unconventional hydrocarbon resource that is most developed.