Five unconventional fuels: geology and environment

Unconventional fuels may present a viable partial replacement for conventional fossil fuel reservoirs (such as sandstone and limestone) in rocks onshore and offshore. These alternative fuels are obtained from distinct sources and employ extraction technologies which are very different to those used to extract conventional hydrocarbons. Oil sands (also known as tar sands or bituminous sands) are loose sand or partially consolidated sandstone containing viscous bitumen. Resources occur in Canada, Kazakhstan and Russia and estimated worldwide deposits represent 2500 billion barrels of oil in place. Oil sands have only recently been considered to be part of the world's oil reserves, as higher oil prices and new technology enable profitable extraction and processing. Converting oil sands to liquid fuels requires energy for steam injection and refining. Methane from coal includes gas recovered from active (coal mine methane or CMM) and abandoned mines (abandoned mine methane or AMM), as well as methane recovered from undisturbed or ‘virgin’ coal seams (usually known as coal bed methane or CBM). Gas from these sources is already produced on a modest scale and exploration is ongoing for further prospects. Gas can also be derived from coal by combustion of underground coal seams in situ to produce synthetic gas (‘syngas’). This process is usually known as 'underground coal gasification' (UCG). This technology is also in its infancy both in terms of engineering the subsurface process and in the understanding of subsurface and surface environmental impacts. Methane hydrates (methane gas trapped in ‘cages’ of water molecules, resembling ice) have been recovered from, or are postulated for, virtually all marine shallow sediment continental margins around the world and a few areas onshore. Volumes of about 2 x 1014m3 methane in‐place have been estimated for this potential resource. To quantify reserve potential and to identify suitable methods of methane extraction, a full understanding of how hydrates are held within sediments is required. A less well known unconventional fuel is subsurface hydrogen. Small flows of hydrogen naturally occur in some mines and in deep oceans associated with abiogenic and biogenic methane, nitrogen and helium. The main geological environment that is promising for exploration is the tectonic remnants of ancient ocean floor known as ophiolites. The main accessible onshore areas are where ophiolites are found tectonically emplaced within fold belts. Though unconventional fuels represent an enormous resource overall, some of the technology is immature and many of the environmental impacts of their exploitation are unknown. Apart from subsurface hydrogen, all are hydrocarbons and thus are constrained in their use in countries which may limit carbon emissions either now or in the future.