Five unconventional fuels: geology and environment

Unconventional fuels may present a viable partial replacement for conventional fossil fuel
\nreservoirs (such as sandstone and limestone) in rocks onshore and offshore. These
\nalternative fuels are obtained from distinct sources and employ extraction technologies
\nwhich are very different to those used to extract conventional hydrocarbons.
\nOil sands (also known as tar sands or bituminous sands) are loose sand or partially
\nconsolidated sandstone containing viscous bitumen. Resources occur in Canada, Kazakhstan
\nand Russia and estimated worldwide deposits represent 2500 billion barrels of oil in place.
\nOil sands have only recently been considered to be part of the world's oil reserves, as higher
\noil prices and new technology enable profitable extraction and processing. Converting oil
\nsands to liquid fuels requires energy for steam injection and refining.
\nMethane from coal includes gas recovered from active (coal mine methane or CMM) and
\nabandoned 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
\nthese sources is already produced on a modest scale and exploration is ongoing for further
\nprospects. Gas can also be derived from coal by combustion of underground coal seams in
\nsitu to produce synthetic gas (‘syngas’). This process is usually known as 'underground coal
\ngasification' (UCG). This technology is also in its infancy both in terms of engineering the
\nsubsurface process and in the understanding of subsurface and surface environmental
\nimpacts. Methane hydrates (methane gas trapped in ‘cages’ of water molecules, resembling ice) have
\nbeen recovered from, or are postulated for, virtually all marine shallow sediment
\ncontinental margins around the world and a few areas onshore. Volumes of about 2 x
\n1014m3 methane in‐place have been estimated for this potential resource. To quantify
\nreserve potential and to identify suitable methods of methane extraction, a full
\nunderstanding of how hydrates are held within sediments is required.
\nA less well known unconventional fuel is subsurface hydrogen. Small flows of hydrogen
\nnaturally occur in some mines and in deep oceans associated with abiogenic and biogenic
\nmethane, nitrogen and helium. The main geological environment that is promising for
\nexploration is the tectonic remnants of ancient ocean floor known as ophiolites. The main
\naccessible onshore areas are where ophiolites are found tectonically emplaced within fold
\nbelts.
\nThough unconventional fuels represent an enormous resource overall, some of the
\ntechnology is immature and many of the environmental impacts of their exploitation are
\nunknown. Apart from subsurface hydrogen, all are hydrocarbons and thus are constrained in
\ntheir use in countries which may limit carbon emissions either now or in the future.

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