As indicated in the coal sector page, thanks to gasification or liquefaction techniques, research is underway to develop coal deposits previously considered non-economically exploitable

In more general terms, alternatives to conventional oil are increasingly being developed, by using resources that are not directly usable at low cost as raw materials.

Thus a wide variety of liquid and gaseous fossil fuels have emerged during the last decades, resulting in a great diversification of the fuel supply, previously dominated by oil and secondarily by natural gas.

The goal is to produce liquid or gaseous fuels in order to replace gasoline and diesel which represent the quasi-incompressible demand of the transport sector (Figure below). In this figure, CNG stands for compressed natural gas. These vehicles are fueled by natural gas contained in high pressure tanks (about 200 bar). The other liquid and gaseous fuels may be grouped under the name of non-conventional oils and synthetic hydrocarbons, or synfuels.

Experts have already begun to include some fuels once considered unconventional, such as shale, in the estimation of proved reserves and the amount of oil produced. It is likely that this trend will continue in the future and that the boundary between oil and gas on the one hand and non-conventional fuels on the other hand will remain unclear and in an evolving state.

Note that this observation explains why the evaluation of oil and gas reserves has recently been significantly reconsidered.

It should be noted that the definitions of these fuels have varied widely over the past decades. Twenty years ago, the term synthesis hydrocarbon included all non-conventional production, but today this term is restricted to fuels produced by chemical means, and there is a preference to call unconventional oils those produced from shale sands or tar sands.

For the sake of simplicity, we will group under the name of non-conventional fuels:

• oil produced from heavy oil and oil shale;

• biofuels, whether first or second generation;

• synfuels;

• liquid fuels from natural gas;

• methane hydrates.

The largest deposits of heavy oil and oil shale are located in Canada and Venezuela.

These include:

• tar sands, crude oil of high viscosity, that it is often impossible to extract the standard way;

• oil shale, which is a sedimentary rock containing a solid organic material which can be extracted with heat in liquid or gaseous form.

Their development may pose some serious pollution problems, as is the case for example for the oil shale industry: to produce one tonne of synthetic oil, 10 tonnes of shale are treated and 2.5 tonnes of water are used. In addition, the process rejects waste volume 20% higher than the inputs (the so-called "popcorn" effect). The environmental impact of these technologies is far from negligible and appropriate solutions must be developed before production can be generalized.

Moreover, the valuation of these resources may require large amounts of energy, particularly for injecting steam at high temperature, so that their overall balance must be carefully estimated, particularly in terms of CO2 emissions.

These are fuels produced from biomass. There are generally two types of biofuels, namely of the first and second generation. Even though their definitions differ among authors, the first usually refers to biofuels that come into direct competition with food crops, while the latter are derived from lignocellulosic materials.

Some countries like Brazil and the United States have launched extensive programs to produce ethanol from sugar crops such as beets or sugar cane. Up to 10% ethanol can be incorporated in fuels for conventional gasoline engines, but its use as pure fuel implies significant changes in the fuel system.

In Brazil, the Proalcool program aims at substituting ethanol from sugar cane for gasoline. The development since 1975 of a refining (distillery) and distribution infrastructure has allowed Brazil, since 1984, to replace approximately 5 million m3 of gasoline with ethanol, and convert 13% of the fleet to this national resource. Twenty years later, 80% of the fleet is "flex-fuel", which allows the consumer to choose, depending on prices, the cheaper fuel.

Biofuel for diesel engines is also produced from rapeseed oil or sunflower oil in Europe and the United States (between 7% and 30% can be added to diesel). This is known as biodiesel.

As of now, the main drawback of these programs is that they have the effect of diverting arable soil from food production, which may represent a high social cost, causing famines in countries that implement them or even globally, a decline in the supply of agricultural products resulting in an increase of their cost.

Gasification or liquefaction processes of coal are relatively old: in 1943, Germany produced up to 4 million tons of synthetic fuel.

Methanol and synthesis gas could also be produced from coal, by combining electrolytic processes and gasification with oxygen.

To limit CO2 emissions, electricity could be provided by a nuclear plant, and methanol and synthesis gas would be used to power gas turbines for peak periods. This way, a power generation fleet could be supplied from only nuclear energy and coal.

These products stem directly from natural gas: they consist of ethane, propane, normal butane, isobutane and natural gasoline / condensate.

Blended with natural gas, they are extracted during gas processing, before transportation to the place of consumption, and are substitutes for refined products from oil, their characteristic being their liquid rather than gaseous form.

Methane hydrates consist of water molecules trapping gas molecules such as methane that can store considerable amounts of gas (e.g. 164 cm3 of methane in 1 cm3 hydrate).

They take the form of flammable ice blocks found in sediments at low temperature and high pressure (ocean or circumpolar regions).

Their development is under consideration, but still poses many technological challenges and could have a significant environmental impact.