What are fossil fuels? They are carbon-based materials that are derived from formerly biological matter that has been compacted and chemically or physically changed into a convenient fuel now found within or on the earth. Conventional wisdom includes materials such as peat, lignite, brown coal, black coal, natural gas (methane) and petroleum. Fossil fuels may take millions of years to form. They are regarded as a finite resource and therefore “non-renewable”, in contrast to “renewable” energy resources, which include wind, solar, wave, hydro, geothermal and biofuels.
There is no doubt that coal in its various forms is a fossil biological material because it is found to contain plant and tree remains that are derived from ancient forests and peat bogs that have later become buried by sediments many millions of years ago. In Europe the coal measures give name to the Carboniferous Period of geological time about 300 to 360 million years ago. In Australia and New Zealand coal measures also occur in much younger rocks, from Cretaceous through to Miocene Age (70 to 20 million years ago). Lignite seams over 100 meters thick are open-cut mined in the La Trobe Valley, Victoria, for electricity power production.
In Scotland and Ireland, crofters in the countryside still cut out the surface peat and dry it for a domestic heating fuel. When peat bogs subside and become buried under mud and sands the overlying pressure of the sediments compacts the organic matter into lignite and then coal. It takes about 10 meters of peat to make 1 meter thickness of coal. Coal increases in rank as water and gases are released during burial. The sequence is peat, lignite, sub-bituminous, bituminous to anthracite coal, the later being almost pure carbon without water or volatiles, having been subjected to high pressures and temperatures of burial (or volcanic intrusion).
The lower rank coals often contain appreciable methane, or “coal-seam” gas which is exploited by drilling into the seams, even though they may be uneconomic to mine by underground means. Up to 10% of the energy can be extracted by this means. In Australia, coal seam gas industry is substantial and supplies ca 30% of domestic and industrial gas supplies for the Eastern states of Queensland and New South Wales.
Methane is a clean burning fuel formed by decomposition of organic matter under anerobic conditions (lacking in oxygen) such as found in ponds and swamps. As a small boy I remember stirring up pond sediment and capturing the bubbles of gas in a jar and applying a match flame to produce an explosive “whoof”. Methane CH4 is structurally the simplest of hydrocarbons and is more stable under ambient conditions (P and T of the earth’s surface) than is petroleum..
Petroleum is a complex liquid solution of hydrocarbons of low to high molecular weight, ranging from propane through to waxes. The crude oil has to be refined to provide us with the useful fuels LPG (propane/butane), gasoline, kerosene, diesel and fuel oil, plus lubricating oils. The huge production of petroleum from drilling of oil deposits in marine sedimentary basins has caused it to be considered a fossil fuel of biological origin, certainly by Western oil geologists. Many Russian geologists regard petroleum to be of abiotic (non-biological) origin and so not to be a fossil fuel, but formed in the upper mantle of the earth, from where it migrates upwards to be trapped in overlying sediments. It is likely that petroleum deposits have several origins, as is the case with other mineral deposits (e.g., gold, iron, copper etc).
The discovery of petroleum ca 1860’s essentially saved the sperm whale from extinction. Prior to this time the whaling industry provided whale oil which was used as an illuminant in lamps and as candle wax., but this was soon replaced by mineral oil (kerosene) and coal oil. However whales were still hunted for their meat and edible oils which are used in margarine production.
The existence of whales, plankton and other oil-rich marine creatures shows that “petroleum-like” hydrocarbons can originate in the oceans and when these creatures die their organic compounds could become incorporated in the sediments below, and on deep burial, to form oil deposits. Where do whales go to die? Fossil whale bones are not found in petroleum deposits. It is all very puzzling.
Another theory of petroleum origin is that it is non-biological and the carbon comes from deep within the earth, either as “primitive carbon” like diamonds, or from recycled carbon (carbonates) in subduction zones associated with mountain building. The Theory of Plate Tectonics is a modern one being accepted as true ca 1960, as a development from the early Theory of Continental Drift which was considered rather far-fetched for a 100 years or more. The volcanic, or igneous origin of petroleum dates back to Plato (ca 400 BC), and the idea was expanded by 19th century geologists (e.g. von Humbolt) and by Russian geologists from ca 1950 onwards.
Plate Tectonic Theory provides more support for petroleum (at least some) being not a fossil fuel. Along the margins of the earth’s tectonic plates are subduction zones (e.g. all along the Andean mountain range) where the continental crust is subducted deep within the earth, 70 to 100 kms or more, and subjected to high temperatures and pressures that result in melting and magma formation. What goes down includes carbon from sediments of limestones, marbles, dolomite, even coal measures. Briefly, it is surmised that by reaction of carbonates (CaCO3) and water, under the reducing conditions (Fe, FeO) of the upper mantle, hydrocarbons are formed in abundance, to eventually migrate upwards through fissure zones to become trapped in overlying sediments. Far fetched you may say ? …. well, so was thought the Theory of Continental Drift.
Today there is a conspiracy by the “global warming fraternity” against using fossil fuels because the combustion of these carbon-based fuels releases “greenhouse gases” (harmless CO2 and water) into the atmosphere, which is considered to be “pollution” and a bad thing for the environment. I do not hold with this view which in aggregate is confused and false logic. The present high standard of living enjoyed by most of us today has been due to our exploitation and technological development of fossil fuels over the past 150 years.
Certainly we should conserve our fossil fuels and not waste them. They provide our cheapest electricity and best transport fuels. This will continue for several hundred years more until nuclear energy becomes our predominant energy source.