The mineral fuels such as coal, petroleum, and natural gas are described as a special type of economic deposit. They show the geochemical accumulation of carbon and hydrogen through processes that were originally biological in nature. Coal is essentially the result of a large accumulation of land plants, while natural gas and petroleum are the results of marine species, though the possibility of any natural gas and petroleum originating in nonmarine conditions cannot be excluded completely.
Origin of Mineral Fuels
The origin of both natural gas and petroleum presents a more difficult problem compared to coal because they are fluids, and therefore they are free to migrate from their place of origin.
Formation of Coal
Relatively, the formation of coal is a straightforward geochemical process, which can be traced readily through its successive stages. The first prerequisite is a geological one: rapid deposition of plant material in conditions that prevent it from decomposing, accompanied by burial in inorganic sediments such as sandstones and shales. In the Northern Hemisphere, the great coal-forming period followed the Devonian Period (from 345,000,000 to 395,000,000 years ago), when the abundant land plants first appeared and have been named the Carboniferous Period (from 280,000,000 to 345,000,000 years ago).
Large areas of Europe and North America were evidently low-lying swamps that hosted lush vegetation throughout this time period. Accumulated in successive layers, this vegetation died and was partly decomposed by bacteria and also other organisms to form peat. The time of bacterial decomposition came to an end with the burial of peat deposits under inorganic sediments, and the subsequent changes to coal were, most notably, mild metamorphism caused by a rise in pressure and temperature.
Chemically, this mild metamorphism was in an excessive part the expulsion of water and carbon dioxide from the coal-forming substance. The major trend in the change from peat via lignite to anthracite and bituminous coal is the increase in carbon and decrease in oxygen content. If it is carried to its ultimate conclusion, the product would be pure carbon in the graphite form. This takes place comparatively rarely, but evidence for it is given as the presence of fewer amounts of graphite in several metamorphic rocks.
Also, coal contains the inorganic material, which appears as ash when it is burned, and some of the coal ashes represent a remarkable concentration of unusual elements.
Possibilities of the Mode of Incorporation
The source of the trace and minor elements and their mode of incorporation in the coal is still not fully understood. There exist three possibilities:
these particular elements were taken up by the plants during their growth;
they were carried into the swamp of coal as the inorganic sediment’s component; or
they were absorbed either at the time of or after the processes of coal-forming from circulating solutions.
The first possibility is not favoured due to the reason growing plants seldom incorporate significant amounts of non-organic elements.
The second possibility is also unlikely due of there is no correlation, or rather an inverse correlation, between trace element concentrations and ash content. This leaves as the third most likely possibility. The presence of an excess amount of carbonaceous matter means the coal-forming environment is one of the highly reducing ones that will favour the precipitation of a few elements; the presence of sulfide ions and hydrogen sulfide will cause the chalcophile element’s (with an affinity for sulfur) precipitation; the complex organic compounds are noted for their chelating, or absorptive capacity for metallic ions. Therefore, many individual reactions are potentially available for the foreign element’s fixation in the coal substance.
Origin of Petroleum
As mentioned above, the petroleum origin is not as readily elucidated as the origin of coal due to the reason petroleum can migrate from the region where it was formed. Indeed, the presence of a commercial oil field suggests that petroleum has been concentrated from a large number of source rocks into a relatively small reservoir.
The fact that petroleum can be almost always found in marine sedimentary rocks has long been considered a basic argument in favour of a marine origin for this particular material. Certainly, it is true that some of the mineral fuels including oil have been found in metamorphic and igneous rocks, but the migration from a sedimentary source bed is the reasonable explanation for these occurrences.
The proof of a marine origin has been forthcoming in recent years by sensitive analyses of recent marine sediments that exhibit; they contain fewer amounts of petroleum hydrocarbons, evidently generated directly by the marine organisms or by their subsequent decomposition.
While natural petroleum is a complex mixture of hundreds of different hydrocarbons, its bulk composition is surprisingly consistent, containing up to 85% carbon and 15% hydrogen. It can include fewer amounts of organic compounds containing sulfur, nitrogen, and oxygen. Its content of the other elements is exceedingly small. Unlike coal ash, petroleum ash is not noted for its content of trace elements. However, some petroleum ash contains significant amounts of vanadium and has been used as a source of this element.