The outstanding feature of all fossil fuels is that they contain a lot of carbon. Coal is especially rich, with up to 95%. The others are mainly hydrocarbons, compounds of carbon with hydrogen, sometimes with other elements present, but even in these the proportion of carbon is high, around 82-87% by weight.
About Coal
Coal was one of the earliest minerals to be be developed in today's technological society, in fact it was one of the main props for the Industrial Revolution, which started in Britain. Britain has considerable coal deposits and a long history of geological discovery, so the nature of coal deposits in that country have become known in great detail.Figure 13.1 (taken from the 1875 Encyclopaedia Britannica) shows the various geological strata found in conjunction with the Coal Measures of different parts of Britain. The actual coal seams vary in thickness from a mere film to as much as 15 metres. In other parts of the world even thicker seams have been found, as in the south of France and in India, up to 60 m thick or more.
Fig. 13.1. Carboniferous strata in Britain
Of course even the rich coal deposits form only a small fraction of the total rock strata, which in the Carboniferous of Britain can be more than 4 km thick. The majority of the rock is made up of typical sedimentary strata, in particular sandstones, limestones, and shales. Although the majority of important coal deposits of the world are of Carboniferous age, some are found in the Permian period which follows, and also in the younger rocks of the Mesozoic and Cenozoic. The younger deposits are usually much less compacted ('brown coal'), have more moisture, and have clearly undergone less conversion from the original plant remains.
Older, more compact coals have little moisture and are richest in carbon, having as much as 95%, the rest being hydrogen, water, and ash. In good coals of any age the ash content is quite low, below 2%. This is similar to the ash content of the above-ground portions of modern plants.
How did coal originate? The answer to this is to be found in any geology textbook, which describes the vast swamps of the Carboniferous Period, with their giant primitive trees and strange animals. That some such plant provenances existed is undoubted -- there are too many well-preserved fossil plants involved in the Coal Measure deposits of the Earth to be able to reject the notion. But the standard swamp picture has a number of serious deficiencies.
Puzzles of Coal Formation
All the sedimentary strata enclosing coal are typical of offshore deposits, deposits laid down in the seas. Limestones are almost invariably of marine origin, sandstones are normally produced on the sea floor from particles washed in by rivers or off the coasts. Shales may be formed from the mud of lakes, but are more typical of offshore seabed areas, beyond the point where the coarser sand particles have already settled.Why should coal seams be enclosed in these typically marine deposits? Why is coal relatively pure carbon, without much trace of any soil remains? Why are 'marine bands', deposits obviously derived from the sea [68], often found within the coal seams? Why are fossil mussel shells often associated with coal? Why are deposits of coal sometimes associated with salt beds?
These questions are even more perplexing when you take into account the relatively small amount of land surface which existed during the Carboniferous, if the approach used in this book is to be believed. Modern high-carbon deposits are being formed on land today, within our swamps and marshes (initially as peat), but their thickness is not great, especially after conversion and compression to a composition and density similar to that of coal.
An answer to some of these difficulties may be found in the following suggestion. Could it have been the case that these vast Coal Measure swamps existed, not on land, but on the surface of the sea? Was coal, and probably much of our oil and gas deposits, formed from material sinking to the floor of shallow seas, at a time when all seas were relatively shallow (because expansion had not then proceeded to the stage where ocean deeps existed)?
Proposition 13A
Most coal deposits were produced by the conversion of plants
which had grown up floating on the surface of the sea
"Impossible!" would be the first response. How could the tall Coal Measure plants, clearly adjusted to fresh water, exist on the sea?
The Quaking Forests
A fascinating and unusual landscape feature can sometimes be encountered which is known as a Quaking Forest. You walk through the pine forests, and suddenly you notice that the trees are swaying, although there is no wind. They only sway where you are walking. The march of the Ents, perhaps, from Tolkien?The explanation for a Quaking Forest is simple. It is a forest which has grown on top of a lake, on a layer of floating plant debris which has gradually accumulated and grown out from the original lake edge. This phenomenon is well known and accepted with some mangrove swamps, growing out from river banks, sometimes completely choking a river. But with a Quaking Forest, there is actually a pocket or lens of water left between the underside of the mass of plant roots and the solid mud which formed the bed of the lake.
It is like a layer of moss growing on the top of a waterbed -- push your finger down into the top and the closer stems bend towards you. As you release your finger, or as waves travel out from where you pushed down, the stems bend and sway, back and forth.
It might be argued that the huge Carboniferous plants were too big to float on the surface of the sea, they would fall over. But, of course, the pines do it now in a Quaking Forest. And, in the densest, tallest, and most prolific rainforests of today, the root systems of the huge trees are surprisingly shallow. They resist falling over partly by developing buttresses, but more importantly through the shelter of their environment protecting them from winds. We have already seen, in NU011, how the Carboniferous conditions were probably of dense, still air under an impenetrable cloud cover which would suppress air movements.
The Floating Swamps
The picture we are building up is perhaps not too different to the accepted swamp scenario, but with one vital difference -- the swamps were not on land, but on the sea. This would explain much. It would explain why the coal seams are interleaved with marine sedimentary rocks, marine shell bands, and occasionally salt beds. It would explain the comparatively wide extent of coal deposits in the land-poor Carboniferous world. It would explain the low ash content of coals, if the plants they were derived from grew in the absence of soils.How about the saltiness of the sea? We have already seen (Propositions 10I, 10J) that the salinity is likely to have increased continously up to the present time, so the seas would have been less salty during the Carboniferous than they are now. Moreover, it would be possible for a thick continuous mass of floating organic material to be saturated with fresh water, even though it was floating on salty water.
Fresh water is less dense than salt water, and under calm conditions it could easily happen that the floating plant layer, soaked in the rain which we have seen was probably falling continually, was stabilized enough so its fresh water did not mix with underlying more salty layers on which it floated. After all, that is not so very different from present conditions where a plant is growing in a soil, wet with fresh water, which overlies a deeper water table where the water is known to be salty.
If the scenario I have painted for the early days of life on Earth is correct, we are looking then at a much smaller Earth, with less land than now, but also much less extensive seas. Instead of the rolling oceans of today, the seas would mostly be relatively shallow interdomain gulfs, perhaps none more than 100 km across, and there would be no deep oceans.
Of course there could still be the conventional shore-line swamps, but these would be only a minor component, blending in continuously with the on-sea swamps. The latter would build up a thicker and thicker layer of plant material, the bottom part of which would break off periodically and sink down to the bottom of the sea. Or possibly whole floating islands could break off, like icebergs calving from a glacier, and later sink further out to sea. In these ways, in quiet times, very thick seams of what was to become coal could be accumulated.
When times were not so quiet, and domains were in active movement, the floating swamps might be washed or blown away. Newly-upraised land would erode and provide abundant sedimentary material to cover the coal. As the interdomain gulfs widened with Earth expansion, these sedimentary layers would be covered with the fine muds of more offshore areas, and perhaps the limestones of the still, warm seas.
Proposition 13B
Coal deposits were laid down in the narrow and shallow
interdomain gulfs produced by early Earth expansion
The Petroleum Story
While coal was the energy mainstay in the early development of modern industry, petroleum is a latecomer in this respect, a child of the 20th Century. During this century it has moved from an energy source of little consequence to be the principal source of our needs. In the present context, petroleum can be taken to include both oil-type sources which are liquid under normal temperatures and pressures, and natural gas.Both types frequently occur together, often with the gas dissolved in the oil, often under very high pressure -- helping to make a self-pumping 'gusher'. Natural gas as a developed energy source is even newer than oil, dating back only to the 1950s. Before this, the gas was usually regarded as an annoying byproduct which was burnt off or otherwise went to waste.
Of course there are instances of practical use of petroleum, dating far back into the past. Natural seepages of oil (asphalt and bitumen) were used in the Middle East by the Sumerians, Assyrians, and Babylonians some 5000 years ago, in building mortar, road construction, and ship caulking.
Petroleum is very commonly associated with salt, and as the use of deep drilled wells was once primarily for the extraction of brine (concentrated natural salt solutions), it has often figured as an unwanted discovery. This was the case with the early Chinese, who around 200 BC drilled a 140 m deep well to extract brine and were annoyed to get gas as well. Subsequently they worked out how to burn the gas and use it to evaporate the brine in making salt crystals.
Even the early work in the United States, where large-scale petroleum extraction was pioneered, had a similar history. In 1819 a well being bored for brine in Kentucky yielded so much black petroleum that it was abandoned in disgust. In 1829 another Kentucky brine well yielded a huge flow of several thousand tonnes of oil, most of which was wasted, although a little was bottled and sold for liniment (as 'American oil'). It was not until 1859 that a well was bored specifically to extract petroleum, in Pennsylvania.
Formation of Petroleum
There are obvious similarities and links between petroleum and coal, and a number of obvious differences. Chemically, petroleum sources are principally hydrocarbons, compounds of carbon and hydrogen, whereas in coals much of the corresponding hydrogen has been eliminated. Both fossil fuel sources are essentially complex mixtures, with no two deposits chemically identical. Coal often has a much higher sulphur content than petroleum, and for this reason has lost favour for domestic use with increasing concern over air pollution.Physically, petroleum sources are fluids whereas coal is a solid, and this has important consequences. As a fluid, petroleum can migrate, and the rocks from which it is extracted are often not the same as the ones in which it was formed. It also means that to be available for large-scale extraction, the petroleum must be 'trapped' in the rocks in some way, as with impermeable layers of clay, shale, or salt around it.
The reservoir rocks which hold the petroleum are mostly sandstones (59%) and limestones, including dolomites (40%), the same typical sedimentary rocks which were associated with coal. Less than 1% of the world's oil has been found in fractured igneous or metamorphic rocks, which typically lack the pore or void space needed to be successful reservoir rocks.
There seems no doubt that, whatever their mode of formation, both coal and petroleum are essentially derived from the remains of living creatures. In Proposition 13A, I made the possibly novel suggestion that coal was formed from the remains of plants growing floating on the seas. It seems very likely that petroleum had a similar origin.
Proposition 13C
Oil and gas deposits were formed from the remains of plants
which had grown floating on the surface of the sea
Amusingly enough, while the coal proposition in 13A may lead to outraged protests, the almost identical one for oil will not -- it is close to the currently accepted view. This is that the major source of petroleum was floating plankton, minute marine plant and animal organisms, which grew in shallow seas.
Fuller details of the reasons for concluding that petroleum has an organic origin, and that its major source was marine plankton, are given in the 1974 Encyclopaedia Britannica article on Petroleum [12]. The paragraph on the origin of petroleum concludes "In spite of the great amount of scientific research ... there remain many unresolved questions regarding its origin".
It does seem possible that, even though Proposition 13C can be regarded as the accepted view, the floating plankton source idea may need modification in two ways. The first is to suggest that what are regarded as 'land' plants formed an important, or even the principal, source of the petroleum material. In other words, these plants grew on floating mats on the sea, just as suggested for the coal deposits. And the remains of plants which are accepted as being of 'land' types are not uncommon in some petroleum deposits.
The second point has more implications; this is the suggestion that the floating mats of material were essentially continuous, forming closed capping layers over the surface of the sea. While these layers may not have been as thick and 'trafficable' as the coal ones, able to support quite tall trees, they still may have been able to effectively seal off the underlying sea from the atmosphere and from normal evaporative processes.
Proposition 13D
The floating layers of plants which provided the source
material for petroleum and coal were able to seal off significant
areas of the seas and prevent normal evaporation
If this Proposition is found to be valid, it has considerable implications for the formation of both fossil fuels and for salt. It is not disputed that the formation of fossil fuels from organic materials needs anaerobic conditions, those where oxygen is lacking. This is because the actual conversion is done by anaerobic bacteria which can only function where there is no oxygen -- these are the organisms responsible for production of marsh gas (methane) from bogs, which lack oxygen under their surfaces.
Clearly sealing off the surface of a shallow sea with organic material would allow its water to become completely anaerobic and enable the conversion of plant remains under the surface to coal and petroleum.
Proposition 13E
Seas sealed from the atmosphere with a floating organic layer
would become anaerobic and foster the conversion of organic
material to fossil fuels
It has always been assumed that rock salt deposits, which are sometimes of great thickness, were formed by conventional evaporation of water from the surface of enclosed lakes or seas. This may well be the case, but it is also possible that they were formed from sealed seas.
If domain movement caused the uplift of a sealed-sea area, or some other change occurred to reduce the rain falling on such an area, it would be expected that the water in the sea would be gradually diminished and would disappear. Even if the floating plant layer was completely dead, water would continuously rise through the sponge-like layer and be evaporated, leaving the salt behind. Once a certain salt concentration was reached, the special properties of such salt solutions for holding thermal inversion layers could accelerate this process, leading to the formation of thick salt layers beneath the organic seal.
Proposition 13F
Some salt deposits were formed by the elimination of water
from sealed-sea areas
Of course this suggestion does match in with the observed association of salt with coal and petroleum deposits.
The Age of Fossil Fuels
We have seen that most black coal deposits were formed in the Carboniferous, with some in the Permian, in the last two periods of the Paleozoic. Most petroleum deposits are found in the rocks of the Mesozoic (63%) and Cenozoic (29%), with only 8% in Paleozoic rocks, and almost none in the Precambrian.Of course petroleum is known to migrate from its strata of accumulation, but these figures do fit in well with Expanding Earth scenarios and other suggestions already made. Coal deposits are mostly in current land areas, whereas oil and gas fields are increasingly being developed on offshore areas, on the continental shelves. In the modern deep ocean beds, which are comparatively young, under 200 my old, no fossil fuel deposits at all are found.
The inference is that the fossil fuel deposits were laid down at the bottoms of the deepest seas which then existed.
Proposition 13G
Fossil fuel deposits were formed at the bottoms of the deepest
seas which then existed, from plant sources floating on those seas
The scenario is this. In Paleozoic times, the only seas which existed were fairly shallow and lying in the new interdomain gulfs, and it was in these that the coal plants were dumped. With increasing Earth Expansion, these areas are now mostly above sealevel. As expansion continued into the Mesozoic, new and lower interdomain gulfs opened up. It was into these that most of the petroleum plants were deposited, and it is these areas which are today mostly lower and at continental-shelf level.
Advancement into the Cenozoic saw the development of the modern deep ocean basins, and with this, changes in conditions to largely eliminate fossil fuel formation. The great floating plant mats died off, unable to survive in the blustery open ocean conditions, increasing salinity, and reduced carbon-dioxide content of the air. Their disappearance meant that the anaerobic conditions needed for plant conversion could no longer be attained in the ever-deepening seas.
The Matter of Sulphur
A minor point mentioned above was that coal deposits often have high sulphur contents, which makes them less suitable than oil or gas for domestic use, because the sulphur can lead to air pollution.Evidence which we have seen so far gives us an explanation for this difference. It seems likely that there was much more sulphur in the air in Paleozoic times, and this too, like the ammonia and methane, was largely eliminated by the start of the Mesozoic.
Proposition 13H
The Paleozoic atmosphere originally contained much more
sulphur compounds, which were largely eliminated by the start
of the Mesozoic
In the primeval atmosphere, sulphur was likely to have been present as hydrogen sulphide, the gas which gives the smell to rotten eggs. This is likely to have been converted to sulphur dioxide, the most common oxide of sulphur, by the start of the Paleozoic (when free oxygen became plentiful).
Both hydrogen sulphide (molecular weight 34) and sulphur dioxide (mw=64) are relatively heavy gases (see Table 11). Because of this, they are not likely to have been appreciably lost into space, but instead incorporated into sulphate sedimentary rocks when the right chemical conditions arose.
(Full list of references at NURefs)
[12]. Encyclopedia Britannica. 1974 edition.
[68]. F H T Rhodes. A evolucao da vida. Ulisseia, Lisboa, 1960.
NU014: Geoprospecting and Mineral Riches
NU012: Death Of The Dinosaurs
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