Just getting here thanks to a heads-up from Hokulele (Thanks, my little Island Princess). As a petroleum geologist, let me weigh in on a few of these topics. I do have one caveat. My knowledge is not completely up to date, having graduated more than two decades ago, but I try to stay reasonably well-informed.
As to inorganic hydrocarbon compounds, yes, they happen. There is a lot of non-organic carbon in the earth and a lot of "laboratories" where heat and pressure can do odd things. The abiogenic theory of petroleum formation had it's most fervent adherent in
Thomas Gold, and he convince enough others to actually try some drilling tests. The most famous of these tests was the drilling project at the
Siljan Ring, an ancient meteorite impact area. A few complex hydrocarbons were found which did appear to have inorganic origin (at least partially), but not in anything close to economic quantities. However, it is widely accepted that the vast majority of complex organic compounds (greater than two carbons) are produced by organic activities. The evidence is simply overwhelming.
Possibly the greatest piece of evidence is found in what are called "source beds", which are organic-rich layers of sediment that can be shown to be creating petroleum
even at this time. What we call "oil shale" is often little more than a source bed that has still retained much of the oil it has generated due to limited access to reservoir rock. But oil does most often migrate upward, since it is less dense than the surrounding rock and the other fluids (mostly water). We have found migration paths complete with trace amounts of oil adhering to migration paths. However, even without finding these tracks, it is possible to run analysis of the oils found in reservoirs and "fingerprint" them, using gas chromatography, mass spectrometry and other tools, and tie them to specific source beds with the same "fingerprints". I used to do this for a living.
Of the inorganic hydrocarbons, methane (CH
4) is by far the most common. Methane has existed in our atmosphere long before life appeared on earth. It is a fairly common breakdown product of numerous inorganic reactions, including the breakdown of carbonate rocks. Ethane (C
2H
6) is another, much more rare breakdown product. Other inorganic hydrocarbons occur by synthesis, rather than breakdown, but only under very rare circumstances of heat and pressure (such as a meteor impact).
Coal and oil, as might be expected, are closely related, and the main difference is the type of organic material that creates them. Coal (which
is, in fact, a rock) is mostly formed from woody plants in swamps and marshes. These were most common during the Carboniferous period when the gigantic tree-ferns were one of the dominant life-forms. Coal is absolutely chock full of fossils, mostly of the tree ferns that created it.
Calamites and
Lepidodendron are two of the most common fossils you will find in coal and the surrounding beds.
The quality of coal varies depending on the burial history.
Anthracite coal is essentially a metamorphic rock in which most of the volatile compounds have been extruded and the remaining carbon compounds are quite pure, relative to other coals. As a result, it burns with high heat and low ash, though it is difficult to light.
Bituminous coal is found as layers of sedimentary rock and varies widely in quality depending on location. It is much more common than Anthracite and much dirtier to burn, especially because of sulfur compounds.
Lignite is very low-grade coal which is of such low carbon content that it is rarely transported, but often used as fuel when it is locally available. And of course,
peat is common around the world, found in peat bogs (where else?) and often cut and burned as fuel. Peat has not undergone lithification, so it is not truly a rock. Give it time.
One common misconception of oil is that it comes from dinosaurs (or animals). This is not totally incorrect, as animals may contribute a small amount of hydrocarbons, but overwhelmingly, oil comes from plants too, mostly planktonic algae and diatoms. They die, they sink, they get buried, they give up their precious bodily fluids.
The most important factor in petroleum formation is basin history, but there are a number of basin types that can lead to petroleum formation. They are not necessarily (as one poster suggested) near tectonic margins. Without going into too much detail, the basin history must provide for (at least) five things in order for oil to be found.
- Source: There must be deposition of organic compounds in the basin. This is not usually a problem as most basins are teeming with life. There are some places though, like the dead sea, which are hostile to life and would not generate significant amounts of petroleum.
- Maturity: Oil is not simply "squeezed out" of the kerogen (oil-producing compounds). It essentially "cooked out" by the heat and pressure of burial. The amount of heat and pressure often determine whether the product is tars and asphalts (very immature), oil (fairly mature), condensate (a mixture of oil and gas), or gas only (extremely mature). Rock maturity can be analyzed by a number of methods, most of which examine the rock, kerogen and vitrinite mineralogy for temperature history.
- Migration paths: There must be a way for the oil to migrate from the source to the reservoir. Source beds cannot usually be produced due to low permeability. Oil shales are an exception, but they require much different technology.
- Reservoir: There must be some sort of rock with permeability and porosity that allows significant volumes of hydrocarbons to "fit in" and "flow out".
- Trap. Something has to stop the hydrocarbons from "migrating" all the way to the surface. Typically these are areas of impermeability, caused by lithology changes. There are a large number of trap configurations, ranging from simple domes to complexly faulted and salt-intruded structures.
Okay, end of lesson. I'll be happy to answer specific questions to the best of my ability.
