Let me begin by pointing out that my research at Oregon State University is primarily related to understanding the mechanisms by which hydrocarbons may be formed on remote planetary moons. The most noteworthy example is Titan, a moon orbiting Saturn.
However, I do follow the research of terrestrial oil and gas formation—it may not be so different from chemical mechanisms occurring elsewhere on other planets and moons. Of course, the biological origin theory would be specific to Earth.
First, the term “fossil fuel” really is an unfortunate misnomer. Regardless whether you subscribe to the biological or abiogenic—also called abiotic—theory of oil and gas formation, fossils have nothing to do with the process. This discussion is rather complicated, so to make it manageable I’ll describe each theory in general terms (accurate, but avoiding unnecessary detail), and then conclude with a comparison and assessment. Even before I begin, though, I want to explain that chemical mechanisms are never proven definitively, rather possibilities are systematically eliminated through careful scientific study (theoretical and experimental) and the remaining possibilities are just that—possibilities.
Biological Theory of Oil and Gas Formation. The biological theory of oil and gas formation dates back to Georg Agricola in 1556 and later to Mikhail Lomonosov in the 18th century. According to this theory, organic material including algae, aquatic microbes, and other sea life, accumulated over eons in oxygen-depleted water, eventually becoming covered with silt. Over time, as this organic-rich matrix was further buried, it was compressed and heated, eventually undergoing unspecified chemical reactions resulting in crude oil and methane (natural gas).
The influence of temperature is important; if the temperature is too cold, the decomposition of the organic matter (called kerogen) will not occur, and if the temperature is too high, the liquid and complex hydrocarbons we call oil will breakdown to methane. The rather narrow range of acceptable temperature is called the oil window and it comprises a temperature range of about 60 to 120 degrees Celsius, corresponding to a depth of about 3 to 6 km.
Supporting the biological theory, oil often has trace-level contaminants of cyclic organic compounds called porphyrins that are also found in living organisms. These so-called biomarkers are argued to be evidence that the source materials must have been biological.
Based on the biological theory of oil production, we expect to find oil at depths up to 6 km or so. We also expect that oil is a finite resource, relatively speaking, since it takes eons for the formation of oil and its subsequent accumulation in reservoirs. In fact, peak oil production has been forecast many times during the last 100 years.
Abiogenic Theory of Oil and Gas Formation. The abiogenic theory of oil production was largely developed by the Russians and Ukrainians, and received a boost in the West in the 1960s following a publication by the American Thomas Gold. There are many variants to the abiogenic theory, but all hold a common theme, that oil (and natural gas) are formed continually through chemical reactions within the Earth, rather than decomposition of dead organisms. This distinction is important, as it would suggest that oil and gas might be more or less uniformly distributed, available in locations (including depths) previously to be barren of oil.
The two most common variants to the abiogenic theory are A) inorganic reactions involving hydrogen and carbon (in various forms) to yield hydrocarbons (oil and gas), and B) extraterrestrial origin; i.e., hydrocarbons deposited on Earth through comet and meteor impacts, as well as during the accretion period when the mass that comprises Earth first coalesced.
My personal opinion is that both are true, but I suspect that the inorganic reactions have a greater contribution to the reserves we know exist on Earth. This is in contrast to Titan, whose abundance of hydrocarbons seems to be an aberration, a local accumulation of hydrocarbon mass during the formation of Saturn and its moons.
There are many natural sources of hydrogen in a reactive form here on Earth. First, there is compelling evidence that hydrogen is dissolved in the iron-nickel core and slowly, over time, this hydrogen is naturally evolving. Hydrogen can also be produced in the mantle from the reaction of water at high temperature with iron-based minerals. Around the world, water-saturated crust is constantly be drawn into the mantle at subduction zones—regions where one continental plate rides over the neighboring plate.
Similarly, carbon is widely available as well. There is strong evidence that elemental carbon exists in significant concentrations within the mantle (we know this from the appearance of diamonds in igneous rocks from ancient volcanoes). And just as water is drawn into the mantle at subduction zones, so is carbon in the form of carbonates (sea shells) and dead organisms. At this point, you may realize that there is some overlap between the biological and abiogenic theories, and you’d be right.
With a proper catalyst (probably iron-containing minerals, this is one of my research areas), elevated temperature and pressure, hydrogen and carbon can be caused to react to form hydrocarbons, both liquids and gases. In fact, Nazi Germany used a version of this chemistry to make synthetic fuels from coal and water.
Analysis. In my opinion, one of the most glaring deficiencies with the biological theory is its lack of specificity. Exactly what reactions are taking place that result in biological remains being converted into a range of hydrocarbon molecules? Furthermore, why is coal so much different from oil? We are fairly certain that coal is formed from anaerobic conversion (under high pressure and elevated temperature) of plant material. The first step in the process is peat, and given enough time and the correct conditions we arrive at coal. But coal is a sold material, and oil is much different; being a blend of hydrocarbons (liquids and gases) it is richer in hydrogen than is coal.
Also, remember the oil window? If oil only forms from decay of biological material within a narrow range of temperature, that temperature corresponding to a depth of up to about 6 km, then why are so many major oil reservoirs found at greater depth? For example, oil is being recovered from massive deposits in the Gulf of Mexico, off the coast of Venezuela, and along the Kola Peninsula in Russia at depths between 10.6 km and 12.3 km. Some of these reservoirs are refilling after having been thought depleted.
Biomarkers are a seductive clue. But there are many plausible explanations for how they end up in oil other than being associated with the source material. Perhaps the most obvious is that these biomarkers are simple contamination that results as the oil migrates through the rock strata. Unless contamination can be ruled out, the whole discussion about biomarkers must be dismissed as scientifically inconclusive.
Oil continues to be discovered and produced at record pace. The dire warnings of peak oil production have proven to be false. Within the decade, the U.S. is poised to overtake Saudi Arabia as the largest producer of oil and gas in the world.
Obviously, this debate is much deeper than presented here. I merely attempted to present what I consider to be the highlights. Logic and reason must prevail; this is at the core of science. This means that theories must be fully developed and tested. At its core, this is the scientific process. My research group at Oregon State University is investigating chemical mechanisms by which hydrocarbons may form from water and mineral carbonates. Whether this is a significantly fast reaction that can explain the formation of oil on Earth is an open question. I would not be surprised if we find, given enough time and research, that many competing reactions are producing oil, and all are taking place every day.