Nearly all petroleum geologists agree that the oil and gas to be found beneath the surface of the earth is there as a result of the death of once-living creatures and the sedimentary entrapment of their carbon molecules.
These creatures are not (especially) the dead dinosaurs of popular imagery. Such imagery became ubiquitous in the U.S. in the mid 20th century, and its echoes remain very much with us. The fact, though, is that on the orthodox account any sort of organic tissue can undergo the sort of anaerobic decomposition that yields kerogen, which in turn yields oil and gas. Further, we owe most of our hydrocarbon fuel to the accumulation of masses of microorganisms, not that of larger and more dramatic beasts.
Oil Geology 101
When a large organism dies on land, it generally begins to compose aerobically, chiefly because microorganisms (especially those dependent on oxygen) start to eat it; that is, to break it down.
But suppose an organism dies and is buried in sediment before it can decay. The easiest supposition is that a sea creature, of any size, dies, drifts to the ocean floor, and is then buried by other stuff sinking on top of it. Our creature will still decay over time, but the decomposition will be anaerobic. As more sediment is piled on top of the old, both the pressure and temperature acting upon the old sediment increase.
For the transition we have in mind, the necessary depth is between 1.25 and 3.75 miles beneath the surface (which may as in our hypothetical be the ocean floor) and the necessary temperature is between 140o and 320o F. Then the organic material turns into kerogen, a sludgy mixture of organic compounds with a very high molecular weight, high enough to render it insoluble by normal organic solvents. Proteins, carbohydrates, lipids, lignin, can all go into the mix and produce kerogen.
As temperature and heat increase still further, and given the right sort of rock, kerogen can transform again into hydrocarbons, either natural gas or crude oil, and can form into reservoirs.
As a general rule, dry gas forms from keragen at higher temperatures than wet gas; wet gas at higher temperatures than oil; oil at higher temperatures than biogenic methane.
Most commonly the reservoir rock that holds these materials is sandstone or limestone. The physical requirements are that the rock must be porous (so it can hold the oil) and it must be permeable (so the oil can flow through it).
But petroleum doesn’t generally bubble its own way to the surface so that a lucky bystander can discover it, shout “I’m rich,” and start planning his career as a philanthropist. The crude generally gets trapped first. Often the trap involves a fold or fault in the sedimentation. That’s the formation that oil exploration companies lust for. Above the trap sits a “cap rock,” something that isn’t porous or permeable: something like chalk, or clay.
The Life and Death of a Heresy
Through much of the third quarter of the 20th century, there was a good deal of talk about another possibility, about a theory that petroleum was not generated by biochemistry but rather by the planet’s mantle. The heretical explanation works, then, not from the surface down but from the deep interior up.
The outer portion of our planet, the “crust” with the continental plates that hold both the continents and the ocean floors, is at its thinnest about five miles deep, at its deepest still only about 22 miles thick. Beneath that, and for a long ways until one gets to the core, sits the mantle.
At the upper level of the mantle, temperatures range from 932 to 1,652°F. Although the mantle itself is not molten, it is predominantly solid rather; it does lend itself to the development of magma, the complex fluid which makes its troublesome surface-world appearances as lava.
The theory of mantle-created petroleum was advanced by, for example, Vladimir B. Porfir’ev and N.A. Kudryavtsev.
Kudtyavtsev argued, among other points, that hydrocarbon-rich parts of the earth’s crust tend to be hydrocarbon rich at many different levels. In his mind, this supported the view that those places were propitious for the upward movement of something derived from beneath the crust, “migration pathways” as they have come to be called.
Thomas Gold, a Cornell University astrophysicist, put forward a variation on this view. Gold conceded that there is biological debris in hydrocarbon fuels, but he disputed the idea that this has anything to do with the fuels’ origins. He contended that microbial life is widespread in porous rock, and that the fuel picks up the debris as it migrates upward from the mantle. Thus, as Gold wrote, the fuels that humans eventually pump out of the ground represent not life reworked by geology (as the conventional view would have it) but geology reworked by life.
Except that they don’t
The consensus in favor of a biotic view of petroleum has more than merely survived the heretical attacks, it has prevailed. The abiotic view may fairly be said to have faded away in recent years.
Part of the problem with the abiotic theory is its chemistry. Methane, for example, is CH4. What abiotic chemical reaction could create this? One possibility is that a molecule of methane (plus two molecules of water) could arise from CO2 plus 4 hydrogen molecules. Formally, one writes that thus:
CO2 + 4 H2 ⟶ CH4 + 2 H2O
The problem, though, is that there isn’t enough abiotic CO2 deep in the crust to make this work in large quantities.
Then there is the matter of empirical evidence. Yes, there do seem to be trace amounts of abiotic hydrocarbons. For example, siderite [a widespread mineral that’s an ore of iron] can decompose in the presence of water. This decomposition does leave hydrocarbons. But, as the Springer Handbook of Petroleum Technology (2017) puts the point, “the contribution of abiotic hydrocarbons to the global crustal carbon budget is considered to be inconsequential.”
The biochemical origin of petroleum outlined above has won the day. Sometimes a consensus becomes a consensus and remains one because it happens to be right.