If you ask your third-grader, "Where does oil come from?" he'll probably gleefully shout, "From dinosaurs!" Ask the little guy again when he's a college senior, and there's a pretty good chance he'll give the same answer. He's on the right track: oil comes from the long, slow decay of dead plants and animals. He's wrong about it coming from the big lizards, though: while they're among the biggest animals ever to walk the Earth, dinosaurs are way, way, way down the list of things that died to make oil.
The most important sources are at the opposite end of the size scale: microscopic plants and animals like algae; most of which were plankton floating in oceans and large lakes. When untold billions of those animals and plants died, their bodies settled to the bottom along with fine sediment, and the whole shebang ended up buried. That's a key to turning dead organic matter into oil: burial. One of the reasons why dead dinosaurs - most land animals for that matter - didn't get turned into oil is that they were exposed to the air and oxidized, just like Grandpa's '68 Chevy rusts away to nothing if you leave it sitting behind the garage long enough.
OK, back to those micro-organisms. Now that they're dead and buried, we have an underwater layer of sediment chock full of dead stuff - what we call organic matter or complex carbohydrates (compounds of carbon, hydrogen, and oxygen). What's it gonna take to make oil out of that? Well, it takes four things:
* First, you need enough organic matter: a good, rich layer has fifteen, twenty, or even fifty per cent organic carbon. As a general rule, layers that have less than four to five per cent organic carbon are too 'lean' to produce much oil. Just like it is with parents, richer is better.
* Second, it takes pressure. You get pressure by stacking lots more rocks on top of your layer.
*Third, it takes heat. Things don't even start until the temperature is somewhere between 120 and 190F (about 50-90 C).
*Fourth, it takes time: lots of time.
The amounts of pressure, heat, and time necessary to "cook" organic carbon into the oil we crave makes a complex balancing act. For example, low temperature over a long time generates as successfully as high temperatures for less time. Two of the three requirements, though, mean that our carbon-bearing layer must be buried. The deeper the better, though not too deep: if things get too hot, the oil can "overcook." Another complication is that different kinds of organic carbon generate their oil at different combinations of temperature and pressure. An entire branch of geology (organic geochemistry) is devoted to the study of this process, which scientists call "thermal maturation."
Geochemists call all this the biogenic theory of oil generation. "Biogenic" means that the raw material to make oil is, or was, living things. A very small number of scientists, mostly in the former Soviet Union, have different ideas.
The theory of "abiogenic" oil (no living things involved) holds that some oil came from primordial carbon compounds buried tens and even hundreds of miles beneath Earth's surface. The theory says that these compounds, such as methane, have been altered by the heat and pressure at these depths to form oil. To date, however; no experimental test of abiogenic generation has found significant amounts of oil. Scientists who do not subscribe to the abiogenic theory say that no such experiment has found any oil at all. Though most scientists are able to keep an open mind about contrary theories, the vast majority of earth scientists believe that abiogenic oil, at best, contributes only a tiny fraction to the Earth's petroleum reserves. Most consider the theory completely unfounded.
So, to review: biogenic oil requires four things, all of which must be present in the proper amounts to generate oil. These four are organic matter, heat, pressure, and time. When we say time, we don't mean on the order of days, weeks, months, or even years. We're talking tens or hundreds of thousands of years; even millions of years if things aren't very warm. The time needed explains why oil is a non-renewable resource: the process is so slow that, for all practical purposes, no new petroleum is being created.