Where Does Oil Come From? Basic Petroleum Geology, Part I

It's tough to wrap your head around the oil industry without a basic understanding of just what oil (petroleum) is and how it gets to consumers. When we see headlines like "ExxonMobil and Shell Post Record Quarterly Profits" on the same day we've paid almost $5.00 per gallon to fill our tanks, anger and disgust are natural reactions - as is suspicion that the companies making all that money are ripping us off; big time. The huge profits oil companies, large and small, make when oil prices rise into the stratosphere don't however, come (as some might believe) from manipulating prices at the pump. Those profits are, instead, the result of a series of happy accidents and calculated risks, just as are the profits from opening a body shop or buying a fast-food restaurant. The big difference between the oil companies and Wendy’s or Joe's Collision Service is that almost everyone knows where beef patties come from and how fenders get dented. Many people, on the other hand, haven't given much thought to how gasoline gets to the pump or even where it comes from in the first place. You want to know a little more? Let's begin at the beginning...
   
If you ask a third-grader, “Where does oil come from?” he'll probably say, "From dinosaurs!" Ask him again when he's a college senior, and he'll probably give the same answer. He's wrong: though they're among the biggest animals ever to walk the Earth, dinosaurs are way, way, way down the list of sources of oil. The biggest sources are at the other end of the size scale: microscopic plants and animals like algae; most of which were plankton floating in oceans and large lakes. When uncounted 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 that dead organic matter into petroleum: 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 a hunk of iron left sitting behind the garage rusts away to nothing with enough time.

Now we have an underwater layer of sediment chock full of dead stuff - organic matter or complex carbohydrates (compounds of carbon, hydrogen, and oxygen). What's it gonna take to make petroleum out of that stuff? It takes four things:

First, it takes a high enough concentration of that organic matter: a "rich" layer can contain fifteen, twenty, or even fifty per cent organic carbon. As a general rule, concentrations of 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.

Third is heat: things don't even start until the temperature is somewhere between 120° and 190°F.

Fourth is time: lots of time.

The amounts of pressure, heat, and time necessary to "cook" organic carbon into the oil we humans crave are a complex system. Lower temperatures for long periods can generate as successfully as higher temperatures for shorter periods. Two of the three variables, though, require that the carbon-bearing layer be buried; and the deeper the better; though not too deep - if things get too hot, the oil "overcooks." Complicating matters is that different kinds of organic carbon generate different hydrocarbons at different pressure-temperature levels. An entire branch of geology (organic geochemistry) is devoted to the study of this process, which scientists call "thermal maturation."

This is the first step in the lifecycle of oil; creation of a petroleum source and its conversion from tiny bits of dry organic matter into the liquid we know as crude oil. Remember, there are four things, all of which must be present in the right amounts to generate oil: organic matter, heat, pressure, and time. And 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. That last explains why fossil fuels such as petroleum are called "non-renewable" resources: the rate of replacement is so slow that, for all practical purposes, no new petroleum is being created.


Where Does Oil Come From? is the first in a series of posts on oil and the oil industry.

Stay tuned. Future installments will cover:

Where do Oil Companies Find Oil?
How do Oil Companies Find Oil?
The Economics of Petroleum Exploration and Production
Refining: It's not all Gasoline
The Economics of Big Oil
The Future of Oil