Oil's origin
Crude oil -- properly called petroleum -- includes natural gas, a flammable fluid usually found with oil. Petroleum is the remains of organic material that was deposited, usually in marine environments, millions of years ago. One seep, in Michigan's Upper Peninsula, comes from billion-year-old rocks, although most commercial petroleum was generated from rocks that are between 65 million and 225 million years old.

Plants use photosynthesis to fix carbon dioxide from the atmosphere into organic forms of carbon -- carbon bonded with hydrogen and other elements. This carbon becomes the basis for our biosphere -- everything living. The details are not all worked out, but here's an oversimplification of how oil forms from organic carbon.

Die. First you live, and then you ____ ? Right. You die. If you're a marine organism, you drop to the ocean floor. If you're not eaten, you may start to

Rot. It's the fate of all -- or most -- flesh. Bacteria decompose you, and you become mere organic gunk on the ocean floor. But before bacteria completely devour you, the mass of sediment piling on top establishes a

Quarantine. Bacteria need oxygen, sulfur or certain other chemicals to do their eating, and the rain of organic matter (and sediment like mud and sand) eventually cuts off the supply of these elements, halting the bacteria in mid-rot. By this time, chemical reactions have trimmed off most elements except for carbon and hydrogen -- the major components of petroleum. Finally, after burial, a small percentage of you becomes a fluid, and you start to

Go truckin'. Oil being lighter than rock and water, you float from the "source rock" into porous "reservoir rock." You may reach the surface and become one of the oil seeps ancient peoples used to caulk their boats and preserve their dead. But sometimes you

Jam. Floating upward, you bang your head against an impermeable "cap rock." The cap restrains the oil, making you fair game for a wildcatter -- an oil driller working unproven fields.

To distill our discussion, oil starts forming when organic junk accumulates and gets covered quickly enough to shut off the supply of chemicals that bacteria need to oxidize the carbon (otherwise, they will burn it all up and form carbon dioxide).

Microbes, explains Richard Kettler of the department of geological sciences at the University of Nebraska at Lincoln, are ubiquitous. "If there's something to eat, if there's a chemical reaction that can run at a reasonable temperature and pressure, you can almost guarantee there will be a life form that will be using that reaction." And living, he stresses, means devouring the organic matter that would otherwise become oil.

We should mention that while a few renegade scientists claim that significant amounts of petroleum has an inorganic origin, the conventional wisdom, as presented above, is pretty convincing:

We can watch organic sediments accumulating at the bottom of the ocean.

Many chemicals in crude oil have a structure characteristic of molecules with organic origins.

Hopanes, a group of hydrocarbon molecules found in petroleum, are made by bacteria, Kettler notes.

Petroleum may also contain molecules of waxes that land plants use to prevent drying, and other markers of biological origin.

Finally, there's the linguistic argument: Tack an "e" onto that organic crud drifting down to the sea floor, and you got crude!

Do you just drill any old where, or is there a science to finding petroleum?

©1999, University of Wisconsin, Board of Regents.