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ALL POST-RIFT FORCES act to deepen the pile of sediments (yellow) accumulating along the continental margins. Rivers deliver sediments to create deep sedimentary basins, like the one shown here in the Gulf of Mexico, that accumulate the source rock that matures into oil and gas after rapid burial and heating (see also: http://domino.research.ibm.com/comm/ wwwr_thinkresearch.nsf/pages/quest198.html).
COURTESY OF ROGER N. ANDERSON

Roger N. Anderson, a professor at the Lamont-Doherty Earth Observatory at Columbia University, explains.

Plate tectonics determines the location of oil and gas reservoirs and is the best key we have to understanding why deserts and arctic areas seem to hold the largest hydrocarbon reserves on earth. But there are other important locations of large reserves: river deltas and continental margins offshore. Together, these four types of areas hold most of the oil and gas in the world today.

Oil and gas result mostly from the rapid burial of dead microorganisms in environments where oxygen is so scarce that they do not decompose. This lack of oxygen enables them to maintain their hydrogen-carbon bonds, a necessary ingredient for the production of oil and gas. Newly developing ocean basins, formed by plate tectonics and continental rifting, provide just the right conditions for rapid burial in anoxic waters. Rivers rapidly fill these basins with sediments carrying abundant organic remains. Because the basins have constricted water circulation, they also have lower oxygen levels than the open ocean. For instance, the Gulf of California, an ocean basin in development, is making new oil and gas in real time today. The Gulf of Mexico is also a great example of new oil and gas formation in a restricted circulation environment (see image at right above).

The same plate tectonics that provides the locations and conditions for anoxic burial is also responsible for the geologic paths that these sedimentary basins subsequently take. Continental drift, subduction and collision with other continents provide the movement from swamps, river deltas and mild climates--where most organics are deposited--to the poles and deserts, where they have ended up today by coincidence. In fact, the Libyan Sahara Desert contains unmistakable glacial scars and Antarctica has extensive coal deposits--and very likely abundant oil and gas--that establish that their plates were once at the other ends of the earth (see image at right).

Plate tectonics is also responsible for creating the "pressure cooker" that slowly matures the organics into oil and gas. This process usually takes millions of years, giving the oil and gas deposits plenty of time to migrate around the globe on the back of plate movements. Because these hydrocarbons are much more buoyant than water, they eventually force their way to the surface. Alternatively, rifting, collisions between land masses, and other tectonic forces can free the mature oil and gas from deep within sedimentary basins and then trap these organic fluids in reservoirs before they escape to the earth's surface. We know these reservoirs as oil and gas fields.