The leaking well off the coast of Louisiana has spilled more than four million gallons of oil into the Gulf and continues to gush at a rate of 210,000 gallons per day. Why is this particular well spewing forth so prolifically, while in other places we have to pump oil out of the ground ourselves?
Because it's under a lot of heavy rocks. Deep deposits, like those in the Gulf, are packed down very tightly by the rock column above them. If they are tapped carelessly, the oil will start to gush out all at once, Beverly Hillbillies -style. Deposits that are closer to the surface are under less pressure, and the oil may need to be pumped from the very first drop. Almost all wells need a little help eventually, as extraction diminishes their internal pressure. Engineers pump water or natural gas into the chamber to force out the remaining crude, which is later separated from the added fluids.
Gulf wells are even more likely to gush than you'd expect, given their depth. Most of them formed under tremendous pressure, far beneath the earth's surface, but a subsequent geologic disturbance has pushed them up to their current location, at 20,000 feet. To control the flow of oil from wells like these, companies pack their drilling hole with a thick, heavy fluid known as drilling mud that maintains pressure on the reservoir. The clay and mineral compound helps prevent blowouts like the one that's now flooding the Gulf.
Even some shallow wells are under enough pressure to make it to the surface without human intervention. Native Americans collected seeping oil to waterproof baskets and caulk boats. Settlers used it to grease their wagon wheels as they rolled along the Oregon Trail. The first commercial oil well, drilled in Titusville, Penn., by Edwin Drake in 1859, was located based on natural seeps. Many of the significant deposits in Texas and California have been found the same way.
Marine seeps are even more common. A single leak near Santa Barbara releases around 2,500 gallons of crude into the Pacific every day. Natural seeps account for 46 percent of the hydrocarbons in our oceans.
Bonus Explainer: Why does the oil slick appear to be orange instead of black? Because the emulsion of oil in water consists of particles of many sizes, separated by water molecules. The way these particles scatter light (and produce color) depends on their size, and their size can change over time as they clump together or separate.
Oil isn't always black, even when it hasn't been mixed with water. Crude oil—that's the unrefined stuff that comes out of the well—can be transparent, yellow-green, reddish, amber, brown, or black, depending on its composition. So-called "light crude," in which the hydrocarbons are mostly free of contaminants, tends toward the lighter end of the scale (naturally). Heavy crude often takes a dark brown or black color on account of the metals mixed in.
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Explainer thanks Christine Ehlig-Economides of Texas A&M University and Michael J. Economides of the University of Houston. Thanks also to reader Brian Colfax.