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Rare mix of geological factors created rich but dangerous reserves

Crude oil from the Deepwater Horizon oil spill washes ashore in Orange Beach, Ala. The gulf's near-unique geology and millions of years of river flow account for the area's exceptionally rich deposits.
Crude oil from the Deepwater Horizon oil spill washes ashore in Orange Beach, Ala. The gulf's near-unique geology and millions of years of river flow account for the area's exceptionally rich deposits. (Dave Martin/Associated Press)

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By Joel Achenbach
Washington Post Staff Writer
Tuesday, July 13, 2010

In the oil business, geologists tell stories. Here was a river, they will say. Here was a shallow sea. Here is where the sea dried up and left only salt. Here is where the sea formed anew, and widened, and deepened, and where sediments from another river, and the carcasses of microorganisms, were deposited, buried, baked, until finally -- the enchanting payoff of the story if you're an eager-beaver oil executive -- the organic matter turned into oil.

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The Gulf of Mexico is full of such stories. Unfortunately, the story of one well, named Macondo, drilled by the rig Deepwater Horizon, has turned into a tragedy.

The geology of the gulf is pretty close to perfect for the creation of oil reservoirs. There are salt sheets and domes that form impermeable caps on oil fields. There are abundant rock formations that have been deformed into hump-shaped strata known as anticlines, natural traps for oil.

"It traps oil and gas beautifully," said Roger Anderson, a Columbia University geophysicist who has long studied Gulf of Mexico geology.

Anderson compares the deep-water gulf to Texas and Oklahoma more than a century ago. The "oil patch" had its famous moments, such as the Spindletop gusher in Beaumont, Tex., in 1901, which blew out at 100,000 barrels (4.2 million gallons) per day, and the Wild Mary well, which spewed out of control for 11 days in Oklahoma City in 1930. That region still produces, but there aren't many big discoveries still to be made in the pincushioned ground.

Not so the deep water. There's oil out there, in reservoirs that can top a billion barrels.

"It's like the old days. It's true frontier," Anderson says.

Shaped by great rivers

It's not the only such place in the world. There's abundant oil in deep water off the coasts of Brazil and West Africa, for example. But the gulf has its own near-unique geology, shaped by the great river that flows into it. The Mighty Mississippi, the Father of Waters, drains almost everything from the Rockies to the Appalachians. Millions of years ago, the Red River, which forms part of the Texas/Oklahoma border, was as big as the Mississippi. These rivers dumped dead organisms into the gulf in prodigious quantities.

Those nutrients help feed thriving ecosystems and some of the richest fisheries in the world. But the gulf is also an isolated sea, almost walled off from the Atlantic Ocean by Cuba and the Florida and Yucatan peninsulas. That means the gulf lacks the deep-water circulation of open ocean.

Bad circulation means lots of anoxic layers, dead zones, places where there's so little oxygen that organic matter doesn't decay. That's great for the eventual creation of an oil field.

"What oil and gas is is undecayed dead organisms. Microorganisms, not dinosaurs. So the small foraminifera and algae that lived in the ocean and lived in the Mississippi River died and got swept out to sea and got buried under all the mud coming out of the Mississippi. As it got deeper and deeper, it got hotter and hotter and got cooked into oil," Anderson said.

Ken Deffeyes, a retired Princeton geologist who once worked for Shell Oil and has written about the gulf, said, "The Mississippi Delta and the Niger Delta are the only two really productive, big deltas in the world. The Amazon, nothing. The Ganges, nothing or very, very little."

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