"Geologists once doubted oil could be found this far from shore. Twenty years ago, as drilling marched seaward across the shallow continental shelf, the oil-bearing sandstone layer seemed to be petering out. Scientists concluded that the sand deposited tens of millions of years ago by great rivers had not spread all the way out on the shelf. But they were mistaken. The sand—and oil—was there all right.
The sand had spilled off the edge of the shelf and down the steep continental slope to the deep-ocean floor. There it had pooled in apron-like deposits that turned into porous rock—perfect for capturing oil oozing from still deeper rock layers. In the 1990s, as hints of these deposits began showing up in seismic data, the vanguard of oil production stepped off the continental shelf, into waters thousands of feet deep. Now giant new fields, the biggest of them Thunder Horse [BP's Deepwater Horizon is / was located here], beckon at 6,000 feet (1,829 meters) and more. At still greater depths approaching 10,000 feet (3,048 meters), says geophysicist Roger Anderson of Columbia University, "there have been a whole series of finds," although they have yet to be exploited.
All in all, oil experts estimate that the deep waters of the Gulf of Mexico will yield more than 25 billion barrels of oil. That's twice as much as in Alaska's giant Prudhoe Bay field, and far more than in any untapped U.S. prospect, including the controversial Arctic National Wildlife Refuge. "There's a major consensus that there's more oil there than you'd ever find in ANWR," says Anderson. With the boost from deepwater wells, offshore oil should increase from a third of U.S. oil production now to more than 40 percent by 2008, before tapering off. But it will still barely ease the American thirst for oil."