It has all the trappings of an energy executive's fantasy: a gigantic oil and natural gas reservoir, potentially a thousand times as rich as Saudi Arabia's, conveniently located in the western United States. But if geologists are reading the rocks correctly, the trove vanished long ago. The cache in southern Utah apparently emptied into the atmosphere about 6 million years ago, and it possibly even warmed up the planet.
The Navajo Sandstone layer exposed on the Colorado Plateau in southern Utah is the remnants of the largest field of wind-blown sand dunes known. Iron paints the sandstone a rich red, yet some sections have been bleached white. Previous work suggests that fluids moving through the sandstone after burial probably stripped out the iron.
The most likely bleaching agents, argue geologists Brenda Beitler, Marjorie Chan, and William Parry of the University of Utah in Salt Lake City, are petroleum and natural gas. More buoyant than other fluids found in the crust, oil and gas tend to move upward through sandstone pores and pockets until they become trapped by impermeable rock. In the December issue of Geology, the team reports that satellite maps of bleached sandstone in the region show that the white rock's distribution appears to match what would be expected from such a scenario.
And a lot of gas it was. By combining estimates of eroded material, the extent of the bleached sandstone, and current natural gas production in the area, the team calculates that the ancient reservoir was probably big enough to boost global temperatures when released into the atmosphere. Erosion and the rapid incision of the Colorado River into the plateau about 6 million years ago may have shaved off the impermeable reservoir caps and released natural gas, which is mostly methane, a greenhouse gas.
Geologists have suggested that bleached sandstone is the result of hydrocarbon fluids, but the studies were confined to small areas. "This study's very interesting," says geologist Peter Mozley of the New Mexico Institute of Mining and Technology in Socorro, because the satellite imagery allows the team to track the whitened rock over the entire state, rather than looking at localized bleaching at individual field sites.