There's almost certainly some out there, and a new U.S. Minerals Management Service resource assessment for the Alaska Outer Continental Shelf due out Jan. 26 will provide an update on what government geologists now think might be in the Beaufort and Chukchi seas.

The 2005 assessment, unlike previous estimates, will focus more closely on the economic potential of Arctic offshore gas, according to Kirk Sherwood, an MMS regional geologist. The last assessment was in 1995, with an update provided in 2000.

Oil has already been found in deeper-water Beaufort exploration wells like Kuvlum and Hammerhead, drilled several years ago by ARCO Alaska Inc. and Union Oil of California. Those discoveries weren't economic, but they may be someday. The important thing is that they confirm that oil is out there in the deeper waters.

Oil has also been found closer to shore in fields like Northstar, which BP is now producing, and Liberty, which is planned for development.

All of the known North Slope fields were found along the Barrow Arch, a broad geologic uplift that forms a crest of rocks running northwest to southeast across the North Slope that is - by coincidence - parallel to the present-day coastline. Seismic mapping indicates that the arch may continue offshore, northwest of Barrow.

Petroleum found in the giant, now-producing Prudhoe Bay and Kuparuk oil fields was formed south of the arch and north of the Brooks Range in the dim prehistoric past, deep in organically-rich sedimentary "source" rocks. The oil gradually seeped upward and northward through layers of porous rocks. Impervious rocks covering the Barrow Arch halted the slow migration of the hydrocarbon fluids. As they accumulated against the layers of underground impervious cap rock, the big oil fields were formed.

Could the same thing have happened north of the Barrow arch? Could there be twins to the Prudhoe Bay and Kuparuk fields out there?

Sherwood thinks there's almost certainly more oil to be found in the Beaufort, even if the geologic conditions might discourage finding huge fields like Prudhoe.

The "fingerprinting" of the oil produced in BP's Northstar field six miles north of the coast suggests that it originated in different source rocks than that of the higher-sulfur oil found in Prudhoe, Kuparuk and other onshore fields, Sherwood said.

"The oil in Kuvlum and Hammerhead are thought by the U.S. Geological Survey to have come from Cretaceous-and Tertiary-age shale rocks of the Canning formation that overlaps the Barrow Arch and descends to great depth to the north," Sherwood said.

But just where and how these source rocks lay is a puzzle. Sherwood admits geologists don't know a lot about the Beaufort Sea because the area has not seen much drilling.

Rock formations provide clues

Economics aside, the conventional wisdom is that the Beaufort and Chukchi seas have good oil and gas potential because the types of rocks that generated and trapped the tens of billions of barrels of oil found in the Prudhoe Bay area extend across most of the region, onshore and offshore.

The Chukchi Sea area is widely regarded as having the better potential - in fact, excellent prospects - because its broad continental shelf extends across thousands of square miles between Alaska and Siberia. The offshore Beaufort is considered somewhat less prospective mainly because the continental shelf is narrower, about 45 to 55 miles wide.

Still geologists know the continental shelf of the Beaufort is underlain by deep layers of sedimentary rocks for about 50 to 60 miles north of the coast, Sherwood said. About 30 miles offshore, a major fault forces the sedimentary rocks downward to more than 30,000 feet, beyond the recording depth of typical seismic exploration surveys and generally beyond industry's technical ability to produce.

The ideal temperature and pressure conditions for the formation of oil and gas occur from 8,000 to 10,000 feet below ground.

What put those rocks in place in the Beaufort Sea, however, appears to be different than what created many of the deeply buried oil source rocks south of the Barrow Arch, which are the source of most of the Prudhoe Bay-area oil.

However, a tremendous thickness of Jurassic, Cretaceous and Tertiary rocks do appear to be present in the subsurface north of the Barrow Arch. These may relate to the oil-source rocks that formed the oil in the Alpine, Kuvlum and Hammerhead deposits as well as many of the oil seeps in the Arctic National Wildlife Refuge.

Many geologists believe several of the thick layers of sedimentary rocks offshore were put there by huge outflows of silt over eons from the Mackenzie River in Canada. "The Canada basin has a tremendous thickness of sediments. It extends for hundreds of miles," Sherwood said.

"The thick sequences of rocks north of the Barrow Arch indicate high rates of deposition of sediments and organic matter that were derived from land, and because of that may be more prone to the formation of natural gas than oil," Sherwood said.

Still, there appears to be scattered segments of much older sedimentary rocks left when continents drifted apart eons ago to create modern-day Alaska and Canada (some geologists think the two were once joined). These older rocks, including some from the Chukchi borderlands, are of the age and depth where oil could have formed, Sherwood said.

Deep water may become less of a challenge

The new 2005 MMS oil and gas assessment will cover the Beaufort continental shelf out to its edges and even some of the Slope as the shelf descends toward the deep polar ocean basin. Water depths in the northern part of the Beaufort assessment reach 500 meters, Sherwood said.

The deep-ocean basin - where the earth's crust is thinner - is not good oil country, Sherwood said. There is often only half a mile or so of sediments atop hard igneous rock, he said.

Whether anyone will want to explore in the Arctic Ocean in depths of more than 500 meters (1,650 feet) is open to question, but Sherwood said the MMS didn't want to leave large areas of sedimentary rock out of its assessment. Industry may very well tackle the challenges given rapid advances in technology in drilling and producing oil and gas in very deep waters of the Gulf of Mexico. In the Gulf the deepwater conditions, except for ocean ice, aren't much different than those found in the Arctic.