How much is left?

USGS assessing undiscovered hydrocarbons in North Slope’s ‘middle ground’

Kristen Nelson

Petroleum News Editor-in-Chief

Millions of barrels of oil have been discovered on Alaska’s North Slope, but how much more could there be, yet undiscovered?

The U.S. Geological Survey has assessed undiscovered oil and gas resources in the Arctic National Wildlife Refuge on the eastern side of the North Slope and in the National Petroleum Reserve-Alaska on the western side, and is now looking at what it calls the “middle ground.”

The middle ground of the North Slope, says Ken Bird of the USGS, is the area between ANWR and NPR-A, north of the Brooks Range and south of the three-mile offshore limit of state land.

It includes all of the fields currently producing on the North Slope, but the assessment the USGS is doing doesn’t include discovered oil and gas, he said. Instead the agency is assessing the amount of resource — in place, technically recoverable and economically recoverable — that remains undiscovered in the area.

Bird, project team leader for the USGS Alaska Petroleum Studies Project, said in a preliminary report to the Alaska Geologic Society in Anchorage March 18 that while the assessments in ANWR and NPR-A “consisted primarily of federal land, the assessments that we’re working on right now are composed primarily of state and Native lands with a relatively small proportion of federal land.”

What is known

The North Slope has been relatively lightly explored, Bird said, with only some 400 exploration wells drilled, probably three-fourths of those in the middle ground area. According to the Alaska Division of Oil and Gas web page, Bird said, there are almost 70 known accumulations across the North Slope, with some 80 million barrels of oil discovered and 20 billion barrels of that believed to be recoverable, along with some 35 trillion cubic feet of recoverable natural gas.

Assessment methodology

“This is our first time to use it in the middle ground area, basically because we have seismic for the first time.”

Plays are volumes of rock with similar geologic features such as petroleum charge, reservoir and trap — features that determine petroleum potential. Bird said the team had not finalized the list of plays it would include, but once that work is completed, probability distributions are run for the characteristics of individual plays and for the distribution of the sizes of accumulations, with a minimum cutoff of 50 million barrels of oil in place.

You can’t just sum individual plays to get a total, because the plays are related to one another, Bird said. “What are the dependencies in terms of the charge, reservoir and trap? If the source fails in one of these plays then it’s going to have an effect on the overlying play, which was relying on the same source rocks.

“Those dependencies are incorporated into the aggregation process,” he said.

An economic assessment is then run, with the resources allocated to geographic blocks so factors of distance to infrastructure can be included in determining economic recoverability.

Major middle ground plays

In the middle ground area, Bird said, the Ellesmerian Ivishak sandstone play is the main reservoir at Prudhoe Bay, and also includes Sandpiper, Northstar, Gwydyr Bay, North Prudhoe, Sag Delta North and Eider. He said it’s hard to find closures in this area that haven’t been drilled, because the Ivishak is the “most intensely explored of plays in this area.”

“The Ivishak has been the primary target of frontier exploration outside of the Prudhoe area,” Bird said, “and most of these (middle ground exploration) wells … the primary target was the Ivishak.”

What the assessment is looking for in undiscovered resource, he said, would be “small Ivishak accumulations.”

There are more discoveries in the Beaufortian than in any of the other mega sequences, Bird said, including the Kuparuk field, the largest at 2.5 billion barrels recoverable, Alpine and recently discovered accumulations in northeast NPR-A.

Brookian deposits include heavy oil deposits at both West Sak and Ugnu.

Non-conventional gas later; Yukon Flats, other basins in the works

But, he said, “on the assessment schedule years down the road, we’ve got unconventional resource evaluations scheduled” including coalbed methane and gas hydrates.

The USGS also has other projects under way in Alaska.

It is collaborating with the Minerals Management Service on a synthesis of petroleum geology and resources of onshore and offshore northern Alaska; completing data compilation, analysis and assessment of the Yukon Flats basin in central Alaska; continuing baseline measurements of gas content of coal in various Alaska basins; and initiating data collection in Cook Inlet basin in preparation for a new oil and gas assessment.