NPR-A study is first in reassessment of North Slope oil and gas potential

USGS using public-domain seismic data and new field work; sees promising fairways for both Alpine-type and Tarn and Meltwater-type prospects

Kristen Nelson

PNA Editor-in-Chief

The U.S. Geological Survey is using old seismic and new field work to prepare a fresh evaluation of the oil and gas potential in the National Petroleum Reserve-Alaska. While the study is not yet complete, the agency sees promising fairways in the NPR-A for both Alpine-type and Tarn and Meltwater-type prospects.

The NPR-A assessment should be completed by the end of the year, David Houseknecht, a member of a multi-disciplinary team working on the project, told the Alaska Geological Society in January. The potential of the entire North Slope will then be reevaluated, using the same techniques.

The USGS has been cash strapped, Houseknecht said, and did not do a lot of new petroleum geology on the North Slope for well over a decade. The 1995 national assessment of the North Slope was largely based on watching activity and updating data bases, “but was not one where we went back and looked at seismic data in a new way and made new interpretations,” he said.

Reevaluation of old seismic

“These seismic lines have gone through a post-stack reprocessing and depth conversion. A lot of them have been merged to form these regional lines that are as much as 300 miles long,” Houseknecht said.

Houseknecht said they are looking for the kinds of plays being developed east of NPR-A, trying to come up with estimates of area enclosure, reservoir thickness and porosity — and then assign some probabilities to those estimates.

The USGS is using sequence stratigraphy in its NPR-A assessment “because of the emphasis now on stratigraphic traps on the North Slope and the importance of trying to estimate such things as reservoir numbers and reservoir volumes,” Houseknecht said.

Alpine-type prospects

The USGS is using seismic, well data and cores in the Beaufortian analysis to define fairways across NPR-A with the potential for ‘sand pods’ which could act as reservoirs.

“Without high-resolution 3-D seismic data, it’s very difficult indeed to estimate numbers and volumes of those sand pods,” he said, but areas with potential are being identified.

For studying the Brookian, the USGS has put a lot of emphasis on outcrop studies, Houseknecht said, looking “for the types of features that need to be explored for if you are looking for … Tarn, Meltwater type accumulations.”

The agency is also using seismic and has identified a Torok turbidite fairway across NPR-A.

The USGS has looked at one outcrop where oil-stained turbidite sands extend for 12 miles at the base of an escarpment, and another turbidite system, heavily oil stained, at least 300 feet thick, highly porous and reasonably permeable.

Houseknecht said based on a volumetric estimate with very conservative dimensions, porosities and water saturations this one alone could contain 500 million barrels in place, with perhaps 150 million technically recoverable.

“There are a number of systems within the Torok,” he said, “that appear to have sizes that would be viable as standalone accumulations in NPR-A and a lot… here that would be viable in series or in satellites.”

“So what we think we have here is a Tarn-type system,” he said.

The USGS isn’t ignoring other possibilities in NPR-A.

“It’s just that with the Alpine play raging and with the Tarn-type play raging, I think we have to at least focus on assessing the stratigraphic plays that we know are viable today, both outside NPR-A and probably inside NPR-A.”

Once the NPR-A assessment is completed, Houseknecht said, the USGS has been charged to update its whole North Slope perspective with this approach.

That will be more challenging for state lands, he said, because the agency does not have access to the seismic data which it has for NPR-A.