Alaska's Nanushuk and Torok Formations: 8.7 Billion Barrels of Undiscovered Oil

A USGS assessment, 2017

What changed

Before 2015, about 150 exploration wells had drilled through the Nanushuk and Torok Formations, on Alaska's North Slope, yet each formation had produced oil from just one small pool — under 10 million barrels of recoverable oil (MMBO). Earlier USGS assessments saw potential for undiscovered oil in both, but expected the pools to be modest (Houseknecht, 2003).

Then came a run of discoveries, 2015–2017, in and near the National Petroleum Reserve in Alaska (NPR-A):

How much oil each will actually yield stays uncertain until development drilling and production begin. But the finds show the pools can be much larger than thought. The USGS team also had new three-dimensional (3D) seismic data for the northeastern NPR-A and neighboring State lands and waters, which let it count and size seismic amplitude anomalies — good stand-ins for undrilled prospects.

So the USGS updated its estimates of undiscovered, technically recoverable oil and gas in conventional accumulations across the whole onshore and State-waters extent of both formations, replacing earlier ones (Bird and others, 2005; Houseknecht and others, 2010).

The six assessment units (AUs) in and around the NPR-A, including State waters. P Pikka, H Horseshoe, S Smith Bay, W Willow, U Umiat. USGS.

The rocks

Together, the two formations are a huge wedge of sediment laid down in a deep-water basin, preserved north of the Brooks Range beneath the North Slope and offshore.

A two-dimensional seismic reflection image. Low in the Torok (green), reflections run nearly parallel to its base — sediment on the basin floor. Higher up they tilt east — sediment on the slope between deep and shallow water. The Nanushuk is yellow. USGS.

The new discoveries are oil pools in stratigraphic traps, concentrated along ancient shelf margins. Their subtle shapes are hard to see in seismic data, but innovative 3D seismic techniques are raising success rates, as these finds show.

Six assessment units

Using older 2D and recent 3D seismic data, wells and outcrops, the USGS divided each formation into three AUs:

The Umiat pool proves oil can sit in the southern structures: found in an anticline in 1946, it is estimated to hold more than 70 MMBO and has never been produced.

One risk: the sandstones in both formations are mostly fine grained, and lose porosity and permeability as they are buried. In the deeper parts of the units, poorer reservoirs could make oil uneconomic — more likely in the Torok, which is buried deeper.

The estimates

For all six AUs together, the mean estimates are:

• Oil: 8,727 MMBO — about 8.7 billion barrels.
• Gas: 24,545 billion cubic feet (BCFG) — about 25 trillion cubic feet — of which 6,992 BCFG is associated gas, found with oil, and 17,553 BCFG nonassociated.
• Natural gas liquids (NGL) in all six, with the Nanushuk Northeast AU alone estimated at a mean of 74 million barrels.

Uncertainty is large — a sign of how early exploration for stratigraphic traps is here. The ranges below run from F95 (a 95-percent chance of at least this much) to F5.

• Oil is concentrated in the two northeastern units, smaller in the northwest and smallest in the south.
• Gas is largest in the Nanushuk Northeast AU, at a mean of 8,689 BCFG in total.

With the older rocks

Older rocks in the NPR-A were assessed in 2010, at 86 MMBO and 14,732 BCFG (Houseknecht and others, 2010). No new wells have reached them since, so there was nothing new to update. Added to the new results, the totals become:

• Oil: 1,754 to 21,859 MMBO, mean 8,813 MMBO;
• Gas: 5,507 to 112,932 BCFG, mean 39,277 BCFG — about 39 trillion cubic feet.

Sources

Based on Assessment of Undiscovered Oil and Gas Resources in the Cretaceous Nanushuk and Torok Formations, Alaska North Slope, and Summary of Resource Potential of the National Petroleum Reserve in Alaska, 2017, by the USGS Nanushuk–Torok Assessment Team, USGS Fact Sheet 2017–3088, U.S. Geological Survey; a work of the United States government in the public domain. The Bureau of Land Management and the Bureau of Ocean Energy Management collaborated in reviewing the data. The map, the seismic section and the table's gas figures are taken from the fact sheet's PDF, which the import had left out.