NPR-A, Alpine yield high quality crude

Kay Cashman

In the late 1940s, when exploration crews working for the U.S. Navy began drilling wells on the eastern edge of the National Petroleum Reserve-Alaska, they burned the high gravity oil they found near Umiat in their diesel engines — without any processing.

Today, lighter crude is considered more valuable in the marketplace because it requires less processing.

“Generally, oil on the North Slope and at Prudhoe Bay is relatively heavy oil because of the chemistry it inherits from its source rocks. However, typically, Alpine oil is 40 degree API gravity; the oil from Umiat is about 35 degrees. Their oil is probably generated from the same type of source rocks as Tarn and Meltwater. … Prudhoe Bay tests at 28 degrees. … West Sak’s heavy oil ranges from 22 to 10 degrees. It has very low gravity partly because it is biodegraded,” Dave Houseknecht of the U.S. Geological Survey told PNA Oct. 17.

Houseknecht is a member of a multi-disciplinary team using old seismic and four years of new field work to prepare a fresh evaluation of the oil and gas potential in the NPR-A. (See story in February edition of Petroleum News • Alaska.)

“We’ve been working out of the Umiat Hilton for the last two years,” he said. “One of the things we are assessing in our study is the potential of finding the kind of high gravity oil found at Alpine and Umiat across the NPR-A.”

Alpine lies just east of the northeastern part of the NPR-A. It is the largest onshore U.S. oil discovery in the past 15 years and is the westernmost oil field on the North Slope.

Houseknecht supplied the following API gravity averages for North Slope fields:

Alpine 40 degrees API

Fiord 32 degrees API

Tarn 36 degrees API

Umiat 35 degrees API

Prudhoe Bay 28 degrees API

West Sak 10 to 22 degrees API

Note: The high numbers = low viscosity or “watery” crude; the low numbers = high viscosity or “thick” oil.