A Shift in Plays On North Slope

Amid concerns over aging fields and declining production, new technology and some surprising geological discoveries are combining to reawaken Alaska’s petroleum promise, experts on the region say.

AAPG member Dave Houseknecht has worked on Alaska North Slope basin analysis and petroleum resource assessment for the U.S. Geological Survey since 1995. Tim Ryherd is a geologist with Alaska’s Department of Natural Resources Oil and Gas Division.

Both agree that oil exploration strategies are changing, and that economic factors are coming together to create a new Alaska boom in natural gas.

Historically, exploration strategy on Alaska’s North Slope was to look for Prudhoe Bay-type traps — pure structural traps or combination traps involving very large structures, Houseknecht said.

Along the way, companies sometimes chanced upon some pure stratigraphic traps — usually too small to be commercially viable unless they were located near existing infrastructure.

In the past six to eight years, two developments altered the traditional approach, he said. They are:

• Advanced geologic and engineering technology allowed companies to consider smaller targets.

"Specifically, 3-D seismic in exploration mode, applied sequence stratigraphic concepts and directional drilling in exploratory and especially in developmental wells really reduced the footprint and expense of developing smaller accumulations," Houseknecht said.

"In the 1990s, suddenly instead of 400 million barrels as a minimum viable field size, we saw companies stepping out on 100 million or even smaller fields," he said.

"Technology allowed them to to start looking at stratigraphic traps as primary objectives," he said.

• A "serendipitous" discovery by Arco Alaska in 1994 — announced in 1996 — "revolutionized North Slope exploration strategy and kicked off the hottest play in Alaska today, usually called the Alpine play," Houseknecht said.

The primary target — a lower Cretaceous turbidite sand — proved unsuccessful. The company took the project deeper because older wells in the general neighborhood had tested non-commercial amounts from upper Jurassic strata, Houseknecht said.

The resulting discovery in the Alpine field, located on the Colville River delta on the eastern northern boundary of the NPRA, had an announced size of 429 million barrels.