HOME PAGE SUBSCRIPTIONS, Print Editions, Newsletter PRODUCTS READ THE PETROLEUM NEWS ARCHIVE! ADVERTISING INFORMATION EVENTS PETROLEUM NEWS BAKKEN MINING NEWS

Providing coverage of Alaska and northern Canada's oil and gas industry
May 2008

Vol. 13, No. 18 Week of May 04, 2008

Going underground on the North Slope

DGGS-led program links foothills surface geology with subsurface well, seismic data; North Slope exploration still in ‘primary phase’

Alan Bailey

Petroleum News

There has been a view on Alaska’s North Slope that oil exploration too far south of the Beaufort Sea coast is a futile operation. Go south of a so-called “line of death” somewhere along the middle of the slope and you’ll cross into territory where the petroleum source rocks dipping south from the coastline become so deeply buried that they are likely to have reached temperatures at which natural gas rather than oil would have formed.

But on the basis of what goes down must come up, those same strata bend back up to the surface along the foothills on the northern side of the Brooks Range, where extensive surface rock exposures enable geologists to examine rocks that only occur in the subsurface to the north.

For several years Alaska’s Division of Geological and Geophysical Surveys has led a team of scientists in a research project to better understand the geology of the North Slope by examining, mapping and measuring rock sections exposed in the foothills region. The team has focused on a swath of territory west from where the North Slope Haul Road emerges from the north side of the Brooks Range, and east of the Haul Road along the sector of the foothills that swings north towards the Beaufort Sea coast.

The DGGS program uses funding from the federal government, the state and industrial sponsors.

Increasing interest

The likely construction of a North Slope gas pipeline coupled with a burgeoning interest in natural gas as a fuel is driving a keen interest in the gas potential of the southern North Slope and the foothills — in the winter of 2007-08 Anadarko Petroleum drilled for gas at Gubik and Chandler in the foothills, in the first gas exploration drilling ever seen in northern Alaska.

In fact, North Slope oil and gas exploration is still in what might be characterized as a “primary phase,” in which the relatively easy discoveries have been made, Bob Swenson, Alaska state geologist and acting director of Alaska’s Division of Geological and Geophysical Surveys, told Petroleum News April 17.

“That’s kind of where the entire North Slope is,” Swenson said.

Swenson compared the area of the Brooks Range foothills between the Chukchi Sea coast and the western boundary of the Alaska National Wildlife Refuge with the front ranges of the Canadian Rocky Mountains.

“We’ve got 25, 30 wells across that entire front range and they have literally tens of thousands of wells and are still finding gas,” Swenson said.

And between the foothills of the front range and the oil fields of the central North Slope lies what geologists term the Colville basin, containing vast quantities of sediment washed down from the emerging Brooks Range to form what is termed the Brookian sequence, the youngest of the petroleum-bearing rock sequences in northern Alaska.

Wealth of information

The DGGS-led team has used foothills rock exposures to discover a wealth of information about the Brookian and other rock strata involved in the North Slope oil fields. But the team has now entered a new phase in which it is using seismic and well data to relate those surface discoveries to the subsurface of the Colville basin.

“That’s been a step change for us in our understanding of how this fits together with any of what the explorers are seeing up in the northern part of the Colville area, as well as explorers in the southern part,” Swenson said.

DGGS scientists have been primarily focusing on the surface geology while Alaska’s Division of Oil and Gas has provided expertise in seismic interpretation, Swenson said. The team is using old 2-D seismic from the foothills and southern North Slope, together with data from the scattering of wells in the region.

The team is taking the detailed geologic sections that the geologists have measured at the surface and tying those sections through the seismic cross-sections to the well logs, Swenson said. The tying of detailed rock stratigraphy into the seismic data will particularly help in seismic analysis in the foothills area, where upending of the rock strata by the growing Brooks Range has made the seismic data very difficult to interpret, he said.

Major payoff

Swenson expects a major payoff from the research in terms of understanding the distribution and quality of oil and gas reservoir rocks under the North Slope. For example, an understanding of how the Colville basin was filled with sediments during Cretaceous times will help clarify reservoir distribution. The research should also shed light on the critical question of how the deep burial of rocks in the Colville basin has impacted the porosity of the reservoir rocks — the ability of the rocks to hold fluids.

“Now with the possibility of a gas pipeline, the geology down on the south side of the Colville basin has become much more interesting and our understanding of the porosity and permeability distributions and sand body distributions … are going to become more and more important all the time,” Swenson said. “So that’s really where we’re pushing right now.”

Swenson said that the team is also extending its investigations up into the younger Tertiary section that includes the strata that reservoir heavy oil deposits in the Prudhoe Bay area.

Key question

A key question for geologists is the source of the sediment that filled the Colville basin, especially as the DGGS foothills research has revealed better quality reservoir rocks than had been thought to exist by people with the “line of death” mindset.

Geologists have long understood that in the vicinity of the National Petroleum Reserve-Alaska the sediments filled the Colville basin from west to east, with the oldest rocks being found towards the west of the region. However, on the east side of the North Slope along the boundary with the Alaska National Wildlife Refuge, the direction of deposition was more south to north. Exactly where that depositional direction changed remains something of a mystery.

And a massive sandstone exposure near the Gilead Creek east of Sagwon on the Haul Road that the DGGS geologists investigated during the 2007 summer field season may provide important clues about that transition in direction.

Another important aspect of the research into the geology of the Colville basin is the tracing of rock units across the basin — the Brookian rocks in the basin are notoriously difficult to track across long distances because of variations in the types of sediment simultaneously deposited in different places. DGGS plans to try using chemical fingerprints for correlating rocks between one location and another, in a technique that has already proved successful in Cook Inlet.

“Where there is a lot of well control we’re finding out that it really does work,” Swenson said. “So we’re going to a pilot program on the North Slope.”

As with all DGGS research, the results of the work will become publicly available to anyone with an interest in North Slope geology.

“It’s pretty exciting. We’re doing some top end work,” Swenson said. “We’ve had a lot of response from the geologic community on the work that we’ve done. It’s very encouraging that we’re on the right track.”






Petroleum News - Phone: 1-907 522-9469 - Fax: 1-907 522-9583
[email protected] --- http://www.petroleumnews.com ---
S U B S C R I B E

Copyright Petroleum Newspapers of Alaska, LLC (Petroleum News)(PNA)©2013 All rights reserved. The content of this article and web site may not be copied, replaced, distributed, published, displayed or transferred in any form or by any means except with the prior written permission of Petroleum Newspapers of Alaska, LLC (Petroleum News)(PNA). Copyright infringement is a violation of federal law subject to criminal and civil penalties.