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
November 2008

Vol. 13, No. 44 Week of November 02, 2008

Looking for the geologic links on North Slope

DGGS-led team is finding evidence for how the geology and petroleum systems in different parts of the North Slope hook together

Alan Bailey

Petroleum News

Editor’s note: This article originally appeared in the Oct. 12 edition of Petroleum News. We are reprinting it because the original version contained an incorrect diagram.

It shouldn’t come as much of a surprise to anyone that Alaska’s North Slope holds some world class petroleum systems in its subsurface geology. But, as oil and gas exploration moves away from the Prudhoe Bay region where major oil discoveries have been made in the past, there are many as yet unanswered questions over how the various rock strata link together across the vast expanses of northern Alaska.

And answers to those questions could prove crucial in determining likely locations for future oil and gas discoveries.

DGGS project

In the past couple of years Alaska’s Division of Geological and Geophysical Services has led a team of geologists working an area of the east-central North Slope, around the Haul Road near Happy Valley, to unravel the regional geologic linkages. In many ways the east-central North Slope is a linchpin area, connecting the geology of the National Petroleum Reserve-Alaska to the west with that of the Arctic National Wildlife Refuge to the east, and with the geology of the Prudhoe Bay area to the north.

The investigation in the Happy Valley area is the continuation of a multi-year DGGS-led geologic investigation that has worked its way along the northern side of the Brooks Range and in the Brooks Range foothills, using surface rock exposures to obtain information about rocks that extend into the subsurface under the North Slope. These rocks include stratigraphic units equivalent to the reservoir and oil source rocks of the prolific North Slope oil fields.

And whereas geologic information obtained by oil companies typically remains commercially confidential, DGGS is able to find and publish public domain geologic information that is available for anyone interested in oil and gas exploration. The division’s North Slope program receives funding from the federal government, the state and from industrial sponsors.

Geologic mapping

In the summer of 2008, as part of the investigation of the Happy Valley area, DGGS geologist Bob Gillis led a surface geologic mapping effort in that area, Marwan Wartes, DGGS program manager for the North Slope and Brooks Range foothills, told Petroleum News Sept. 22.

“That was really a big focus for most of the time we were out there, collecting that basic geologic information,” Wartes said.

The mapping area included the Lupine No. 1, Echooka No. 1, Ivishak No. 1 and Aufeis No. 1 wells, all drilled between 1972 and 1975, and was close to the Susie well, drilled in 1966.

There is also some publicly available seismic data for the area immediately north of where the DGGS team did its mapping, Wartes said.

“That is a big, big help in deciphering the stratigraphy and the approximate (rock) thicknesses we’re likely to be dealing with,” he said.

Working from discontinuous rock outcrops along river cuts and, in some cases, picking out major geologic features using aerial photographs, the team measured and recorded detailed information about the rocks.

It’s actually been known for many years that the area includes sandstones with deep oil stains. And some of these stains can be smelled from as far away as 100 yards, Wartes said. The geologists engaged in the 2008 fieldwork also found many instances of light oil staining of the sandstones, he said.

“It further corroborates that there’s a pretty prolific petroleum system and it’s a matter of finding the right combination of factors to get it trapped somewhere,” Wartes said.

There are also many places where freshly broken sandstone exhibited a petroleum odor.

“There were literally dozens of those occurrences that we’ve made note of,” he said.

Brookian trends

Using the mapping results and data from the old wells, the geologists were able to confirm some major geologic trends in what is known as the Brookian sequence. The Brookian is Cretaceous and Tertiary in age. It is the youngest of the oil and gas bearing rock sequences in northern Alaska.

The Lower and Middle Cretaceous strata of the Brookian thin by many thousands of feet from southwest to northeast in the area of the 2008 mapping. That thinning probably marks the early uplift of the mountains of the Brooks Range, with detritus from the mountains being dumped along the north side of the range, Wartes said.

The strata that lie immediately above these older Brookian rocks are relatively thick in the east-central North Slope but thin considerably to the north where the major North Slope oil fields are located. That thinning is largely associated with a major geologic feature called the Barrow Arch, an area of uplifted strata that is associated with most of the major oil fields.

The thickening of the Brookian to the south results from the deposition of the material from the Brooks Range into a subsiding basin — Wartes likened the effect to the filling of a moat running along the north side of the Brooks Range.

“The further south you go off the Barrow Arch, the more you’re getting into a much, much thicker sedimentary basin,” Wartes said.

Hue shale

But the intriguing point about this particular package of rocks is that it is in part equivalent to the Hue shale, a prolific oil and gas source rock. And, whereas in the area of the North Slope oil fields the Hue shale is a relatively thin formation, in the east-central North Slope the time-equivalent rock interval opens out into a thick sequence of sandstones with interspersed, organic-rich shales. That combination of potential sandstone reservoirs and shale source rocks could give rise to a very interesting oil and gas play, Wartes said.

The Late Cretaceous and early Tertiary rocks that lie above the equivalent of the Hue shale have characteristics that indicate a west to east transition from an ancient land environment into an ancient marine basin. And those rocks on the east side that were deposited in marine conditions include the Schrader Bluff Formation. The Schrader Bluff Formation is of particular commercial importance because it is also found in the Prudhoe Bay area where it reservoirs huge amounts of viscous oil, including the Milne Point Schrader Bluff pool, and the oil pools for the Orion and Polaris fields.

The DGGS work is helping clarify the detailed nature of these reservoir rocks, Wartes said.

“It’s important to have a good model for how the sands are distributed in the reservoir,” he said.

But given the huge geologic variations across that linchpin east-central North Slope area, a key objective of the geologists has been to verify the correlations between disparate rock units across northern Alaska. Mapping done in the past tended to result in a plethora of names for the various units, including different names at different locations applied to the same units. The confusing nomenclature has compounded the difficulty in correlating the rocks across the region.

A couple of years ago North Slope geology experts Gil Mull, Dave Houseknecht and Ken Bird proposed a revised and simplified nomenclature which the DGGS team is now testing.

“It really did extremely well,” Wartes said. “… It made several predictions in terms of where we ought to see certain units, such as (ancient) regional flooding surfaces, and that certainly came out to be true.”

Field tour

In June DGGS hosted its annual show-and-tell field tour to show industry sponsors of the division’s North Slope program some of the geologic features that the DGGS-led team is investigating. In excellent weather conditions over a period of two days a small fleet of helicopters ferried representatives from ConocoPhillips, Anadarko, Petro-Canada, FEX, ENI, the U.S. Geological Survey (including USGS Director Mark Myers), Alaska’s Division of Oil and Gas and DGGS to 10 locations in the east-central North Slope. BP, Shell and BG also sponsored the program this year but were unable to attend the field tour.

Printed seismic sections were presented at some locations, so that tour participants could discuss how rock observed at the surface related to rocks found underground in wells.

“I think that went over very well,” Wartes said, adding that oil company geologists often only work on subsurface geology.

But despite the progress that the DGGS-led team has made, many unanswered question about the “missing links” between the rock stratigraphy at opposite ends of the North Slope remain.

“It wasn’t at all clear how some of those units correlated and we don’t quite have the answer yet ourselves,” Wartes said. But, in that east-central part of the Slope, the team is working right in the area where the key changes in the geology occur, he said.






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.