The annual North Slope show-and-tell tour, organized by Alaska’s Division of Geological and Geophysical Surveys, has become something of an institution among North Slope geologists. And this year’s tour at the end of June attracted a large attendance.
Although originally intended to show sponsors of the DGGS North Slope research program the work that the division is engaged in, the annual tour provides anyone interested in North Slope geology an opportunity to visit notable geologic sites in the company of some of the foremost experts on the regional geology.
The only qualification for attendance is that you arrange your own field helicopter transportation, DGGS geologist Marwan Wartes told Petroleum News July 20.
Such was the level of interest this year, that on June 28 no less than seven helicopters converged on Galbraith Lake, where this year’s DGGS field program has been based. And, thanks to some beautiful weather and careful air traffic control, the helicopter fleet succeeded in ferrying a large group of enthusiastic geologists around eight sites over the following couple of days.
“We had to make sure that you could safely land seven helicopters (at each site), including a couple of larger ones like a Bell 212,” Wartes said.
This year’s attendees included 21 industry geologists from Anadarko, Petro-Canada, BG, ConocoPhillips, FEX and ENI. The U.S. Geological Survey attended in force, with USGS Director Mark Myers and Deputy Director Bob Doyle, in addition to four USGS North Slope specialists including geologist Dave Houseknecht.
Wartes and Division of Oil and Gas geologist Paul Decker led the tour, with veteran Alaska geologist Gil Mull also providing his insights into the geologic features visited. State geologist and Acting Director of DGGS Bob Swenson and Acting Director of the DO&G Kevin Banks also attended.
The mix of geologists, some of whom were more experienced with investigating the subsurface geology, while others were more familiar with surface rock outcrops, led to some stimulating and productive discussions, Wartes said.
Multi-year programDGGS has been leading a northern Alaska geologic research program since the mid-1990s. Using a combination of funding from the federal government, state government and industry sponsors, the program has involved geologic mapping and other geoscience investigations in the Brooks Range foothills, where rocks exposed at the surface provide invaluable information about the subsurface geology of the North Slope petroleum province.
The foothills themselves exhibit significant petroleum potential, especially for natural gas.
As part of the program the DGGS-led team has carried out detailed mapping along a swath of territory in the foothills west from the Haul Road. That mapping and the associated investigations have involved research into a wide range of rock formations covering almost the complete gamut of northern Alaska stratigraphy.
And in 2006, the team undertook some detailed mapping in the area of the Kavik gas field, east of the Haul road and just west of the Arctic National Wildlife Refuge.
For its 2007 field season the team has been working in the Sagavanirktok and Phillip Smith quadrangles within 30 miles of the Haul Road, north of the Brooks Range mountain front, investigating the stratigraphy of what is known as the Brookian sequence. These investigations form a necessary prelude to some planned geologic mapping and possible shallow coring in the Sagavanirktok River area in 2008.
“To do that (mapping) we really need to have the stratigraphic foundation that we developed this summer,” Wartes said.
And in a departure from its normal agenda, the 2007 show-and-tell tour only visited Brookian outcrop locations.
“This year was just a little bit different. … We were strictly looking at the Brookian,” Wartes said.
The Brookian is the youngest of the major sedimentary rock sequences in northern Alaska and includes the reservoirs for the Nanuq, Tarn, Meltwater, Tabasco and Badami oil fields in the central North Slope. The viscous oil deposits being developed in the Schrader Bluff formation also occur in the Brookian sequence.
Potential Brookian reservoirs and traps are becoming a major exploration focus in the National Petroleum Reserve-Alaska and the southern North Slope, as well as offshore under the Beaufort Sea (Shell’s Beaufort Sea Sivulliq prospect has a Brookian reservoir).
Complex stratigraphyThe Brookian rocks formed during the Cretaceous and Tertiary periods of geologic history, when the emergence of the Brooks Range caused vast quantities of sand, mud and other sediments to pour to the north into what geologists term the Colville basin. These sediments started filling the Colville basin from west to east, and eventually spilled over the northern edge of the basin and down the subsea slope of the Beaufort Sea.
The complex pattern of sediment deposition in the Colville basin has resulted in wide variations in rock type from one location to another and has rendered the Brookian stratigraphy notoriously difficult to interpret. And the use of different names for the same rock formations at different locations has compounded the stratigraphic confusion.
In fact, rocks that appear similar in outcrop may have formed at distinctly different ages.
“It’s a thorny problem, especially with discontinuous outcrops separated by many miles, and you know that there are some structures between them that you can’t unravel with the existing seismic,” Wartes said.
In 2003 geologists Gil Mull, Ken Bird and Dave Houseknecht published a revised Brookian nomenclature, reconciling some of the inconsistencies in the existing names — the DGGS-led team wants to apply that new nomenclature in its 2008 mapping.
“We were making a real effort this summer to integrate that new nomenclature into the outcrop belt,” Wartes said.
One particular difficulty is a major difference in Brookian nomenclature between the Canning River area at the eastern end of the foothills and the Umiat area to the west. There has always been a major discontinuity in nomenclature somewhere near the Haul Road, even though the rocks themselves don’t change at that point, Wartes said.
By working in a region where the geologic structures are relatively simple, well north of the complex rock deformation associated with upheavals of the Brooks Range mountains, the DGGS-led team has been able to use publicly available seismic sections to clarify the relationships between some rock outcrops seen at the surface. That use of seismic data, combined with extensive rock sampling for the identification of tiny age-determining fossils in the rocks, is helping the team make sense of the east-west nomenclature disconnect.
The team has made much progress in understanding better the locations of rock-type transitions and the appropriate distinctions between rock formations, Wartes said. Cooperative work involving seismic and petroleum geology expertise from DO&G and surface geology expertise from DGGS expertise has proved particularly fruitful in this effort, he said.
“It’s been quite a boon to this whole program — the integration of the Division of Oil and Gas and the state survey,” Wartes said.
Exciting aspectAnd the linkage between seismic data and surface outcrops proved to be a particularly exciting aspect of the show-and-tell tour — Dave Houseknecht and DO&G geologist Paul Decker, both of whom have substantial expertise in the seismic data from the region, were on hand during the field stops to comment on how the surface rocks appear to related to the regional geology, Wartes said.
“We made a strong effort for a lot of these (tour) stops to put them in the context of the regional seismic stratigraphy,” Wartes said.
For example, the tour visited two sites on an unnamed creek east of the Haul Road, between the Sagavanirktok and Ivishak Rivers, where there is a particularly extensive section through the upper Cretaceous (Gil Mull has coined the informal name “Sagashak Creek,” a combination of the two river names, for this creek). Rocks exposed in the sides of the creek valley show the characteristics of sediments deposited near or on the floor of the Colville basin while seismic data enable these deeper basin rocks to be traced through the subsurface to other parts of the evolving basin.
And some of the rocks along the “Sagashak Creek” show evidence of a petroleum system.
“Locally there’s some pretty significant oil staining,” Wartes said.
Locations such as this enabled geologists on the tour who were new to the region to see elements of the North Slope petroleum systems in surface exposure.
“Some of the newer companies coming in haven’t seen a field example of the various source rock facies,” Wartes said.
And the various stops during the two-day tour enabled geologists to see different aspects of the regional geology.
“There was a stop or two that was not necessarily something that we were working on but we feel it’s very representative or illustrates a concept,” Wartes said.
For example, a stop at Slope Mountain, next to the Haul Road about 8 miles south of pump station 3 of the trans-Alaska oil pipeline, enabled people to examine a particularly extensive exposure of rocks characteristic of the Colville basin edge in the Cretaceous Nanushuk formation.
Understanding the basinA puzzling rock exposure at a stop east of a place called Icecut on the Haul Road provided a focus for a discussion about the manner in which the Colville basin evolved. The rock at this location contains coarse rock detritus and pebbles that seem to indicate deposition from a river system. Yet this location looks to be well east of the edge of the ancient basin margin. Does the existence of this particular rock at this particular place indicate that deposition into the basin at this point had swung from a west-to-east direction to a south-to-north direction?
DGGS geologist Dave LePain has documented evidence to the south, near Galbraith Lake, for a change of deposition direction of this type. But no one has established how far north that north-facing shelf edge extended. And that’s not just an academic question — the answer has serious implications for finding petroleum sand reservoirs in the Brookian.
“From an exploration perspective it’s really pretty important to know where that shelf margin was at any given time, to predict sand accumulations and such,” Wartes said.
It all helps to home in on places where difficult-to-find Brookian petroleum traps are most likely to exist.