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Geological ‘show and tell’ in Umiat Potential reservoir rocks in the Cretaceous and in the Lisburne formation spark industry interest Alan Bailey & Kay Cashman Petroleum News Contributing Writer
This year’s annual field meeting about the state of Alaska Division of Geological and Geophysical Surveys’ geological fieldwork in the Brooks Range foothills took place in mid-July. Nearly 30 people traveled to Umiat on the North Slope of Alaska to review the results of the 2003 field season.
“Each year at the end of our field season we’ve been inviting each of the sponsoring companies to send representatives up to get a ‘show and tell,’” Gil Mull, a geologist with the Alaska Division of Oil and Gas, told Petroleum News. “We show them what we’ve been doing ... there’s a brief field tour to some of the field locations.”
Representatives from ConocoPhillips, Anadarko, EnCana, Petro-Canada and Pioneer Natural Resources, all of whom are sponsors of the DGGS North Slope field studies, came to the meeting, along with independents Fred James and Randy Ray, who are also sponsors. Dave Houseknecht from the USGS is collaborating closely with DGGS and led part of the tour. Total was also a sponsor of the field program this year.
Mark Myers, director of the Alaska Division of Oil and Gas, and Tom Irwin, commissioner of the Alaska Department of Natural Resources, also attended the meeting.
“It was particularly worthwhile having Tom Irwin from DNR there,” Mull said, “because that gave him a chance to meet and talk with the industry folks ... and to get a feel for their interests on the North Slope and in the projects DGGS has been working on.” Started four years ago The state’s foothills program in the central Brooks Range foothills started about four years ago.
By investigating rock outcrops along the northern thrust belt and foothills of the central Brooks Range, geologists in the program team have been discovering information about the rock stratigraphy and the oil and gas source and reservoir potential of the rocks. All of this information provides valuable insights into the subsurface geology and its economic significance, especially for people doing seismic exploration.
As well as investigating the stratigraphy and economic potential of the surface rocks, the team is preparing one inch to one mile geological maps of the region. The mapping component of the program receives federal funding under the U.S. Geological Survey StateMap program.
“We have a lot of different elements to the program,” Ellen Harris, project manager for the Kanayut River StateMap, said, “and probably about half of it this year was funded through the StateMap (program).” Potential reservoir rocks in the foothills Field excursions in the “show and tell” this year focused on some surface exposures in the foothills south of Umiat — an area where Dave LePain and Rocky Reifenstuhl of the DGGS have been studying the stratigraphy.
The exposures display sediments in the Brookian sequence, deposited in a broad basin when the Brooks Range was forming around 100 million years ago. The sediments include sandstones and conglomerates with good reservoir potential in some locations.
For example, Dave Houseknecht led one field excursion to view some sandstone channels in a lower Cretaceous layered sandstone and shale deposit called a turbidite. The channels display evidence of having contained an oilfield when the rocks were still buried in the subsurface. Subsequent uplift and erosion has exposed the channel and has caused the oil in the rocks to degenerate.
“This particular turbidite channel has been filled with mostly dead oil — oil that has lost most of its volatiles,” Mull said. But exposures of such rocks on the surface can provide valuable insights into similar subsurface relationships that might become potential exploration objectives, he said.
A field excursion led by Rocky Reifenstuhl of DGGS included a visit to exposures of the late Cretaceous Tuluvak formation, which is thought to be another potential oil and gas reservoir in the subsurface.
“It’s a really excellent, excellent sandstone and conglomerate horizon that we think might very well have some good prospectiveness in the subsurface to the east” Mull said.
The state geologists have also been studying the Nanushuk formation, the reservoir rock for the oil deposits that are found at Umiat.
“We’ve examined that in fair detail in past years, so we didn’t lead a tour to it this year,” Mull said. “It’s one (horizon) that’s reasonably considered to be prospective.” Mapping near Anaktuvuk Pass The mapping component of the state’s field program has been working west from the Dalton Highway around Galbraith Lake and Atigun Pass since 2001. This year, the mapping efforts focused on an area north and east of Anaktuvuk Pass.
One of the more notable findings from mapping by Paige Peapples of DGGS turned out to be the extent of potential reservoir rocks in the dolomites of the Lisburne group, Mull said. These rocks form the reservoir for the Lisburne field and date from the Carboniferous period about 300 million years ago. Uplift during the formation of the Brooks Range thrust the rocks to the surface along the north front of the Range.
The reservoir potential of the Lisburne Group in the Brooks Range thrust belt seems to have piqued industry interest — some oil industry geologists have also been studying these rocks, Harris said.
“There was a lot of interest this year in the Lisburne and ... the reservoir potential of the Lisburne,” Harris said, “...and they’ve been working to the east along the mountain front in the Lisburne.”
Marwan Wartes, a Ph.D. student from the University of Wisconsin, and Rocky Reifenstuhl of DGGS have also been studying some Cretaceous conglomerates in the foothills — the Fortress Mountain formation. Reifenstuhl has also been working with Ken Ridgeway from Purdue University of Ohio on this study. Where next? In past years, the DGGS foothills team has made some exciting discoveries of economic interest both east and west of the Dalton Highway. However, the original plan for the program was to study the surface geology all the way from Dalton Highway to the DeLong Mountains at the far western end of the Brooks Range. And, as this year’s field season demonstrates, there can be valuable findings anywhere along the north of the range.
So, there’s a perennial question of the future direction of the study.
“We’re currently putting StateMap proposals in again and ... deciding which way we’re going to go, to the east or west,” Harris said. “We tend to follow where industry interests are.”
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