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Looking for oil and gas in the Yukon Flats Geologists from the DGGS and the U.S. Geological Survey are taking a renewed interest in the Interior Alaska’s Yukon Flats basin Alan Bailey PNA Contributing Writer
With so much attention on the exploration of the North Slope and the Cook Inlet, it's easy to forget that there are other potential oil and gas basins in and around the Alaska. Take, for example, the Yukon Flats basin that extends beneath the Yukon River, between the trans-Alaska pipeline and the Canadian border. Other than some preliminary exploration by Exxon and Texaco in the mid-'80s, industry has shown little interest in this 12,000-foot thick sequence of sedimentary rocks. Yet, it seems likely that the basin contains extractable hydrocarbons.
The Alaska Division of Geological and Geophysical Surveys is investigating the Yukon Flats basin as part of an Interior basins program that includes the Nenana basin and the Copper River basin, Rocky Reifenstuhl, a geologist with the Alaska Division of Geological and Geophysical Surveys, told Petroleum News Alaska recently. DGGS wants to evaluate the oil and gas potential of these basins and has done some initial field work and rock analysis in the Yukon Flats area, Reifenstuhl said.
The U.S. Geological Survey is also assessing the oil and gas potential of the area. Faulted basin The Yukon Flats basin sits in a segment of the earth's crust that has dropped between two major faults. The two boundary faults extend east-west along the northern and southern flanks of the basin. The area has filled with river deposits and other land-based sediments since the crust starting sinking at the start of so-called Tertiary time, about 65 million years ago.
Gravity and seismic surveys indicate a deep basin, especially in the southern part of the area.
“The indications from the seismic are that there's between 3,000 meters and 4,500 meters of Tertiary fill in the basin,” Reifenstuhl said. Hydrocarbon indications And there's evidence that the sediments in the basin include hydrocarbon source rocks. For example, geologists have found tasmanite, a form of oil shale, in the northeastern part of the basin. Tasmanite is a strange, elastic rock that you can light with a match — people sometimes call it “blubber rock,” Reifenstuhl said.
Exxon exploration between 1984 and 1986 included some seismic lines and some shallow wells around the perimeter of the basin. Exxon was looking for hydrocarbon source rocks but the company has not released the results of its work to the general public. Texaco also shot some seismic lines in the Yukon Flats in the mid-1980s.
The U.S. Geological Survey drilled a test well in 1994 and hit some gas generating coal at a depth of about 1,200 feet.
“About at the base of the section they hit a 29-foot big coal section, which is mid-Miocene coal,” Reifenstuhl said. “They pulled it out and it was popping and cracking and diffusing gas.”
A couple of years ago, a 10-mile seismic line, run by DGGS in conjunction with the Kansas Geological Survey, confirmed the existence of coalbeds at shallow depths. This particular seismic survey did not penetrate the deeper rocks and added little definition to the stratigraphy of the basin. Rock sampling Last September Reifenstuhl, travelling by helicopter with Rick Stanley of the U.S. Geological Survey, visited some of the sparse surface outcrops in the Yukon Flats area. The two geologists collected representative rock samples for analysis.
“We probably looked at 16 to 20 Tertiary exposures,” Reifenstuhl said.
Laboratory analysis of the rock samples revealed a thermal history that could support oil and gas generation.
“You'd look at them and say 'OK, you can't discount this basin because of an over-mature Tertiary section',” Reifenstuhl said. “The Tertiary section says 'yes, it's possible for this rock to contain oil and/or gas'.”
The permeability and porosity of the rocks also include appropriate ranges for oil and gas reservoirs, although volcanic action may have clogged some of the sediments along the margins of the basin.
“The porosity ranges from 2 percent to about 26 percent, with an outlier of about 39 percent,” Reifenstuhl said. “They're nice looking fluvial, graded stream deposits.”
It all adds up to a strong possibility of hydrocarbon accumulations.
“What we're going to be looking for is probably shallow gas, possibly oil in the deepest part of the basin,” Reifenstuhl said. “There's a lot of coal in the section.”
Defining the stratigraphy DGGS is seeking funding to carry out a detailed investigation of the stratigraphy of the basin. With few wells and sparse 20 year old seismic, the geology of the area remains imperfectly understood — Reifenstuhl thinks that a portfolio on the reservoir characteristics of the rocks would provide an invaluable starting point for companies interested in exploring the area.
Rivers, cutting down through the land surface, have exposed hundreds of meters of rock in some places.
“What we're hoping to do is to go out to some of these Tertiary sections ... and actually do a more detailed stratigraphic analysis of the section,” Reifenstuhl said. “... ultimately we'd like to put together a small portfolio on the potential reservoir characteristics of some of the Tertiary sections.”
Meanwhile, Rick Stanley in the U.S. Geological Survey plans to publish later this year an assessment of undiscovered oil and gas in the basin, based on what information is currently available.
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