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Upping the ante for the Yukon Flats USGS raises estimate of top of the oil window to 6,000 feet; Doyon re-assesses prospects on land close to trans-Alaska pipeline Alan Bailey Petroleum News
It seems that the more that’s known about the Yukon Flats basin in Alaska’s Interior, the more potential the basin appears to have for substantial oil and gas resources. In the latest chapter in a series of investigations, the U.S. Geological Survey has raised the estimated level of the ceiling of the oil window in the basin from about 8,000 feet to 6,000 feet below the surface, thus substantially increasing the volume and extent of rock in which oil could have been generated.
New possibilities And that’s giving Doyon Ltd., the Alaska Native regional corporation for the region, food for thought when it comes to exploration and development possibilities in the basin, James Mery, Doyon’s senior vice president, lands and natural resources, told Petroleum News Nov. 21.
“We’ve looked at several new suites of prospects,” Mery said. “… We’re in the process of assessing what all this means.”
The Yukon Flats consists of an approximately 15,000-square-mile lowland area around the Yukon River between the trans-Alaska oil pipeline and the Canadian border. A sedimentary basin with petroleum potential, the Yukon Flats basin, lies under the flats.
USGS has done its new Yukon Flats evaluation by plugging inferred rock properties, likely heat flow rates from the subsurface and estimated surface temperatures into a computer model. By running the model for the likely period of formation and development of the basin, geologists have evaluated the thermal history of rocks at different current depths within the basin. And that thermal history has enabled them to estimate the current depths at which oil and gas might have been generated.
The geologists ran the model using four different plausible theories for how the basin may have formed. But, despite the fact that these theories require the basin to start to develop at two significantly different times, the estimated depths in the current basin for oil and gas generation turned out to be very similar — that consistency adds credence to the evaluation results.
2004 assessment USGS published an oil and gas assessment for the Yukon Flats basin in 2004. That assessment determined that there might be more oil and gas in the basin than had previously been thought. However, USGS viewed the basin as likely to be more prospective for natural gas than for oil, but with the potential for some oil in the deepest sections of the basin.
The assessment estimated that technically recoverable natural gas reserves could lie somewhere in the range from zero to almost 15 trillion cubic feet, with a mean of about 5.5 tcf. The corresponding figures for oil consisted of a range of zero to almost 600 million barrels, with a mean of about 173 million barrels. The natural gas liquids estimates range from zero to 350 million barrels, with a mean of almost 127 million barrels.
The assessment involved a re-evaluation of some Yukon Flats seismic data shot in the 1970s and 1980s, and concluded that the basin attains a maximum depth of 25,000 feet, a significantly greater depth than previously thought. The assessment confirmed that the deepest section of potential oil and gas bearing strata sits close to the middle of the southern part of the basin.
The seismic lines used by USGS in its assessment focused on that deep, south-central part of the basin. However, in 2005 researchers published an interpretation of gravity data that indicated that the deep south-central basin forms one of a series of sub-basins distributed around the complete Yukon Flats basin. Most of the sub-basins appear to have depths in excess of 8,000 feet, the depth below which USGS originally thought temperatures would have been high enough to generate oil from organic material in the rocks.
New assessment In February 2008 Mery told Petroleum News that Petrotechnical Resources of Alaska had completed another assessment of the oil and gas potential of the Yukon Flats basin and had come up with an estimated 300 million to nearly 1 billion barrels of oil, and perhaps 15 tcf of natural gas.
“We clearly think that this area is permissive of at least a couple, maybe more, Alpine-sized fields,” Mery said at the time.
PRA, as part of its analysis, also addressed the question of whether the petroleum source rocks in the basin are capable of generating oil, rather than gas — geologists have tended to consider non-marine rocks of the type found at Yukon Flats to be more gas prone than oil prone. Tom Walsh, a principal partner in PRA, told Petroleum News in February that the shale source rocks in the basin were likely to have been deposited from lakes and that lake-deposited sediments in similar geologic environments elsewhere in the world had proved to be effective oil sources.
“That’s given us a fair amount of confidence that this (Yukon Flats) basin is very prospective, just compared to other lacustrine basins around the world,” Walsh said. “... A lot of these Tertiary lacustrine basins are more oil prone than gas prone.”
Now, the latest USGS findings that raise the top of the oil window to 6,000 feet have upped the ante further by broadening the areas of the various sub-basins that might be oil prone — the sub-basins narrow at depth, a bit like huge, buried pudding bowls, so that the higher the level at which oil generation can occur, the wider the oil generation “kitchen” becomes.
And that now makes several of the sub-basins quite prospective, including a basin near Stevens Village that is only about 35 miles from the trans-Alaska oil pipeline.
Land swap The evolving perspectives on oil and gas prospects in the Yukon Flats are raising some interesting questions about a land swap that Doyon has been trying to engineer with the U.S. Fish and Wildlife Service, the agency that manages the Yukon Flats Wildlife Refuge. The 8.6 million-acre refuge occupies a substantial portion of the Yukon Flats lowlands, including the land containing the deepest part of the Yukon Flats basin.
Doyon and some Native village corporations own a patchwork of surface and subsurface land amounting to about 2 million acres inside the refuge perimeter boundary.
The proposed land swap, the subject of a 2004 agreement in principle between Doyon and Fish and Wildlife, would entail a shuffling of land tract titles to bring the deepest and most prospective part of the basin into Doyon ownership. For a number of years Doyon has been looking into the possibility of oil and gas development in the Yukon Flats basin as a means of generating income for the corporation’s Native Alaska shareholders and to generate economic activity and employment for the Yukon Flats communities.
Controversy But the land swap proposal has proved controversial among the Yukon Flats communities. Some local residents see the potential for oil and gas development to bring economic benefit to a region hard hit by escalating fuel costs. Others view oil and gas development as a threat to their traditional subsistence way of life and worry about the potential for environmental damage.
Fish and Wildlife has been addressing concerns about the land swap by preparing an environmental impact statement for the swap. However, complexities in the process for valuing the land potentially involved in the swap have resulted in delays in EIS development, and the final EIS is not now expected to be completed until the fall of 2009.
Meantime, the new geologic findings have greatly expanded the area of land thought to be prospective for oil and gas. At the time of the land swap agreement in principle, Doyon was looking at 200,000 acres of prospects, 90 to 100 miles from the trans-Alaska oil pipeline. Now there is the possibility of 800,000 acres of prospects, with some quite close to the pipeline and some in land originally identified for transfer to the U.S. Fish and Wildlife Service, Mery said.
“We’re re-assessing things and we’ll continue to do that,” Mery said. “We’re still very interested in the land exchange. We think it has some real possibilities.”
But while work on the land swap EIS continues, Doyon has been talking to people about exploration possibility on existing Doyon Yukon Flats land and the corporation will be promoting that land at the 2009 North American Prospect Expo, Mery said.
“We’re actively promoting the property right now,” Mery said.
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