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Vol. 22, No. 22 Week of May 28, 2017
Providing coverage of Alaska and northern Canada's oil and gas industry

Promising for oil

Nenana basin well found multiple oil shows; focus moving to northern part of basin

Alan Bailey

Petroleum News

The Toghotthele No. 1 well, drilled by Doyon Ltd. in 2016 in the Nenana basin, found multiple oil shows, Gerry van Kooten, geological consultant with Petrotechnical Resources Alaska, told the American Association of Petroleum Geologists Pacific Section annual meeting on May 23. The well, a few miles west of the town of Nenana in Alaska’s Interior, was drilled to a depth of 11,375 feet. There were 20 to 30 oil shows at depths between 6,400 feet and the bottom of the well, van Kooten said.

“This is really good evidence that oil is present in the Nenana basin,” he said.

The well also encountered some gas, relatively dry gas higher up in the well but wetter, with condensate components, deeper in the well.

Oil migration

While the well did not encounter any trapped oil, the oil shows indicate that oil has migrated through the rocks at some time. The rocks penetrated by the well exhibit too immature a thermal history to have generated oil from organic material that they contain. And there is evidence that the rocks have been uplifted by perhaps 3,000 feet at some time in the relatively recent past, a phenomenon that would suggest that the oil may have migrated through the rocks before the potential trapping structure that is now seen would have formed, van Kooten said.

Doyon is now moving the focus of its exploration to the more northerly part of the basin, where the uplift impacting the more central and southerly section of the basin does not appear to have happened to any significant extent. Doyon has just completed a 3-D seismic survey in that more northerly area, to ferret out some detailed information about leads that it identified from previous 2-D seismic.

An Interior basin

The Nenana basin, a large sediment-filled basin southwest of the city of Fairbanks, has long been thought to be prospective for natural gas. The basin is conveniently located near the road and rail transportation corridor between Southcentral Alaska and the Interior, thus making the basin an attractive target for potential oil or gas development.

For several years Doyon has been spearheading exploration of the basin. The corporation’s exploration program included 2-D seismic surveys in 2005 and 2012, a 3-D seismic survey, conducted in the winter of 2014-15 and three exploration wells: the Nunivak No. 1 in 2009, the Nunivak No. 2 in 2013 and, most recently, the Toghotthele well. Cook Inlet Region Inc. partnered with Doyon in the drilling of that last well.

All of these wells are located in the central part of the basin, to the west of Nenana. Doyon has in the past said that the concept has been to find oil and gas traps in a central saddle of the basin, where hydrocarbons may have flowed upwards from the basin’s deeper sections.

The wells have been targeting sands within the Healy Creek formation of the Usibelli group, a sequence of rocks of relatively young Tertiary age laid down from a pre-existing system of rivers. Older rocks under the Healy Creek formation exhibit poor reservoir quality and appear close to the economic basement of the basin, van Kooten said.

Van Kooten said that the Nunivak No. 1 well had encountered a small number of oil shows, in a thin zone near the base of the Healy Creek formation. Doyon has previously reported that the Nunivak No. 2 well encountered about 400 feet of gas-bearing rock in what appeared to be a depleted gas reservoir - the reservoir contained too much water to be commercially viable.

The sands where the gas was found show excellent reservoir quality, van Kooten commented.

Coal source

The abundant coal seams in the basin appear to be the primary source of both the oil and the gas. Van Kooten said that analyses of the organic and hydrogen contents of shales, coaly shales and coals from the basin have provided insights into the source rock potential of these rock types. The shales have low source potential, while the coal and coaly shales have good potential, van Kooten said.

Coal is generally associated with natural gas generation. However, there are situations where coal is known to have generated oil. Van Kooten cited as an example coal-generated oil that is produced from the Gippsland basin in Australia. Scientists from Alaska’s Division of Geological and Geophysical Surveys and the U.S. Geological Survey have conducted laboratory tests on coals from central and southern Alaska and have demonstrated that these coals generate oil when heated.

Van Kooten said that the geothermal gradient in the Nenana basin appears to be relatively high and would put rocks deep in the basin within the temperature window where oil could form. Two basin models have indicated the possibility of the relatively recent generation of several billion barrels of oil and several trillion cubic feet of gas in the basin.

Basin structure

PRA consulting geophysicist Jon Konkler showed gravity maps that demonstrate the broad structure of the basin floor, with particularly deep sections in the basin’s northeastern and southeastern quadrants. Various seismic surveys conducted across the basin have provided more detailed insights into the basin’s internal structure.

The mapping of thickness changes in the Nenana gravel that occupies the upper level of the basin indicates that, while there has been significant uplift and erosion in the more southerly part of the basin, the northern part has seen some relatively recent subsidence, with the Paleocene and early Miocene section of the basin fill entering the hydrocarbon maturation zone at depths in the range 12,000 to 24,000 feet. That now makes the northern sector of basin a particularly interesting exploration target.

The higher probability of continuous hydrocarbon migration pathways in the north reduces the exploration risk in that region, Konkler commented. A cluster of leads that Doyon’s team is now investigating are close to the deep “hole” in the northern basin, he said. Using its recently acquired 3-D seismic data Doyon is looking at what it refers to as the Totchaket East lead. A seismic section across this lead indicates the existence of a potential hydrocarbon trapping structure.



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