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DGGS geologist advocates drilling Holitna basin
Patricia Jones Petroleum News Contributing Writer
A former Alaska state geologist who has studied carbonaceous mudstones, shales and coals taken from fault outcrops near the Holitna basin advocates drilling to determine gas and hydrocarbon potential of the remote region. (See related story in this week’s North of 60 Mining section.)
Plans by Holitna Energy to punch one or two drill holes in the southwest Alaska basin this winter are “right on track,” said Dave LePain, who is in the process of producing a third report on the Holitna basin for the Alaska Division of Geological & Geophysical Surveys.
Existing state-distributed aeromagnetic and gravity data, combined with analysis of carbonaceous samples gathered in recent years suggests the basin contains potential targets for gas accumulations, both conventional and coalbed methane.
“There are a couple bull’s eyes that correspond with gravity lows,” LePain told Petroleum News in an Oct. 23 phone interview from Wisconsin. “Until you prove the stratigraphy … it’s pretty speculative.”
LePain left state employment this summer and has yet to complete a final geochemistry analysis report on 50 to 60 samples taken in the Holitna basin in the summer of 2002. An initial review of the samples indicates high levels of total organic carbons, LePain said. Follow-up samples gathered The samples were gathered in an effort to follow up on two of about a dozen samples gathered during the 2000 field season, which surprisingly showed potential for hydrocarbon accumulations.
Sampling carbonaceous rocks from the Holitna basin, which is a teardrop shaped formation that starts south of Sleetmute on the Kuskokwim River and stretches for about 70 miles along the Farewell fault, isn’t possible because of extensive ground cover.
Instead, LePain’s field crews have gathered rocks from outcrops along the fault, several miles to the northeast of the potential gas basin.
Those samples show organic-rich mudstones and shales, in thick layers that are “good news for a gas point of view,” LePain said.
The question is, he added, are those similar layers of organic material contained within the Holitna basin, and if so, “…is it deep enough to generate the hydrocarbon conversion?” Past work discounted region Industry exploration geologists working for ARCO, Unocal and Sohio (now BP) independently considered and rejected the Holitna basin during the 1980s.
Older rocks did not show potential for hydrocarbon or gas source rocks, LePain said, although the younger, Tertiary strata did.
“All three acknowledged the potential source in the younger rocks, but how are you going to get it to market, even if you have a significant volume,” LePain said. “Back in the 1980s, it was not economic …. all three independently walked away from the region.”
Another consideration was the potential resource size. The Holitna basin, speculative as it is, could contain from 50 to 100 billion cubic feet of gas, LePain said, which compares to Alaska’s North Slope fields known to contain amounts of gas measuring in the trillion cubic feet accumulations.
“That (smaller size) is just not attractive to commercial operators,” he said.
The Holitna basin scenario dramatically changes, though, with a local demand for energy, which could be provided by development of the Donlin Creek gold mine. Some negative characteristics LePain’s past work and reports on the Holitna basin do contain some negative traits, including layers of sandstone in between the organic rich shales.
“Those sandstones don’t appear to have a lot of porosity in the outcrops,” LePain said. The trap potential of the upper layers is also poor, based on outcrops, creating a potentially leaky reservoir, he said.
The organic rich shales could possibly be both source rock and reservoir rock, LePain added. “Depending on the fracture and volume, there could be gas storage within the matrix,” he said. “Until you put a hole in the ground, you just don’t know.”
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