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Alaska coal as potential oil source Tests on coal samples have demonstrated that the coal can produce oil as well as gas if heated to appropriate temperatures Alan Bailey Petroleum News
People generally think of coal as a source of natural gas. But there are types of coal that produce oil as well as gas when heated to some appropriate temperature. Recent laboratory tests conducted by the U.S. Geological Survey and Alaska’s Division of Geological and Geophysical Surveys have confirmed that some Alaska coals are of the oil-bearing variety, a finding that may have important consequences for the oil and gas potential of some of the state’s hydrocarbon bearing basins. For example, Doyon Ltd. is exploring for oil and gas in the Nenana basin, where the hydrocarbon source rocks are coal and coaly shales.
On May 23 during the Association of American Petroleum Geologists Pacific Section annual conference Paul Lillis from USGS talked about the research into the oil potential of Alaska coals.
Lillis said the scientists conducting the research had used four coal samples, two from the Nenana basin and two from the Chickaloon formation in the Matanuska Valley. The tests involved heating the coal samples in the presence of water to 360 degrees C over a period of 72 hours, thus raising the thermal maturity of the coals from a relatively immature level to late- or post-maturity. The thermal maturity determines the propensity of the coals to generate oil and gas. Prior to heating the samples the coals were tested for their hydrogen indices, an important parameter in determining the potential for organic material to form oil when heated.
Generated waxy oil The scientists found that all four coal samples generated waxy, low-sulfur oil at yields of 59.5 to 101.4 milligrams per gram of organic content. Gas chromatograms of the oil demonstrated the presence of hydrocarbons typical of oils that are generated from coals that originate from non-marine woody material. No algal material was observed in the coal samples, Lillis said.
Coals typically have hydrogen indices that appear too low for effective oil production - hence the common assumption that coal will produce gas rather than oil. However, it is well known that the hydrogen index of coal initially increases when heated, thus potentially making the coal more oil prone, Lillis said. In the case of the Alaska coal, this phenomenon appears to have particularly taken place in the case of the Chickaloon samples, he said.
The technique used for the experiment, called hydrous pyrolysis, tends to exaggerate oil production by as much as 50 percent. But, with sufficient burial time at appropriate temperatures, Alaska coals could generate waxy oils, Lillis said. The experiments have illustrated that the coals will expel the oil, rather than keeping the oil trapped. And the oil produced did resemble oils that have come from coal sources in various basins around the world, Lillis said.
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