Oil from Alaska coal
USGS, working with coal samples provided by DGGS, extracts liquid hydrocarbons
For Petroleum News
Scientists from the U.S. Geological Survey and Alaska’s Division of Geological & Geophysical Survey have successfully extracted oil from Alaska coal samples.
Coal is known as a source of methane, the main component of natural gas, but getting oil is something new.
Liquid hydrocarbons were extracted from four Alaska coal samples in tests, DGGS Director Steve Masterman told the House Resources Committee in a recent overview hearing. Each sample “generated quite a lot” of oil, Masterman said
In a Jan. 31 interview, David LePain, a DGGS petroleum geologist, said the division worked in a partnership with the USGS to do the tests using a relatively new procedure - hydrous pyrolysis - in a USGS laboratory in Lakewood, Colorado, near Denver.
Petroleum geochemists from the USGS actually conducted the hydrous pyrolysis experiments.
Few commercial laboratories in the U.S. currently have the ability to conduct hydrous pyrolysis tests, but the government lab in Colorado does have the capability and the geochemists with the expertise.
Coal samples from AlaskaThe two agencies worked together, but LePain said he wants it to be clear that “the USGS is doing the heavy-lifting” on the project.
DGGS supplied the coal samples, with two from the Wishbone Hill area in the Matanuska Valley and two from the Healy area in the Interior, near where Usibelli Mine Inc. operates a coal mine.
All samples were from surface outcrops of coal, LePain said. Characteristics of the oil are still being analyzed by the USGS. Preliminary results may be made available later this spring at a meeting of the Pacific section of the American Association of Petroleum Geologists, he said.
What is still not understood, however, is just how oil is formed in a coal bed. Coal is formed when prehistoric plant material accumulates and is gradually buried, compressed and heated over geologic time. Oil is generally formed when marine sediments containing algae and other marine organic material settle to the sea bottom and are gradually buried and heated over time.
Some methane formed in coal occurs through a “biogenic” process caused by bacteria feeding on the compressed organic material in the coal. Oil is usually created through a “thermogenic” process caused by heat and pressure when sediments are buried at depth.
Complicated processBut oil from coal is likely to be more complicated, however. “Certain coals in Alaska, in the Holitna, Nenana, North Aleutian/Bristol Bay, Susitna, Cook Inlet and Gulf of Alaska basins, if buried deeply enough to reach the right level of thermal maturity, might have potential to generate significant quantities of liquid hydrocarbons,” DGGS said in a briefing paper on the tests.
Initially, tests were first done using a process called Rock-Eval pyrolysis, a relatively simple procedure where coal samples are heated in the absence of water and measurements are taken of hydrocarbons being expelled.
But those tests did not really replicate conditions found in nature, deep underground, the white paper said. In addition the Rock-Eval pyrolysis appeared to overestimate the amount of hydrocarbons released.
To get a better picture, the science team switched to tests using hydrous pyrolysis, done with water. Hydrous pyrolysis is a time-consuming test procedure but it more closely duplicates natural conditions underground.
“Hydrocarbons generated using the ‘HP’ procedure more closely resemble compounds generated under natural subsurface conditions, in part by conducting the experiment in water-saturated conditions,” the white paper said.
Coal samples are placed in a reactor vessel with water and heated to temperatures between 260 degrees and 365 degrees Celsius for 72 hours.
The tests did result in oil being generated and expelled from the coal.
LePain said just how the coal produces oil is still not understood. In many coal deposits the coal is interspersed with layers of shale, coaly shale. Both types of rock contain the organic material and it’s uncertain whether the oil is coming from the coal or the mudstone. “It’s the same type of organic material, so it’s not easy to distinguish,” he said.
Another mystery is just how the oil is expelled from the coal. In the case of methane, when coal seams are depressured by the withdrawal of water, a common technique in production of coal-bed methane, the gas desorbs from the coal matrix and can flow to wells.
With oil, it too appears to accumulate in pores in the coal but since there is little permeability in coal matrix (connections between pores) unlike a porous and permeable sandstone, it is difficult for the oil to flow. Also, due the chemical attraction between the coal and hydrocarbon molecules, it tends to adhere to the coal surfaces.
Oil somehow does unlock itself, however - the samples generated in the tests show that - although the mechanism is still unclear. “We don’t know how to explain this right now,” LePain said.
Near-term applicationsWhile it seems unlikely that coal seams could become a source of commercial oil production in the near future, the research by DGGS and the USGS does have near-term practical applications, said Paul Decker, who heads the resource evaluation group in the state Division of Oil and Gas.
“Understanding the ‘fingerprint’ of this oil (which could come when the analyses of the oil samples is completed) will allow us to correlate this to conventional oil” and better understand the origins of the oil.
The most important near-term impact of these experiments could be that Alaska’s coaly Interior basins, traditionally considered gas-prone, might warrant additional exploration for oil, Decker said.
The USGS is quite interested in this because a primary responsibility of the federal agency is evaluating undeveloped basins and their potential oil and gas resources.