Montana Governor and First Lady's Math and Science Initiative
Coal, Oil, and Natural Gas
Mile Post 646 on US Highway 2, Culbertson Rest Area
The Fort Union Formation was deposited not long after
the dinosaur extinction, about 65 million years ago.
Rivers originating in mountains to the west and south
carried abundant sediment to the nearby shallow inland sea
to the east, crossing the subtropical, swampy coastal plain of
eastern Montana on their way. As the rivers shifted, the swamp
vegetation and peat were covered with thick deposits of sand,
silt, and clay. In time, it compacted to form coal. The sand, silt,
and clay cemented and compacted into sandstone, siltstone, and
mudstone. Eastern Montana has an estimated 200 billion tons
of coal that developed from the vegetation of these swamps.
Geologists classify much of the coal buried in northeastern
Montana as lignite, which has a low energy content. Most
of the coal is buried under layers of the soft rock in the Fort
Union Formation, but is locally exposed in hills and road cuts.
The scattered red rock of the Fort Union Formation is clinker.
Clinker forms when lightning strikes, wildfires, or spontaneous
combustion causes the coal to ignite and burn. The intense
heat of the burning coal bakes the adjacent sedimentary layers
creating the dense, hard red rock.
Northeastern Montana is part of the Williston Basin, a
slightly irregular, round depression centered in North Dakota
that slowly subsided over hundreds of millions of years.
Sediment that accumulated in the depression is now up to
16,000 feet deep in the basin. The Williston Basin is a major oil
and natural gas province because it has all the right components
and conditions for oil and gas formation: organic-rich deposits
such as black shale to generate the oil and gas, reservoir rocks
where it accumulated such as porous sandstone, and geologic
traps. The traps are faults, folds, and lateral changes in the
rock that keep the oil and gas from moving out of the area of
accumulation.
Coal, Oil, and Natural Gas
