Wyoming
Bighorn Basin appears on the right side of this NASA image, east of the Beartooth Mountains and the Absaroka Range.
Gazing from an airplane window down on arid Wyoming, it’s hard to believe the entire region was once covered in dense, swampy vegetation. Over geologic time, this plant matter decomposed into kerogen, a sticky substance that breaks down further into petroleum and natural gas products, which in turn have made Wyoming rich in fossil fuels. The sedimentary layers that store such reserves are widespread throughout the region.
One of the most productive places for petroleum is the Bighorn Basin in north central Wyoming. It is technically a structural basin—the opposite of a raised dome—where warping of the crust has produced a large-scale depression in the landscape. From the air, look for a large, dry swale enclosed by the Bighorn Mountains to the east, the Pryor Mountains to the north, the Beartooth Mountains and Absaroka Range to the west, and the Owl Creek and Bridger Mountains to the south. Over time, this depression has collected sediments that now measure more than 20,000 feet thick, dating from the Cambrian Period to the Miocene Epoch, between 500 and 5 million years ago.
The main reservoirs of oil are contained in porous layers of sandstone and limestone from the Mesozoic Era. The 500-foot-thick Tensleep Formation (sandstone), which dates to the Pennsylvanian Period, around 300 million years ago, is the most-tapped oil source in the region. More reserves are found within the Madison Formation (limestone), 330 million years old; the Phosphoria Formation (shale), 260 million years old; and the Frontier Formation (sandstone), 120 million years old.
Coal is also plentiful in the Bighorn Basin; over 50 million tons have been excavated since the early 1980s. As much as 18 million tons of coal is estimated to remain, but because much of this coal lies under 3000-plus feet of sediment, only a fraction of it is considered economically recoverable. These reserves date to the late Cretaceous Period (99 to 65 million years ago) and are mainly contained within the Fort Union Formation. Much of the coal found in the Bighorn Basin is bituminous, which is relatively soft and sooty and doesn’t burn as hot as denser anthracite. Bituminous coal is a sedimentary rock, but the much rarer anthracite is metamorphic—produced when heat and pressure transform existing bituminous rock.
Where Mesozoic layers of sedimentary rocks are found, so are dinosaur bones. The Cloverly Formation in the Bighorn Basin has been an important source for Cretaceous Period fossils since the early 1900s.
Folds in the sedimentary layers of the Bighorn Basin created traps and pockets where oil and natural gas collected over time.
You might fly over the Bighorn Basin en route to Jackson, Wyoming, or Billings, Montana.
