Reading Salt Creek's Landscape
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Anticline & Domes
Salt Creek Oilfield is a classic "anticline." Being oval in shape, this type of anticline is also called a "dome." Soft, porous stone that originally capped the anticline has eroded away, leaving a crater-like center of impermeable, gray Cody Shale encircled by a horseshoe-shaped escarpment of Shannon Sandstone (a remnant of what eroded from the top of the dome). Embedded with marine fossils, this sandstone escarpment is called the "rimrock" and is visible from Highway 259.
How to find and Oilfield
Salt Creek Oilfield lies in the southwest corner of the Powder River Basin. Due to compression of the earth's crust 60 million years ago, the Big Horn Mountains began to rise on the west and the Black Hills on the east, forming a depression between them about 250x100 miles. Compression also caused the earth's crust to fold within the basin. Elevated folds (hills) are called anticlines,
while the valley between them are synclines.
Erosion of anticlines and synclines creates lines of isolated buttes and knobs, which can be seen on Interstate 25 between Midwest and Casper. Dating to the Cretaceous Period 70-130 million years ago, these sandstone
features mark the southwest edge of the Powder River Basin.
The same geologic activity that shaped Wyoming's rugged mountains and basins also endowed these features with rich mineral deposits and transformed ancient plants and organisms into fossil fuels: petroleum, coal, and natural gas. Understanding how landscape features were formed, geologists predict locations of minerals. To a geologist, the Salt Creek escarpment indicates possible pools of oil down below, since oil and gas accumulate under the dense, exposed Cody Shale. You can drill anywhere in the 20,000 acres of the Salt Creek field and find oil.
Movement of oil Underground
Geologists exploring for oil and gas look for anticlines, whose arched shapes create natural traps for hydrocarbons. Oil and gas are lighter than rock and water (think of oil floating on water). From the deep levels underground where oil and gas form, they migrate slowly upward through permeable layers of rocks (rocks that allow fluids to pass through them). When they are trapped beneath a layer of impermeable stone, they accumulate to form a pool.
How Oil is Formed
Most Salt Creek oil derives from Cretaceous sea life: microscopic algal plants called diatoms
lived 150 million years ago in seawaters that covered this area. When the algae died, they collected on the sea floor and were buried by layers of sand and sediment. As more layers of detritus built up, the diatoms were buried deeper underground. A combination of pressure from the weight of sediment above and heat from the earth's core transformed the plant material first into a waxy substance known as kerogen and from there into gas and oil.
Oil and gas also accumulate in geologic features that are not anticlines. Called stratigraphic traps,
they commonly occur on the pinched-up edge of a sloped, porous oil or gas-bearing layer of sandstone or limestone capped by a layer of impermeable rock, usually shale. With most of the Powder River Basin anticlines depleted of oil, production of oil in this region now focuses on stratigraphic traps.