The Williston Basin: An Ancient Sea Under the Plains

Beneath the wheat fields and oil pads of western North Dakota lies the Williston Basin, a broad sedimentary bowl that formed over hundreds of millions of years as ancient seas advanced and retreated across the interior of North America. The Bakken shale sits roughly two miles down, sandwiched between layers of dolomite, and was deposited during the Devonian and Mississippian periods, roughly three hundred sixty million years ago, when this latitude was warm and shallow ocean. The organic material from that ancient sea, compressed and heated over geological time, became the oil that touched off the twenty-first-century boom. The basin also holds the Madison limestone formation, a deep aquifer whose water runs from fresh along the western edge to heavy brine in the middle of the basin.