The Williston Basin is one of the most structurally simple basins in the world and home to one of the largest oil discoveries of our time – the Bakken Shale. The basin takes the shape of a saucer or bowl being the deepest near Williston, ND, and thinning at its shallow margins. The maximum thickness of Phanerozoic rocks is 16,000 ft. (The Phanerozoic Eon is known for its abundance of fossils) That depth is also referred to as the Precambrian surface. Not many wells have been drilled below this level and the Precambrian rocks do not outcrop. With that, the basins origins are not well understood
While the basin is not well understood, that is changing with the onset of the Bakken oil boom. The Bakken Shale was first discovered over 50 years ago when a well was drilled on the play’s namesake’s property – Henry Bakken. Not much activity ensued, but various companies attempted to develop the oil bearing formation. There wasn’t much success until horizontal drilling incited a short boom in the early 1990s. Even that didn’t last long and the play was largely forgotten until a partnership between Richard Findley, Lyco Energy and Halliburton drilled a successful well in the Elm Coulee Field of Richland County, MT, in 2001. The Elm Coulee Field proved economic two years later and rising oil prices motivated operators to expand into North Dakota. EOG went on to discover the Parshall Field in 2006 and operators have largely been connecting the dots since.
Geology of the Bakken Shale Play
The Bakken Shale is a rock formation from the Late Devonian, Early Mississippian age that is estimated to hold as much as 24 billion barrels of recoverable oil. The play extends into parts of Montana, North Dakota, Saskatchewan and Manitoba. The Bakken formation consists of three layers: an upper black shale layer, middle silty-dolomite, and a lower black layer of shale. The shale layers are petroleum source rocks as well as seals for an oil reservoirs in the dolomite layer and in Three Forks or Sanish formations. The North Dakota Industrial Commission, in a 2010 study estimates the underlying Three Forks Formation could yield 1.9 billion barrels of reserves, but even that number is likely conservative.
In what has now become a famous research paper, Leigh Price estimated the Bakken Shale contained between 271 and 503 billion barrels of oil. With a mid-point of 400 billion barrels, the assumption of 24 Billion bbls recoverable only achieves a 6% recovery factor. If 400 billion barrels is truly a good estimate, there is significant opportunity for technological innovation to increase recoverable reserves by billions of barrels.
Three Forks Geology
The Three Forks Formation is a dolomite that underlies the Bakken Shale. The formation is sourced by the Bakken and produces across parts of North Dakota and Montana. The play is a secondary target in some areas and has proven even more prolific than the Bakken in some areas near the Nesson Anticline.
What is the Sanish Formation?
Many people called the Three Forks the Three Forks Sanish Formation early in the development of the play. The Sanish sandstone is an informal unit that sits atop the Three Forks in certain portions of the basin. The Sanish Formation is primarily targeted in the Antelope Field in McKenzie County and by Whiting in the Sanish Field in Mountrail County.
Other Plays in the Williston Basin
Other producing formations include the Madison, Duperow, Red River, Tyler, and Spearfish. As seismic studies and wells are completed, operators are becoming equipped with the information needed to develop prospects in many of the legacy conventional plays in the region. In 2012, Whiting Petroleum began targeting the Red River play in some areas.