Williston Basin and Bakken Shale: Montana and North Dakota
Williston Basin laterals are predominantly drilled in two target formations: The Middle Bakken Member and the Three Forks Group. The Middle Bakken is located at 9,500- to 10,500-ft TVD and sandwiched between the Upper and Lower Bakken Shales that serve as the source rock for the Middle Bakken Member. The dolostone of the Middle Bakken is mostly homogenous with 15,000-psi UCS. Middle Bakken thickness varies across the Williston Basin, which is centered around the western half of North Dakota. Composition also varies, driven largely by the porosity of the rock comprising clay, quartile, and calcite content.
The Three Forks Formation sits below the Middle Bakken from 10,000- to 11,000-ft TVD. Fed by the Lower Bakken Shale, Three Forks is a mix of dolomite layered with mostly sand and clay, and there are laminations as thin as 1 in. Accessing these formations begins with upper sections of the vertical section through soft and sticky shales and sands comprised of the Pierre Shale and Mowry Formations, followed by the Dakota Formation, which is a mix of sands, shales, and salts. Below the Dakota is highly variable in compressive strength from below 5,000 to more than 30,000 psi, typically encountered in the transition into the Piper Limestone and in the very difficult Minnelusa Group. These punishing transitions finally subside below the Kibbey Formation limestone cap, where salt and anhydrite content increase. The final section is drilled through hard limestone known as the Mission Canyon and Lodgepole Formations, notoriously difficult to drill because of their consistent 20,000 UCS.