Depositional Environment Of The Top Four Members Of The Three Forks Formation In Northwestern North Dakota, Williston Basin And Its Relation To Variables In Oil Production

The Three Forks Formation (Upper Devonian) is present in the subsurface in the western two-third of North Dakota and conformably overlies the Birdbear Formation and underlies the Bakken Formation. The Formation reaches its maximum thickness of 270 feet (82 meters) in the center of the basin. The Three Forks Formation is composed of dolostone, mud, fine sand, and limestone. This research included a detailed study of core samples and some petrographic analysis of thin sections. Six lithologic units were recognized in the subsurface including: laminated beds, massive mudstone, massive sandstone, brecciated beds, mottled beds, and massive limestone. Groundwater controlled dolomitization caused magnesium to replace calcium resulting in the large amount of dolomite. This is common in sabkha environments. Pyrite, derived from sulphate reducing bacteria in hypersaline waters, contributed to the colors and the patterns in the dolomite. Six fining upward sequences occur in the formation and constitute the six members of the Three Forks Formation. These members involved fluctuating waters depositing sandy dolomite and mud in alternating layers. Longer periods of mud deposits occur due to lack of sand supply. Overall the environment was likely an evaporite tidal flat-sabkha with hypersaline warm waters. The Three Forks is economically important formation due to oil production. Different variables such as natural fracture zone, oil saturation, diagenesis, and pyrite concentration can contribute to locating more profitable locations. The areas recommended for drilling based on these factors include member 6 of Divide County, northeastern McKenzie County and southeastern Williams