Conodonts of Bakken Formation (Devonian and Mississippian), Williston Basin, North Dakota: ABSTRACT

Abstract

The Bakken Formation is a thin (maximum 145 ft, 45 m), clastic unit in the subsurface of Williston basin in the United States and Canada. The formation consists of three informal members which display an onlapping relationship. A lower, radioactive, pyritiferous, noncalcareous is overlain by a light to dark gray, dolomitic siltstone to calcareous sandstone and an upper similar in lithology to the lower shale. The finely laminated, organic rich, shales of the Bakken Formation were deposited in an anoxic, marine environment that was apparently offshore, sediment-starved, and below wave base. The middle member contains bedding features and fossil evidence indicative of a slightly dysaerobic, current-influenced, marine environment. The upper and ower shales are considered important source rocks for hydrocarbons in the Williston basin. The Bakken is similar in lithologic character and stratigraphic position to other black shale units deposited on the North American craton during the Late Devonian and Early Mississippian. The Bakken was initially considered entirely Mississippian in age. Paleontologic study of regional physical equivalents and analysis of the macrofauna in Saskatchewan has suggested that the Bakken is actually both Devonian and Mississippian. Conodonts were obtained from cores of the Bakken in an effort to determine the age of the formation in North Dakota and to assess the oil generation potential. Nearly 700 conodonts have been recovered, but are unevenly distributed within the Bakken Formation. A majority was obtained from thin (approximately 0.5 cm), fossil-rich beds within the upper shale. Conodonts from the top of the upper reveal a Mississippian (Kinderhookian) age and are here assigned to the Lower Siphonodella End_Page 1341------------------------------ crenulata Zone. Only rare, fragmentary conodonts have been found in the middle member. Conodont evidence from the middle of the lower suggests a late Devonian (Famennian) age (Upper Polygnathus styriacus Zone) for this member. Conodont color has been established as a geothermometer in carbonate rocks. Color alteration indices of conodonts from the Bakken range from 1.5 to approximately 2.5 and indicate a pattern of increasing temperature with depth. These results suggest possible hydrocarbon generation from shallower depths than has been reported previously for the Bakken. The lack of agreement in interpreted hydrocarbon generation depths may be due to, among other things, the clastic nature of the Bakken Formation. End_of_Article - Last_Page 1342------------