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Stratigraphy and Sedimentology of the Upper Bakken, Cottonwood Canyon and Lower Banff Section: Complexities Associated With Fine-Grained Depositional Systems in a Tectonically Active, Low-Accommodation Setting, and Their Implications on the Bakken Petroleum System

Abstract

Fine-grained depositional systems accumulating shale, silt, and very fine sand comprise a major portion of the Bakken petroleum system. With the proper subsurface conditions, these deposits form the source and much of the reservoir in the Bakken-Three Forks play. Understanding the detailed stratigraphy and sedimentology of these depositional systems provides critical insights into the distribution and quality of both source and reservoir rocks and their hydrocarbon production capability. Complicating this understanding is the tectonically active, low-accommodation setting that characterizes deposition. This type of setting leads to laterally extensive depocenters that shift with time, resulting in a section partitioned into several sequences separated by regional unconformities. This study focusses on one of these sequences, which contains the Upper Bakken and the laterally equivalent Cottonwood Canyon and Lower Banff.Depositional systems that comprise the Upper Bakken/Cottonwood Canyon/Lower Banff sequence are laterally extensive spanning the Williston, South Alberta, Big Horn and Carrington Basins, and adjacent outcrops. This sequence can be subdivided into a succession of three system tracts: 1) a dolomitic lower system tract characterized by silty dolostone and organically lean gray shale, 2) a siliceous middle systems tract characterized by proximal siltstone and very fine-grained sandstone grading into distal organic-rich, black, cherty shale, and 3) a calcareous upper system tract characterized by proximal silty/sandy limestone grading into distal organic-rich, black shale.The lower system tract deposits provide minimal contribution to the Bakken petroleum system as they thin and lapout through the Big Horn Basin and are absent in the Williston and South Alberta basins. Conversely, the proximal siltstone and sandstone of the middle system tract deposits serve as reservoir in the South Alberta Basin, while the laterally equivalent distal black shale serves as source rock in the Williston Basin. Additional source rock is provided by the distal black shale of the upper system tract deposits. Erosion, however, greatly limits the distribution of this system tract with the black shale relegated to primarily the Williston and Carrington Basins. The Carrington Basin contains the thickest organic-rich shale section but is shallow and likely immature.