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The Williston basin has been portrayed as the archetype intracratonic basin, wherein tectonics have played only a minor role in geologic history. This may be demonstrated in the subtle manner in which tectonics have controlled the locus and development of most of the producing reservoirs in the Paleozoic sedimentary sequence.
The pattern of the initial sedimentary sequence covering the crystalline basement is that of a westerly to southwesterly thickening wedge. Limited penetration of the Deadwood sequence has restricted full understanding of its hydrocarbon potential. However, it forms the foundation for overlying Upper Ordovician Red River production focussed along the North Dakota-Montana border. The structural features on which most Red River production occurs show near classic effects of active structural development during deposition of the reservoir beds. Peritidal facies represent minute variations in depositional topography and, therefore, respond to the smallest increments of uplift. This causes sharp variations in distribution of reservoir and nonreservoir facies within a given structural featur .
Major unconformities mark tectonic episodes at the close of the Silurian and near the close of the Devonian. Each of these events is marked by uplift and leveling around the periphery of the basin and significant shifts in depositional patterns within the basin. Late Ordovician through Silurian sedimentation is reflected in basin-centered patterns of thickening. Facies patterns, including the distribution of reservoirs and seals, reflect this. The Devonian onlaps across an unconformable surface and has formed a wedge with significant northward thickening. Facies patterns of both Middle and Upper Devonian units reflect a tectonic and sedimentary connection into the Alberta basin.
Mississippian seas flooded across the late Devonian unconformity and deposited thick sequences of carbonates and evaporites again in a predominantly basin-centered pattern. Rapid subsidence and rapid sedimentation caused a filling of the basin, and a strong regressive pattern of facies developed. Peritidal sedimentary facies dominate in development of reservoir and seal. Traps are thus influenced by very subtle tectonic-topographic variations throughout the basin.
Late Paleozoic to early Mesozoic uplift, tilting, and erosion, followed by deposition of early Mesozoic seals has had a profound effect on the distribution of hydrocarbons in the previously developed traps. A more complete understanding of this final episode and its relation to source rock maturation and migration holds the key to much of the future development of the basin.
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