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Upon superficial comparison, the Madison Group (Mississippian) and the Bighorn Group (Ordovician) in the Williston basin appear to be similar sequences of carbonate mudstones and wackestones capped by evaporite-carbonate alternations. Detailed studies demonstrate significantly different depositional styles.
The Madison Group is an example of a deep-water sediment-starved basin that was filled in by turbidites derived from a ringing carbonate shelf. As the basin filled, the Madison was capped by a basinward prograding
sabkha sequence. Correlation of log markers demonstrates considerable bottom topography along prograding clinoform ramps in the lower Madison and the irregularity of the advancing evaporite complex in the upper Madison. Many marker horizons pinch out against the clinoform slopes or the prograding evaporites, leaving few regionally traceable markers below the Polar interval. The Madison has a high potential for multiple reservoir development and for multiple stratigraphic traps where pinch-outs and lateral gradations occur.
In contrast, log markers in the Bighorn Group extend regionally. The lithologies represented by the markers are also consistent regionally. Several discrete, nonlaterally intergrading events in the upper Bighorn are marked by sharp transitions upward from burrowed, mud-rich carbonate through laminated dolomudstone to anhydrite. The regional persistence of lithofacies, their relatively uniform thickness, and the long distance correlation of log markers indicates both long and short term depositional stability over nearly uniform bottom topography. Deposition took place in a very shallow sea that graded to a carbonate marsh or swamp environment over the entire Williston basin region. Reservoirs are developed at consistent stratigraphic horizons, and the possibility of stratigraphic trap is limited under these conditions.
Just as Madison starved-basin depositional styles in the Williston basin are consistent with Mississippian depositional styles in the Illinois basin and elsewhere in North America, Ordovician depositional styles, representing shallow stable conditions, extended across the Transcontinental arch into the Illinois basin and elsewhere. Contrasting styles of sedimentation and reservoir development in the same basin at different times require different exploration strategies. These contrasts must be considered when developing multiple objective programs.
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