A seismic study of the northern Williston Basin
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Major stratigraphic and structural features of a section across the northern Williston Basin were imaged by using integrated seismic reflection data sets. The information for this investigation consists of about 20,000 field shots distributed along a 1,300 km profile. The field observations were donated by 22 oil companies. The seismic line spans the northern part of the Williston Basin from southwestern Manitoba through southern Saskatchewan to southeastern Alberta. Special wavelet processing and near-surface correction procedures were implemented because of data-dependent aberrations. A wavelet extraction method was designed through synthesis of numbers of spatially distributed sonic logs. A datum transformation technique, called "correlation datuming", was developed to resolve problems of near-surface velocity anomalies. The final regional seismic section was prepared by utilizing a reflection horizon near the top of the Colorado Group (Cretaceous) as a datum to help differentiate structural features. The introduction of this stratigraphic datum transferred the time delays of shallow structures to the surface and eliminated the deeper apparent structures such time delays cause. Through the recognition of a number of strata terminations, the analysis of the seismic stratigraphy of the regional sections allowed identification of 10 major sequences: Sauk, Tippecanoe, Devonian, Mississippian, Jurassic, Mannville, Lower Colorado, Upper Colorado, Montana, and Tejas. In addition, the Montana sequence was further divided into 7 sub-sequences named A to G. The seismic evidence suggests that the tectonic development of the northern Williston Basin advanced in three stages: a passive margin during Sauk time; a stable intracratonic basin from Tippecanoe to Upper Colorado time; and a phase of gentle tectonic deformation from Montana time to the Present. There are nine major subsidence and five major erosional events recognized from the analysis of seismic unconformities. All the major erosional cycles were consequences of periodic and incongruous uplift and tilting of the western and eastern flanks, relative to the central area of the basin. Based on the relations of the strata to unconformities and thickness changes, 15 distinct structures were identified on the regional seismic section. Most of these structures have not been correctly understood in the past. Some of them were not even reported in the earlier geological literature. For example, the present regional information indicates that the Regina Structure is an important component of the northern Williston Basin. In Sauk time, this structure was uplifted, and separated the basin-centre area into western and eastern portions, which later developed into two basinal centres. From Ordovician to Early Cretaceous, steps developed from one centre to another. Local thickness increases in Mississippian to Lower Cretaceous sequences are attributed to dissolutions of the Prairie Evaporite. In most of Montana time, the Regina Structure was a positive unit relative to the surrounding area. The overall tectonic movement of the Regina Structure was similar to that of the Nesson Anticline of North Dakota, which is a major oil producer in the Williston Basin. This suggests that the hydrocarbon potential of the Regina Structure may be far above its present recognition.