ABSTRACT
The Williston basin is a structurally simple intracratonic sedimentary basin that straddles the United States-Canada border east of the Rocky Mountains and that contains an almost continuous stratigraphic record since the Middle Cambrian. Based on the wealth of data generated by the oil industry, the regional-scale characteristics of the flow of formation waters were analyzed for the Canadian side of the basin, and integrated with previous studies performed on the American side.
Several aquifers and aquifer systems identified in the basin were separated by intervening aquitards and aquicludes. The Basal, Devonian, and Mannville (Dakota) aquifers are open systems, being exposed at the land surface in both recharge and discharge areas. Recharge takes place in the west-southwest at relatively high altitude in the Bighorn and Big Snowy mountains and at the Black Hills and Central Montana uplifts, whereas discharge takes place in the east and northeast at outcrop along the Canadian Precambrian shield in Manitoba and the Dakotas. The Mississippian and Pennsylvanian aquifer systems are semi-open, cropping out only in the west-southwest where they recharge, but discharging in the northeast into adjacent aquifers through confining aquitards. The Lower Cretaceous Viking aquifer is a partially closed system, being confined by Cretaceous aquitards except for a small recharge area exposed at the Black Hills uplift and narrow discharge area in the Dakotas. The Upper aquifer is unconfined, with groundwater flow being driven by local topography, whereas the flow in all the other aquifers is regional in nature, being driven by basin-scale topography and characterized by normal hydraulic heads. The intervening aquitards seem to be strong, allowing little cross-formational flow. On regional and geological scales, the entire system seems to be at steady-state, although locally transient flow is present in places due to water use and hydrocarbon exploitation, and to some erosional rebound in the uppermost confining shales.
Fresh meteoric water is present in the western and shallower parts of the basin, and brines are found in the Paleozoic aquifers in the central and eastern parts of the basin. This shows that the basin has not been completely flushed by meteoric water. Some cross-basinal inflow from the Alberta basin is apparent along the northwestern margin of the Williston basin, particularly for the Devonian and Viking aquifers. Hydrocarbons generated in the deeper, thermally mature part of the Williston basin have generally migrated within the same units updip north and northeastward, their buoyancy-driven secondary migration being enhanced by the northeastward flow of formation waters. On the western flank of the basin, the interplay between the northeastward structural downdip direction and the northeastward flow of formation waters creates conditions favorable for hydrodynamic oil entrapment.
