REGIONAL-SCALE FLOW OF FORMATION WATERS IN THE WILLISTON BASIN

The Williston basin is a structurally simple intracratonic sedimentary basin that straddles the United States-Canada border east of the Rock y Mountains and that contains an almost continuous stratigraphic recor d since the Middle Cambrian. Based on the wealth of data generated by the oil industry the regional-scale characteristics of the flow of for mation waters were analyzed for the Canadian side of the basin, and in tegrated with previous studies performed on the American side. Several aquifers and aquifer systems identified in the basin were separated b y intervening aquitards and aquicludes. The Basal, Devonian, and Mannv ille (Dakota) aquifers are open systems, being exposed at the land sur face in both recharge and discharge areas. Recharge takes place in the west-southwest at relatively high altitude in the Bighorn and Big Sno wy mountains and at the Black Hills and Central Montana uplifts, where as discharge takes place in the east and northeast at outcrop along th e Canadian Precambrian shield in Manitoba and the Dakotas. The Mississ ippian and Pennsylvanian aquifer systems are semi-open, cropping out o nly in the west-southwest where they recharge, but discharging in the northeast into adjacent aquifers through confining aquitards. The Lowe r Cretaceous Viking aquifer is a partially closed system, being confin ed 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 loc al 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 scal es, the entire system seems to be at steady-state, although locally tr ansient flow is present in places due to water use and hydrocarbon exp loitation, and to some erosional rebound in the uppermost confining sh ales. Fresh meteoric water is present in the western and shallower par ts 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 inf low from the Alberta basin is apparent along the northwestern margin o f the Williston basin, particularly for the Devonian and Viking aquife rs, Hydrocarbons generated in the deeper, thermally mature Dart of the Williston basin have generally migrated within the same units updip n orth and northeastward, their buoyancy-driven secondary migration bein g enhanced by the northeastward flow of formation waters. On the weste rn flank of the basin, the interplay between the northeastward structu ral downdip direction and the northeastward flow of formation waters c reates conditions favorable for hydrodynamic oil entrapment.