HYDRODYNAMICS OF FORMATION WATERS IN THE NORTH DAKOTA WILLISTON BASIN

ABSTRACT The hydrodynamic characteristics of formation waters in the petroleum-producing formations within the North Dakota Williston Basin were calculated from: 1) drill stem test results; 2) original field pressures for older fields; and 3) the chemical analyses of formation waters from DST samples or produced waters. Approximately 1100 estimates of reservoir pressure and 900 chemical analyses from all units were considered reliable. Pressure and chemical analyses were reported from strata ranging in age from Cretaceous to Cambrian. However, only four stratigraphic units had enough data to allow an estimate of their hydrodynamic characteristics: the Madison Group (Mississippian), Duperow Formation (Devonian), Interlake Formation (Silurian) and Red River Formation (Ordovician). Potentiometric surfaces were calculated for each of the four units. The Madison potentiometric surface shows steep slopes in the southwestern part of North Dakota, and has much lower slope and relief in the deeper part of the basin. Flow directions are north to north-northeast. For the Interlake and Red River formations, the potentiometric surfaces are roughly similar. They show steeper slopes and northeast flow directions in the southwest, but have such little relief in the centre of the basin that they appear to be nearly hydrostatic. The potentiometric surface of the Duperow Formation has no steep slopes and seems to be very close to hydrostatic. The surfaces calculated for all four units are at sufficiently different elevations to indicate hydrodynamic separation from one another. The modern flow pattern probably is no older than the Cretaceous. Prior to that time, brines were probably much more widespread in the four Palaeozoic formations, and conditions were hydrostatic. After the Cretaceous uplifts, and the beginning of water influx from recharge areas to the south and southwest, conditions became hydrodynamic, reducing the area occupied by brines. Madison oils were probably generated during the Late Cretaceous-Early Cenozoic in a largely hydrostatic setting. Oils in strata older than the Madison would have been generated somewhat earlier, also under hydrostatic conditions.