Hydrogeological Characterization of the Bakken Aquifer, Williston Basin (Canada-USA)

Abstract

The Bakken Formation is the most productive formation in the Williston Basin and one of the most important tight oil plays in North America. Oil in the Bakken Formation has been shown to migrate from the mature, central portion of the Williston Basin outwards towards the less mature portions of the basin, and north into Canada. A clear understanding of the hydrodynamics within the Bakken Formation is crucial for development; however, its regional hydrogeology, and the influences of regional groundwater flow on hydrocarbon migration in the Bakken Formation have been relatively poorly studied. A detailed hydrogeological and hydrochemical investigation of the Bakken Aquifer was conducted across the entire Williston Basin. Hydrochemical results show that within the Bakken Aquifer, salinity and water composition are variable. Total dissolved solids concentrations range from less than 10,000 mg/L to over 300,000 mg/L. Salinities are highest in the central portion of the basin and decrease radially outward. Formation waters in the Bakken Aquifer are dominantly Na-Cl type waters, however, some Na-SO4 waters are also present. Sodium-Chloride-Bromide systematics indicate that Bakken Aquifer brines originated from paleoseawater enrichment, halite dissolution, and the mixture between both. Results show a relation between waters with 250,000 mg/L TDS and the high resistivity anomaly found in the Bakken Formation. Hydrogeological results show a large closed potentiometric high located in the center of the Bakken Aquifer with hydraulic heads greater than 1,400 m. Hydraulic head values generally decreasing to less than 400 m in the northeast. Water driving force analysis reveals significant density dependent flow effects are present in northeast Montana and southeast Saskatchewan. Pressure depth analysis shows a large area with greater than hydrostatic formation pressures. Conditions return to near hydrostatic formation pressures outwards from the overpressured area. Production data has been combined with hydrogeological interpretations to identify hydrodynamic effects on hydrocarbon migration and accumulation. Production behaviours from Middle Bakken oil wells were overlaid on UVZ maps. Results show that areas predicted by UVZ analysis as being migration pathways or hydrodynamic traps had favorable hydrocarbon production behaviours compared to elsewhere in the formation. This study demonstrates that a complete understanding of the regional hydrogeology and hydrochemistry is imperative to determine the true flow direction of formation waters in deep saline aquifers. In addition, a sound understanding of the impacts of groundwater flow on hydrocarbon migration and accumulation can be used to further develop the economic potential of the Bakken Formation.