REGIONAL HYDROGEOLOGY AND HYDROCHEMISTRY OF THE BAKKEN FORMATION IN THE WILLISTON BASIN

The Bakken Formation is currently the most productive oil-producing formation in the entire Williston Basin and one of the most productive conventional and unconventional oil plays in North America. Oil in the Bakken Formation has been shown to migrate from the central portion of the basin (currently in the oil window), outwards towards the less mature portion of the basin and north to Canada. A clear understanding of the migration pathways is crucial for the further development of this resource; however, the influences and implications of groundwater flow and hydrochemistry in the Bakken Formation remain unclear.

Pressure and chemistry data from both the Canadian and U.S. portions of the basin have been obtained from multiple sources. Data were compiled into a database and culled to remove data affected by nearby production, injection, and hydraulic fracturing operations. Potentiometric surface maps, water driving force maps, and water chemistry distribution maps have been generated from the remaining data. Results reveal a large central area with greater than hydrostatic formation pressures transitioning outwards towards distal areas of the basin where conditions return to near hydrostatic pressures. Water composition and salinity within the Bakken formation is variable throughout the Williston Basin with Total Dissolved Solids ranging from less than 10,000 mg/l, to over 300,000 mg/l towards the center of the basin. Formation waters are dominantly Na-Cl type however, Na-SO₄ type formation waters occupy the northwest of the study area near the transition into the Bakken/Exshaw Formation of the Alberta Basin. Brine origin as determined from Na-Cl-Br systematics preformed on Na-Cl type formation waters reveal that Bakken Formation brines are of multiple origins. Time-series well production (oil, water) profiles in the different pressure/hydrochemical areas appear to correlate to the local hydrogeological conditions.

The present distribution of formation waters in the Bakken Formation reflects a complicated mix of original connate brines mixing with salt-dissolution waters transported by a regional groundwater flow-system, and this appears to play a role in the observed well production profiles.