BAKKEN FORMATION WATER SALINITY AND THE ROLE OF ION FILTRATION, WILLISTON BASIN

Post-depositional ion filtration may have been an important process in producing highly saline pore water in the Bakken Formation. Upper and lower members of the Bakken are organic-rich black shales that were deposited in deep, anoxic environments. The middle unit is mostly a fine-grained, commonly dolomitic, siltstone deposited in a shallower environment. Water from wells producing oil out of the Bakken Formation is about an order of magnitude more saline (27 to 34 percent) than modern sea water (3.5 percent). Concentrations of sodium, potassium, chloride, and bromide, respectively, are 8, 17, 10, and 14 times larger than sea water values. Calcium and strontium are enriched by a factor of 40 and 200, but magnesium was depleted probably by diagenetic precipitation of dolomite in the middle siltstone. Reduction of sulfate by a factor of 10 led to the precipitation of pyrite in shales. Evaporation of sea water in tidal flats and dissolution of evaporates are commonly proposed mechanisms to explain saline water in deep formations. The latter is the prevailing explanation of saline waters in carbonate and some clastic reservoirs in the Williston Basin, but neither process adequately explains the salinity and element ratios of Bakken Formation water. More likely, pore water was trapped in the middle Bakken by the low permeability upper and lower shales. Overpressures developed during compaction forced some the pore water through the tight shales which functioned as ion filters. Solutes were concentrated in the residual pore fluid remaining in the middle unit of the Bakken Formation.