Characterization and origin of brines from the Bakken-Three Forks petroleum system in the Williston Basin, USA

Brine (also referred to as ‘produced water’) samples were collected from 28 wells producing oil from the Late
Devonian-Early Mississippian Bakken and Three Forks Formations in the Williston Basin of eastern Montana and
western North Dakota. The samples were analyzed for major ions, trace metals, stable isotopes, and strontium isotopes.
The brines in these formations are highly saline with total dissolved solids averaging 308 g/L, almost ten
times the salinity of modern seawater. Relative to modern seawater, the brines are enriched approximately 10 to 20
times in [Na], [K], [Cl], and [Br]. Greater enrichments of 100 to 400 times in [Li], [B], [Sr] and [Rb], and 2,000
to 10,000 times in [Cs] and [Ba] are probably due to water-rock interaction (WRI). WRI is further indicated by
87Sr/86Sr values typically between 0.710 and 0.711—considerably larger than marine values of 0.7081 to 0.7083
during this depositional interval. Bakken Formation sediments were deposited in a stratified water column with salinity
increasing with depth. The deeper water may have been saturated in calcium carbonate and possibly gypsum,
but there is no evidence that halite saturation had been attained. Therefore, brines may have been introduced into
the Bakken Formation from the underlying Devonian Prairie Formation or from the overlying Charles Formation
before these brines were diluted or replaced by meteoric water. Alternatively, salinity of the native pore water was
increased by membrane filtration driven by overpressuring within the Bakken Formation.