A diagenetic shale source for lithium in oilfield brines: Insights from the Devonian Bakken–Three Forks system, Williston Basin

Lithium (Li) concentrations in oilfield brines can exceed those expected from evaporative concentration of precursor seawater, yet the processes responsible for this enrichment remain poorly constrained. Brines from the Late Devonian Bakken Formation (Williston Basin, USA) have a median Li concentration of 50.1 mg/L, which is approximately 250 times greater than that of unmodified seawater (~0.2 mg/L). Integrated brine major- and trace-element geochemistry, shale lithogeochemistry, and mass-balance modelling demonstrate that lithium is sourced from the Bakken shales and released during burial. Strontium isotope ratios preclude a significant contribution from external fluids. Lithium concentrations in the Lower Bakken shales decrease from ~90 ppm along the basin margins to ~70 ppm in the thermally mature interior, corresponding to a loss of approximately 20% of the bulk lithium. Monte Carlo mass-balance calculations using formation thickness, porosity, water saturation and shale Li content, show that diagenetic lithium loss from the shales can reproduce Bakken brine Li concentrations, but cannot account Three Forks Formation brines, which requires additional sources. Similar geochemical relationships are also found in the Duvernay–Leduc system (Alberta) indicating that thin, Li-bearing shales can act as regional sources that charge neighboring reservoirs. This source–sink framework links shale diagenesis to lithium-rich basinal brines and improves the prediction of Li distribution in sedimentary basins.