A MECHANISM FOR LIMITING DIFFUSION IN SEDIMENTARY BASINS

Geochemical studies of brines have suggested that ancient evaporatively-concentrated brines have been preserved in sedimentary basins for hundreds of millions of years, but hydrogeologic studies have emphasized the difficulty of preserving brines and large salinity gradients over long periods. Even diffusion, which is the slowest mechanism for solute transport, can drive significant solute loss over millions of years. The presence of ancient brines and large salinity gradients in modern basins suggests that diffusion is somehow limited. We suggest that abnormally-pressured beds can act as hydrodynamic barriers to diffusion over geologically significant periods if the abnormally-pressured bed is drained (that is, surrounded by normally-pressured beds). Analytic solutions based on sediment loading and unloading demonstrate that this effect should be considered in beds with a compressibility exceeding 10^-8 Pa^-1, with a thickness of 100 m or more, or a sedimentation rate exceeding 10^-5 m/yr.

Conditions favorable to the formation of drained abnormally-pressured beds appear common in sedimentary basins. Flow patterns associated with abnormally-pressured beds appear robust in the presence of heterogeneity and discontinuities, unlike membrane filtration, which has been called on previously to explain large salinity gradients across clay beds. Calculations suggest that thick underpressured shales in the Alberta Basin may currently limit diffusive transfer between deep brines and the overlying freshwater topography-driven flow system. In the Illinois Basin, drained overpressured beds may have limited solute transport across the New Albany Shale until approximately 250 Ma. It is unlikely, however, that overpressures could have persisted long enough to explain concentration gradients observed in the modern basin. These gradients may instead reflect relatively recent halite dissolution above the New Albany Shale.