The maintenance of high salt concentrations in interstitial waters above the New Albany shale of the Illinois Basin

Abstract

Much controversy centers on whether groundwater flow velocities in basin‐scale circulation are large (tens of centimeters per year or more) or small (a few millimeters per year or less). Resolution of the controversy is particularly important in sedimentary basins in which salt beds are unknown but interstitial brines are pervasive. An example of one such basin is the Illinois Basin. The total dissolved solids (TDS) content of interstitial waters above the Upper Devonian New Albany Shale in the Illinois Basin increases with depth at an average rate of 15 wt% km−1. Mass transport calculations show that if no salt dissolved above the New Albany, upward diffusion alone should have lowered the salinity gradient over a 200 m.y. time span (i.e., since the Mid‐Triassic) to between 10 and 30% of the present value. The high salinity gradient found today requires either that halite dissolved slowly in rocks above the New Albany Shale over the past 200 m.y. or that the paleosalinity gradient, say, 200 m.y.B.P., was much greater than the value today. The flushing out of brines by meteoric water was simulated in a rectangular cross section of a generic basin 500 km wide and 1.5 km deep with a freshwater head gradient of 0.06% across the top. Simulations were run using the computer code PORFLOW (Runchal, 1987) which models coupled transport of fluid, heat, and dissolved salt. The simulations of brine flushing suggest that if topography‐driven flow occurred for time spans of several million years, then the preservation of brines in rocks above the New Albany shale could only have been possible if macroscopic flow rates (flow rates averaged over large rock volumes) were very small, on the order of 10 −3 m yr−1or less. When the horizontal permeability was 10−13 m2(10−1 darcy) and the vertical permeability 10−14 m2, brines that initially filled the basin were flushed out and replaced by fresh water in less than 5 m.y., despite the fact that the initial concentration gradient assumed was almost twice the present day value. For such high permeabilities, even very high salt dissolution rates equivalent to the removal of 4.7 m of halite column per million years were insufficient to maintain high concentrations. When the permeabilities were lower, 10−15 m2 along the horizontal and 10−16 m2 along the vertical, high dissolved salt concentrations were maintained for longer times. Even so, about half of the dissolved salt initially in the pore waters was removed after 50 m.y.