REGIONAL-SCALE GROUNDWATER FLOW AND BRINE MIGRATION IN THE MIDCONTINENT USA: COLORADO, KANSAS, AND MISSOURI

Brines are common in many sedimentary basins and play a role in a wide variety of geological processes including formation of ore deposits, petroleum migration, and chemical sedimentary diagenesis. The purpose of this study was to use brines as a natural tracer to investigate large-scale fluid flow and solute transport systems in the Midcontinent sedimentary basin east of the Rocky Mountains, U.S.A. We first mapped the modern distribution of brines along a cross-section of the basin. A two-dimensional finite element numerical model that solves the coupled partial differential equations governing variable-density fluid flow, heat transport, solute transport, and geochemical reactions was then used for simulations of groundwater flow along an 800 km cross-section that extends from northeastern Colorado through central Kansas into southwestern Missouri.

The new salinity profile showed high salinity in close proximity to Permian-age halite-dominated evaporite deposits. This distribution is consistent with previous geochemical findings that suggest salinity was derived from the subsurface dissolution of halite. In simulations, two large-scale flow systems were found to coexist at a quasi-steady-state after about 60 million years: topography-driven flow and buoyancy-driven flow. The development, size, strength, and hydraulic continuity of the flow systems was found to be strongly dependent on evaporite permeability. Dissolution of halite and transport of Cl- produced a concentration distribution that closely matches the mapped distribution of salinity in the basin. Results from this work suggest that thermohaline convection has the potential to drive geochemical exchange with the crystalline basement rocks and is responsible for dissolution observed along the eastern margin of the evaporites.