Variable-density flow in the midcontinent basins and arches region of the United States

Basins in the midcontinent basins and arches region of the United States are typical of many sedimentary basins containing a diverse assemblage of lithologies that have been deformed at various stages in their geologic history and now transmit fluids of varying density. On the basis of the synthesis of geologic data from more than 500 wells, specific-gravity data from more than 1800 samples, fluid-pressure data from nearly 40 deep wells, and hydraulic-conductivity data from core analyses of more than 35 wells, the midcontinent basins and arches region is conceptualized as a layered, heterogeneous, hydrologically mature region containing 11 hydrostratigraphic units that have undergone various periods of deformation and comprise a sequence of aquifers and confining layers with fluid-density variations ranging from freshwater to brine. An interpretative, three-dimensional, steady state, variable-density, finite difference flow model was constructed to organize the field data and to evaluate various controls on regional flow patterns. Simulation results indicate that flow directions in the shallow hydrostratigraphic units are controlled by local surface elevations. The deeper flow systems, however, show the influence of regional structural features, such as the Cincinnati Arch, that control the locations of regional groundwater divides. The amount of downward cross-formational flow into the older hydrostratigraphic units is greatest along the area of the Cincinnati and Findlay Arches where the Mt. Simon Sandstone is relatively shallow and fracturing and faulting are more common. Spatial variations in fluid density are a significant influence on vertical cross-formational flow in regions where salt-bearing units are present.