VARIABLE-DENSITY FLOW IN THE MIDCONTINENT BASINS AND ARCHES REGION OFTHE UNITED-STATES

Basins in the midcontinent basins and arches region of the United Stat es are typical of many sedimentary basins containing a diverse assembl age of lithologies that have been deformed at various stages in their geologic history and now transmit fluids of varying density. On the ba sis of the synthesis of geologic data from more than 500 wells, specif ic-gravity data from more than 1800 samples, fluid-pressure data from nearly 40 deep wells, and hydraulic-conductivity data from core analys es of more than 35 wells, the midcontinent basins and arches region is conceptualized as a layered, heterogeneous, hydrologically mature reg ion containing 11 hydrostratigraphic units that have undergone various periods of deformation and comprise a sequence of aquifers and confin ing layers with fluid-density variations ranging from freshwater to br ine. An interpretative, three-dimensional, steady state, variable-dens ity, finite difference flow model was constructed to organize the fiel d data and to evaluate various controls on regional flow patterns. Sim ulation results indicate that flow directions in the shallow hydrostra tigraphic units are controlled by local surface elevations. The deeper flow systems, however, show the influence of regional structural feat ures, such as the Cincinnati Arch, that control the locations of regio nal 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 re latively shallow and fracturing and faulting are more common. Spatial variations in fluid density are a significant influence on vertical cr oss-formational flow in regions where salt-bearing units are present.