PRELIMINARY MODFLOW SIMULATIONS OF UNDERPRESSURE IN GEOHYDROLOGIC UNITS IN THE GREAT PLAINS DURING LARAMIDE, AND LATE CENOZOIC EPEIROGENIC, UPLIFT AND TILTING ‒ PART1: THE SINGLE AQUIFER SCENARIO

A series of nine cross-sectional models was constructed using the U.S. Geological Survey’s MODFLOW groundwater-flow modeling program. These exploratory models have different geometries representing the progressive uplift and tilting of the Great Plains, associated with Rio Grande rifting. Each model is comprised of 10 layers that represent Cretaceous to Paleozoic geohydrologic units along a cross section from the Rocky Mountains to the eastern Kansas border. The objective is to understand present-day underpressure zones as they relate to possible carbon dioxide sequestration. Based on drill-stem tests, five of the geohydrologic units have regionally integrated pressures, with the degree of underpressure increasing westward and downward (Nelson et al, AAPG Bulletin 14169, in press). Eight geologic time intervals are modeled from 27 Ma to the present, with an additional interval from 70 to 27 Ma to define the starting hydraulic heads at 27 Ma. Uplift rates are slow to moderate for the period of 27 to 4.5 Ma, and increase for the period of 4.5 Ma to the present. The total uplift at the Rocky Mountain front is 1.6 km. The geologic history was one of nondeposition from 27 Ma to 18 Ma, then deposition of the Ogallala Formation as a vast alluvial apron across the Plains from 18 to 4.5 Ma, followed by erosion from 4.5 Ma to the present. A special case is presented of a single aquifer in the Wolfcampian (Wc) Permian unit with a hydraulic conductivity (K) of 10^˗3 meters/day (m/d) in the east and a lower K zone of 10^˗4 m/d in the west that represents the Las Animas Arch structural high. Aquitards above and below the Wc were assigned K values of 10^˗6 m/d. Water enters into the Wc from the Golden fault at the western boundary and exits at the eastern boundary which is open to the atmosphere. Initial heads are at sea level values at 70 Ma, when the Cretaceous Interior Seaway prevailed. For each of the nine geologic time intervals, the initial heads are the heads from the previous time interval plus the physical uplift. Preliminary model results show that the observed underpressure could have developed as a result of large-scale eastward tilting, discharge to the surface at the eastern boundary, and multiple transmissivity barriers that restrict recharge to the system from both the Golden Fault on the western boundary and from the surface.