2014 Rocky Mountain Section AAPG Annual Meeting

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Comparison of potentiometric surfaces of the Cretaceous Dakota Group with surfaces of Paleozoic formations in the Midcontinent of the United States


Hydrocarbons are produced from oil and gas reservoirs in the Denver Basin and in the Anadarko Basin. Near the eastern flank of the respective basins, fresh water is produced from aquifers contained within the same stratigraphic units that produce hydrocarbons. Formation pressure measurements from the reservoirs and aquifers across the basins were obtained through drillstem tests in oil and gas wells and hydraulic head measurements in water wells. Combining these two sources of data, we have created a set of potentiometric surfaces for Mesozoic and Paleozoic stratigraphic units across the Midcontinent of the United States. The potentiometric surfaces of six Paleozoic hydrologic units in the Midcontinent exhibit the following characteristics: (1) contours are generally oriented north-south, indicating that the hydraulic gradient is generally from west to east across Kansas and northern Oklahoma; (2) recharge occurs in the Ozark Plateau of western Missouri; (3) a potentiometric low occurs in eastern Kansas and Oklahoma where the Great Plains aquifers mix with recharge from the Ozark Plateau; and (4) a steeper gradient (more tightly spaced contours) is located at the Kansas-Colorado border suggesting a reduction in permeability associated with the Las Animas uplift. The Mississippian potentiometric surface is much flatter than the other surfaces, and indicates that the regional permeability of this unit is substantially greater than other Paleozoic units. All Paleozoic units are underpressured in western Kansas and eastern Colorado, as indicated by the separation between land surface elevation and potentiometric elevations. The Cretaceous potentiometric surface indicates a zone of recharge in southeastern Colorado and northeastern New Mexico with a broad arc of discharge extending from central Kansas into eastern Nebraska and southwestern South Dakota. From the recharge area, flow in Colorado is northward into the underpressured Denver Basin, then eastward into Nebraska. Although recharge is present in the Denver Basin, the complexity of the flow network does not allow a static head to be maintained throughout the system. Thus, the potentiometric surface of the Cretaceous (Dakota Group) differs substantially from the Paleozoic formations in flow direction and discharge locations, indicating that the Cretaceous (Dakota) hydraulic system is isolated from the Paleozoic units. Isolation is attributed to Permian evaporite sequences.