Fluid migration and regional pressure development in the Midcontinent of the United States
Our construction of potentiometric surfaces for Paleozoic and Cretaceous (Dakota Group) strata, based on drillstem tests and water well levels, reveals the pressure control exerted by outcropping strata. The Dakota Group is isolated from land surface by the overlying Pierre Shale, and Permian evaporites isolate the deeper Paleozoic aquifers. Gradually, halite beds in Permian evaporites dissolved, creating a highly saline plume of formation water that expanded downward because of its high density. As the plume expanded, mixing with fresher waters created a wide range of salinity values across the Midcontinent, especially in Kansas and Oklahoma. With erosion and outcrop exposure, each aquifer equilibrated with atmospheric pressure, causing an adjustment of hydraulic head from east to west, with each potentiometric surface responding over time to regional flow properties. Laramide thrusting, in combination with narrow outcroppings along the Front Range of Colorado, restricted recharge to the now-static confined aquifers, which are presently underpressured. As a state of underpressure developed, gas migrated in response to the changing pressure regime, most notably filling the Hugoton gas field in southwestern Kansas. The timing of underpressure development was determined by the timing of outcrop exposure and tilting of the Great Plains. Thus, the disposition of water and hydrocarbons in the Midcontinent has been driven by a combination of tectonics and erosion. An improved understanding of regional hydrology has significance for underpressured oil and gas reservoirs in the western part of the Midcontinent, for the potential sequestration of carbon dioxide, and for injection of waste water.
AAPG Datapages/Search and Discovery Article #90193 © 2014 Rocky Mountain Section AAPG Annual Meeting, Denver, Colorado, July 20-22, 2014