Abstract
Potentiometric surfaces for Paleozoic strata, based on water well levels and selected drill-stem tests, reveal the control on hydraulic head exerted by outcrops in eastern Kansas and Oklahoma. From outcrop in the east, the westward climb of hydraulic head is much less than that of the land surface, with heads falling so far below land surface that the pressure:depth ratio in eastern Colorado is less than 5.7 kPa/m (0.25 psi/ft). Permian evaporites separate the Paleozoic hydrogeologic units from a Lower Cretaceous (Dakota Group) aquifer, and a highly saline brine plume pervading Paleozoic units in central Kansas and Oklahoma is attributed to dissolution of Permian halite. Underpressure also exists in the Lower Cretaceous hydrogeologic unit in the Denver Basin, which is hydrologically separate from the Paleozoic units. The data used to construct the seven potentiometric surfaces were also used to construct seven maps of pressure:depth ratio. These latter maps are a function of the differences among hydraulic head, land-surface elevation, and formation elevation. As a consequence, maps of pressure:depth ratio reflect the interplay of three topologies that evolved independently with time. As 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. Explorationists in western Kansas and eastern Colorado should not be surprised if a reservoir is underpressured; rather, they should be surprised if it is not.
Highlights
Oil and gas wells drilled beneath the high plains of western Kansas and eastern Colorado encounter pressures less than hydrostatic, with pressure:depth ratios less than a freshwater gradient of 9.8 kPa/m (0.433 psi/ft)
Four critical factors related to topographic, geologic, and hydrologic features are important for the creation and preservation of underpressure present in the Midcontinent: (1) eastern outcrops are exposed at a low elevation relative to the rest of the system (Figures 1, 3, 4, 16), (2) surface topography rises away from the down-gradient outcrop in the east (Figures 3, 16), (3) low permeability layers isolate the deep confined aquifer from the water table (Permian evaporites in Figures 8 and 9 and Pierre Shale in Figures 4 and 15), and (4) restricted recharge in the west is due to limited surface exposures and faulting
Outcrop control of potentiometric surfaces is shown by the maps of Figures 14 and 15, with water well and outcrop locations showing the general area in which each hydrogeologic group has equilibrated with atmospheric pressure
Summary
Oil and gas wells drilled beneath the high plains of western Kansas and eastern Colorado encounter pressures less than hydrostatic, with pressure:depth ratios less than a freshwater gradient of 9.8 kPa/m (0.433 psi/ft). This study maps the potentiometric surfaces and pressure:depth ratios for the seven hydrogeologic units depicted on the stratigraphic column of Figure 5: (1) Cambrian-Ordovician-Silurian, (2) Mississippian, (3) Desmoinesian, (4) Missourian, (5) Virgilian, (6) Permian (Wolfcampian), and (7) Lower Cretaceous (Dakota Group).
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