AN OVERPRESSURED DEEP BASIN AND AN UNDERPRESSURED HINTERLAND: THE GREATER ANADARKO BASIN OF OKLAHOMA, TEXAS, AND KANSAS

The greater Anadarko basin exhibits two disparate hydrologic regimes in close proximity, as defined by pressure measurements from drillstem tests in numerous oil and gas wells. An overpressured (pressures greater than hydrostatic) deep basin in rocks of Mississippian and Pennsylvanian age is bounded on the north and west by strata with pressures that are less than hydrostatic. The pressures from drillstem tests are used to construct potentiometric surfaces providing map views of the two regimes. From east to west, hydraulic head increases by several hundred feet in each rock formation, but elevation increases by thousands of feet. The low hydraulic gradient is attributed to good connectivity to the discharge point and restricted recharge. The apparent underpressuring phenomenon, which also increases from east to west, is a consequence of the vertical separation between surface elevation and hydraulic head.

Overpressuring in the deep basin has persisted since Permian time, but underpressuring in the hinterland developed relatively recently as a response to downcutting and exposure of Pennsylvanian strata in central Oklahoma. The discharge locus lies along and east of the north-south trending Nemaha fault zone, which lies at a lower elevation than the underpressured hinterland to the west. West of the Nemaha fault zone, the entire system is isolated from surface by a 1,000-ft thick cap of Permian evaporites and shales. Hydrologic modeling is required to better understand the system and explore the possibilities for carbon sequestration.