Compartmentalization of Overpressured Reservoirs in the Anadarko Basin

Z. Al-Shaieb, J. Puckette, and A. Close
School of Geology, Oklahoma State University, Stillwater, OK

Basin-scale compartments are areas of abnormally pressured reservoirs that are isolated from surrounding normally pressured intervals. This unique configuration evolved by the sealing of abnormally pressured intervals and the maintenance of hydrostatic conditions in the surrounding rocks.

The sealing of the overpressured interval was controlled by distinct mechano-chemical processes that were coincident with basin subsidence. Within the Anadarko basin, these systematic processes resulted in the formation of seals that define the basin-scale compartment termed the mega-compartment complex (MCC). The top of the complex is relatively horizontal, cuts across stratigraphy, and is identified by the first occurrence of abnormally pressured reservoirs. The base of the complex conforms to stratigraphy and follows the Mississippian-DevonianWoodford Shale. Seal formation was directly related to non-equilibrium diagenetic processes that occurred during the rapid subsidence phase of the Pennsylvanian Orogenic episode. The silica cement phase was the major early diagenetic event that initiated a protoseal at depths around 6000 ft (1800 m) and a temperature of 60 degrees C. Therefore, the overpressuring of theMCC is directly related to generation of hydrocarbon.With continued burial and heating, fluids migrated toward the lower pressure gradient in the southern bounding fault zone. Precipitation in the vicinity of the fault generated a lateral vertical seal. Convergence of the top and basal boundaries resulted in the complete isolation of the complex.

Normal pressure below the basin-scale compartment was maintained by hydraulic continuity between surface outcrops and subsurface reservoirs. Abnormal pore pressure generated during burial by thermal expansion or other processes was dissipated throughout the reservoir and ultimately equalized with the outcrop-connected hydrostatic regime.

AAPG Search and Discovery Article #90903©2001 AAPG Mid-Continent Meeting, Amarillo, Texas