Three Levels of Compartmentation within the Overpressured Interval of the Anadarko Basin
Zuhair Al-Shaieb, James O. Puckette, Azhari A. Abdalla, Patrick B. Ely, 1994. "Three Levels of Compartmentation within the Overpressured Interval of the Anadarko Basin", Basin Compartments and Seals, Peter J. Ortoleva
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Deep basin pressure compartments can be classified on the basis of their size, stratigraphy, and pressure regimes. Detailed investigations of the geologic setting and pressure gradients of numerous reservoirs in the Anadarko basin reveal the presence of three distinct levels of compartmentation.
Level 1 is a basinwide feature known as the megacompartment complex (MCC). This complex is an overpressured volume of rocks that is completely enclosed by seals. It is approximately 241 km (150 mi) long, 113 km (70 mi) wide, and has a maximum thickness in excess of 4877 m (16,000 ft). Gas reserves of the overpressured reservoirs within the MCC are speculated to be approximately 20 tcf. The other two compartmentation levels are further subdivisions of the internal volume of the MCC. Level 2 compartmentation consists of multiple, district-, or field-sized configurations within a particular stratigraphic interval. These compartments are 32 to 49 km (20 to 30 mi) long, 19 to 32 km (12 to 20 mi) wide, and 122 to 183 m (400 to 600 ft) thick. Their reserve estimates can exceed 2 tcf. Examples of this type are the upper Morrowan Chert Conglomerate reservoirs in the Cheyenne/Reydon field area. Level 3 consists of a single, small field or a particular reservoir nested within Level 2. These compartments are generally 3 to 6 km (2 to 4 mi) long, <1.6 to 5 km (<1 to 3 mi) wide, and 3 to 30 m (10 to 100 ft) thick. Reserve estimates range from <1 to several hundred bcf. Individual channel-fill reservoirs of the “Pierce” chert conglomerate represent adequate examples of Level 3 compartments.
The hierarchical classification of compartments has important implications for petroleum exploration and development. Recognizing the upper and lower boundaries of the MCC is essential to safe drilling practices. The similar pressures within Level 2 compartments allow the prediction of approximate reservoir pressures across trend-size areas of the basin (tens to hundreds of square kilometers). Delineating Level 3 compartments improves reservoir boundary prediction and exploration and development strategies.
The integration of tectonic history, stratigraphic relationships, facies distribution, thermal history, and diagenetic patterns of seal zones suggests that the three levels of compartmentation evolved during the Pennsylvanian orogenic episode. This occurred during the rapid subsidence phase of the orogeny over a period of approximately 30 million years.
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Basins worldwide exhibit an unexpected degree of hydrologic segregation. There can be regions of a sedimentary basin that are isolated from their surroundings by a relatively thin envelope of low-permeability rock with an interior of sufficiently high permeability to maintain a consistent internal hydrostatic fluid pressure gradient. These have been named pressure compartments. Presure compartments have several remarkable features, just one of which is that internal fluid pressures can greatly exceed or be significantly less than any regional topographically controlled hydrologic head or drain. This publication contains 30 chapters that take detailed looks at pressure compartments in general, and detail case studies of these compartments in specific basins, such as the Anadarko and Gulf of Mexico. The volume also looks at other considerations in sedimentary basins such as hydrodynamic and thermal characteristics, and mechanical properties of rock.