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Cretaceous shales in the Laramide basins of Wyoming (LBW) below ~8000-9000 ft (2440-2740 m) typically are anomalously pressured. In the basin centers, the top 1000-2000 ft (305-610 m) of the anomalously pressured zone is transitional and typically occurs within upper Cretaceous shales (Steele, Cody, or Lewis). The overpressured zone [~2000 ft (610 m) thick] persists down to the lowermost organic-rich Cretaceous shale. Typically, the rocks below these shales in the Powder River and Wind River basins are normally pressured. The top of the anomalously pressured zone is identified by marked increases in sonic transit time, hydrocarbon production index, clay diagenesis (smectite to illite), and vitrinite reflectance. Many of the affected organic-rich shales are characterized by bitumen-filled microfractures.

In the LBW, the major difference between pressure compartmentalization in Cretaceous sandstones and shales is one of scale. The overpressured Cretaceous shales in each of the basins comprise a basinwide, dynamic pressure compartment. In contrast, the Cretaceous sandstones within each basin are subdivided stratigraphically and diagenetically into relatively small, isolated pressure or fluid-flow compartments [largest dimension 1-10 mi (1.6-16 km)] within the shale section.

The driving mechanism of pressure compartmentalization in both the shales and sandstones is the generation and storage of liquid hydrocarbons that subsequently partially react to gas, converting the fluid-flow system from a singlephase regime to a multiphase regime in which capillarity controls permeability.

In a single-phase, water-dominated system, internal and external Stratigraphie elements (ranging from paleosols along unconformities to transgressive shales) act as low-permeability rocks with finite leak rates. These elements evolve diagenetically during progressive burial (smectite altering to illite; kaolinite to chlorite). As more liquid hydrocarbons are generated and the oil-to-gas reac-tion proceeds, the system becomes saturated with hydrocarbons. This results in the expulsion of free water and greatly increased displacement pressures, which cause the low-permeability elements acting as fluid-flow barriers to form capillary seals. Three-dimensional closure of the seals results in fluid compartmentalization and anomalous pressure and, thus, the formation of an anomalously pressured gas accumulation. In the sandstones, three- dimensional closure of capillary seals above, below, and within a sandstone results in isolated fluid-flow or pressure compartments within the sandstone. In a few cases, faulting bounds sandstone compartments by emplacing low-permeability carbonate- or quartz-cemented rocks adjacent to them.

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