Chapter 12

Anomalously Pressured Gas Compartments in Cretaceous Rocks of the Laramide Basins of Wyoming: A New Class of Hydrocarbon Accumulation

R.C. Surdam
Z.S. Jiao
H.P. Heasler

Institute for Energy Research, University of Wyoming Laramie, Wyoming, U.S.A.

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.