--> Abstract: The Regional Pressure Regime in Cretaceous Sandstones and Shales in the Powder River Basin, by R. C. Surdam and H. P. Heasler; #91012 (1992).

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ABSTRACT: The Regional Pressure Regime in Cretaceous Sandstones and Shales in the Powder River Basin

SURDAM, RONALD C., and HENRY P. HEASLER, University of Wyoming, Laramie, WY

Cretaceous shales in the Powder River basin below a depth of approximately 8000 ft +/- 2000 ft are typically overpressured. The top of the overpressurized zone is transitional 500-1000 ft thick and occurs within the Steele Formation; the overpressured zone (~2000 ft thick) begins in the Frontier Formation and persists down to the Fuson shale. Toward the basin margins, the overpressured shale section is wedge shaped with the top of the overpressurization approximately horizontal and the base parallel to the Fuson shale. The overpressured Cretaceous shale section in the Powder River basin is a basin-wide pressure compartment.

In contrast, the Cretaceous sandstones are subdivided into relatively small isolated pressure, or fluid flow compartments (i.e., largest dimension 1 to 10 mi). Sandstone compartmentalization is the result of internal stratigraphic elements (i.e., paleosols along unconformities). These internal stratigraphic elements are low-permeability rocks in a single phase flow system with finite leak rates, but evolve diagenetically into impermeable seals with discrete displacement pressures as the flow regime evolves into a multiple phase fluid flow system (i.e., by addition of hydrocarbons).

The major difference between pressure compartmentalization in Cretaceous sandstones and shales is one of scale. In both cases the appearance of hydrocarbons drives the transition from single phase (i.e., H2O) to multiple phase fluid flow; the hydrocarbons activate capillary seals and grossly increase displacement pressures in low permeability rocks. When hydrocarbons saturate the compartment the integrity of the three-dimensional bounding capillary seals is established.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)