Pressure Compartmentalization in the Cretaceous Section of the Wind River Basin, Wyoming

Zun S. Jiao, Ronald C. Surdam, Hanqin Zhao

Sonic velocity and velocity-derived pressure profiles reveal that the Cretaceous shale section below a present-day depth of approximately 10,000 ± 1,000 ft in the Wind River Basin is a basinwide pressure compartment which can be subdivided into a transition zone (about 1,000 feet thick) and a hard overpressure zone. This shale section became a regional pressure compartment probably at or before the time of maximum burial (about 10 Ma). However, the Cretaceous sandstones within this overpressured compartment are subdivided stratigraphically and diagenetically into relatively small, isolated pressure or fluid-flow compartments. Reservoirs within the upper transition zone of this compartment are normally pressured or underpressured.

Rock properties change fundamentally at the onset of overpressuring. In shales, changes are indicated by rapid gradient increases of the sonic transit time, illitization, production index, transformation ratio, vitrinite reflectance, nuclear magnetic resonance properties, displacement pressure, and sealing capacity. Hydrocarbon generation, storage, and reaction (oil to gas) is the driving mechanism of overpressuring in both shale and sandstone, and converts the fluid-flow system from a single phase (water) to a multiphase (gas/oil/water) regime; the boundaries of the pressure compartment are capillary seals.

Following conversion of the fluid-flow system, the sealing capacity and displacement pressure of the hydrocarbon/pressure seal increase significantly with increasing burial depth and progressive clay diagenesis. The overpressuring caused by hydrocarbon generation and reaction can reduce the effective rock stress, and therefore the rate of compaction, resulting in a significant preservation of porosity and permeability within the overpressured compartments.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995