Abstract: Thermal Maturation, Diagenesis, and Abnormal Pressure in Cretaceous Shales in Laramide Basins of Wyoming

Ronald C. Surdam, Zun S. Jiao, Jie Liu, Han Q. Zhao

The spatial configuration of the pressure regimes of four Laramide basins (Powder River, Bighorn, Wind River, and Washakie) have been delineated by sonic velocity and velocity-derived pressure profiles. These profiles reveal that the Cretaceous shale section below a present-day depth of approximately 9,000 ft in these Laramide basins typically is anomalously pressured. The anomalously pressured section occurs on a basin-wide scale and consists of a transition zone approximately 2,000 ft thick overlying a hard overpressured zone more than 3,000 ft thick. Typically, in the basin center, both the top and bottom of the anomalously pressured compartment are parallel to stratigraphic boundaries. Toward the basin margins, the compartment is wedge shaped as the boundaries cut across stratigra hic units.

The basin-wide overpressuring of the Cretaceous shales can be closely correlated with thermal maturation levels, clay diagenesis, and petrophysical characteristics of the shales. The top of the anomalously pressured zone is noted by marked increases in sonic transit time, degree of illitization, carbon aromaticity, vitrinite reflectance, displacement pressure, and sealing capacity.

The anomalously pressured Cretaceous shale sections in these Laramide basins are a basin-wide, dynamic pressure compartments. The driving mechanism of overpressure in the shales is the generation and storage of liquid hydrocarbons that subsequently react to gas, so that the fluid-flow system is converted from a single phase (water) to a multiphase (gas/oil/water) regime, with the boundaries of the abnormally pressured compartment being capillary seals. These multiphase, gas saturated, anomalously pressured compartments in the Cretaceous shale section are generally overlain and underlain by single-phase fluid-flow systems.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994