Chapter 14

Characteristics of Anomalously Pressured Cretaceous Shales in the Laramide Basins of Wyoming

Z.S. Jiao
R.C. Surdam

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

Most of the anomalously pressured Cretaceous shales in the Laramide basins of Wyoming (LBW) are overpressured and form basinwide, dynamic pressure compartments. The driving mechanism of the overpressuring is the generation and storage of liquid hydrocarbons that subsequently react to gas. This results in the conversion of the fluid-flow system from a single- phase to a multiphase regime in which capillarity controls the relative permeability, resulting in elevated displacement pressures within the shales.

The boundary between the normally pressured (i.e., pressure gradient ranging from 0.433 to 0.444 psi/ft, single-phase regime) and anomalously pressured (i.e., pressure gradient 0.433 psi/ft or 0.444 psi/ft, multiphase regime) Cretaceous shales is marked by significant differences in the geochemical and geophysical properties of the shales. The top boundary of the anomalously pressured zone is characterized by a marked decrease in sonic velocity and significant changes in the production index (PI), clay diagenesis (illite/smectite), vitrinite reflectance (R_o), nuclear magnetic resonance spectra (NMR), aromaticity, and displacement pressure of the Cretaceous shales. In this paper, we document and attempt to characterize the fundamentally different geophysical and geochemical properties of the anomalously pressured Cretaceous shales below the pressure boundary in the LBW.