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Abstract
Chapter from:
AAPG Memoir 67: Seals, Traps, and the Petroleum System, Edited
by R. C. Surdam
(Publication Subject: Oil Methodology, Concepts)
AAPG Memoir 67: Seals, Traps, and the Petroleum System. Chapter 14: Characteristics of Anomalously Pressured Cretaceous Shales in the Laramide Basins of Wyoming, by Z.S. Jiao and R.C. Surdam, Pages 243-253
Copyright © 1997 by The American Association of Petroleum Geologists. All rights
reserved.
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.
ABSTRACT
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 (Ro), 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.
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