Chapter 13

Stratigraphic Controls on the Development and Distribution of Fluid-Pressure Compartments

R.S. Martinsen

Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, U.S.A.

Studies in the Powder River Basin of Wyoming indicate that the boundaries of individual fluid pressure compartments commonly correspond to the boundaries of various stratigraphic elements (e.g., lithofacies and unconformities). This finding leads to the question: "What, then, is the difference between a stratigraphic trap and a fluid pressure compartment?" The fundamental difference is that conventional stratigraphic and structural traps have incomplete capillary seal closure, whereas fluid pressure compartments (of the type observed in the Powder River Basin) have complete capillary seal closure. Most structural traps and many stratigraphic traps are not completely bounded by low-permeability rocks and have discrete spillpoints. They cannot achieve complete capillary seal closure and are incapable of becoming pressure compartments. Even the many stratigraphic traps (and fewer structural traps) that are enclosed by low-permeability rocks do not all comprise pressure compartments. In order for a hydrocarbon reservoir to be enclosed by capillary seals, not only must it be enclosed by low-permeability rocks, it must be either completely filled (lack a hydrocarbon-free water contact) or enclosed by low-permeability rocks that contain multiple fluid phases. These conditions are more likely met in reservoirs completely enclosed within, or closely associated with, mature source rocks. The distribution and characteristics of fluid pressure compartments observed in the Powder River Basin of Wyoming are discussed within the framework of these types of stratigraphic variables.