ABSTRACT: Stratigraphic/Diagenetic Sealing of Pressure Compartments: Muddy Sandstone, Powder River Basin, Wyoming and Montana

MARTINSEN, RANDI S., ZUN SHENG JIAO, and WILLIAM P. IVERSON, University of Wyoming, Laramie, WY

The Lower Cretaceous Muddy Sandstone (Viking Formation equivalent) is a thin, but complex, stratigraphic unit containing a variety of anomalously pressured compartments. One or more lowstand surfaces of subareal exposure and erosion (LSE), numerous transgressive surfaces of submarine erosion (TSE), and varying lithofacies compartmentalize the Muddy Sandstone on at least three scales. The first level of compartmentalization is defined by the relief along the LSE surface or surfaces, which, in places, exceeds the preserved thickness of underlying deposits and physically divides the Muddy, both vertically and laterally, into older and younger sequences. The second level is defined by the intersection of shales above the TSEs with the LSE (either by onlap or truncation), and the third lev l results from variations in lithofacies. Whereas many of the compartments comprise classic stratigraphic traps consisting of shale (seal) encompassing sandstone (reservoir/compartment), compartments exist wherein sand is juxtaposed against sand without benefit of intervening shales to serve as seals. In these situations, the seal consists of a paleosol developed beneath the LSE. Analyses indicate the paleosol typically has 5-15% porosity, less than 0.1 md permeability, pore throat radii less than 0.01 micrometers, and can withstand approximately a 2000 psi pressure differential, which is the approximate pressure differential observed within the compartment from which the data were obtained. This sealing capacity, which is similar to that observed in such classic seal types as shales and anhydrites, is the product both of early and shallow diagenesis during subareal exposure and late diagenesis during deep burial.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)