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Abstract


 
Chapter from: M 61: Basin Compartments and Seals 
Edited by 
Peter J. Ortoleva

Authors:
D. B. MacGowan, Zun Sheng Jiao, Ronald C. Surdam, and F. P. Miknis

Methodology and Concepts


 


Published 1994 as part of Memoir 61
Copyright © 1994 The American Association of Petroleum Geologists.  All Rights Reserved.
 

Chapter 21

*
Formation Water Chemistry of the Muddy Sandstone and Organic Geochemistry of the Mowry Shale, Powder River Basin, Wyoming: Evidence for Mechanism of Pressure Compartment Formation

D. B. MacGowan
State University of New York
College of Fredonia
Fredonia, New York, U.S.A.
Zun Sheng Jiao
Ronald C. Surdam
University of Wyoming
Laramie, Wyoming, U.S.A.
F. P. Miknis
Western Research Institute
Laramie, Wyoming, U.S.A.



*

ABSTRACT

In the Powder River basin, pressure compartmentation has been linked to the establishment of multiphase fluid-flow systems. The transition from a single-phase to a multiphase fluid-flow system is driven by liquid hydrocarbon generation and its subsequent reaction to gas. As a consequence, pressure compartments in this basin should be related to changes in formation water chemistry, thermal maturation of organics, clay diagenesis, and other geochemical reactions associated with progressive burial. To test this, measured and calculated pressure anomalies were studied in relation to changes in formation water chemistry, clay mineralogy, kerogen structure, carbon aromaticity, vitrinite reflectance, and organic-matter production indices. The results indicate that fundamental changes in formation water chemistry, rock inorganic geochemistry, and organic geochemistry occur between about 8000 and 10,000 ft (2400 and 3000 m) present-day burial depth, coincident with a major change in the formation pressure regime, the onset of abnormal pressure, in the Muddy Sandstone. The results also indicate that the onset of abnormal pressure is coincident with the generation, migration, and reaction to gas of liquid hydrocarbons. Thermal modeling, organic geochemistry, and 

 

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