--> Abstract: Pressure Compartment Seals: Evidence For Their Origin From Formation Water Chemistry of the Muddy Formation and Organic Geochemistry of the Mowry Shale, Powder River Basin, WY, by D. B. MacGowan, Z. S. Jiao, and R. C. Surdam; #91017 (1992).

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ABSTRACT: Pressure Compartment Seals: Evidence For Their Origin From Formation Water Chemistry of the Muddy Formation and Organic Geochemistry of the Mowry Shale, Powder River Basin, WY

MACGOWAN, DONALD B., ZUN SHENG JIAO, and RONALD C. SURDAM, University of Wyoming, Laramie, WY

In the Powder River basin, pressure compartmentalization is linked to the establishment of a multi-phase fluid flow system. The transition from a single-phase to a multi-phase fluid flow system is driven by liquid hydrocarbon generation and reaction to gas. As a consequence, measured and calculated pressure anomalies are related to changes in formation water chemistry, clay mineralogy, and organic geochemistry. Analyses of produced formation water chemistries from the Muddy Sandstone, as well as kerogen nuclear magnetic resonance, anhydrous pyrolysis, vitrinite reflectance, and x-ray diffraction of clays from the Mowry Shale indicate that a fundamental change in the rock-water system and the inorganic and organic geochemistry occurs between about 8000 to 10,000 ft of present-day buria depth; this coincides with a major change in the formation pressure regime (e.g., onset of abnormal pressure) in the Muddy Sandstone.

The data from this study indicate that the onset of abnormal pressure is coincident with the generation, migration, and reaction of liquid hydrocarbons. Thermal modeling, organic geochemistry, and pressure measurements suggest that abnormal pressures have existed in the Muddy and Mowry for geologically significant periods of time (i.e., >10 m.y.). Further, geochemical modeling suggests that rupture of boundary seals accompanied by a significant pressure drop will result in calcite precipitation and, consequently, seal restoration. These considerations differentiate the type of pressure anomalies seen in the Muddy Sandstone from those resulting from compaction or hydrodynamic disequilibria.

 

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