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Chapter 21
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Formation Water
Chemistry of the Muddy Sandstone and Organic Geochemistry of the Mowry
Shale, Powder River Basin, Wyoming: Evidence for Mechanism of Pressure
Compartment FormationD. 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.
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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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