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The AAPG/Datapages Combined Publications Database
AAPG Special Volumes
Abstract
Mitch
Harris, and G. P. Eberli
2004. The American Association of Petroleum Geologists. All rights reserved.
Sequence-stratigraphic and Paleogeographic Distribution of Reservoir-quality Dolomite, Madison Formation, Wyoming and Montana
Langhorne B. Smith Jr.,1 Gregor P. Eberli,2 Mark Sonnenfeld3
1New York State Museum, Albany, New York, U.S.A.
2University of Miami, Miami, Florida, U.S.A.
3iReservoir.com, Greenwood Village, Colorado, U.S.A.
ACKNOWLEDGMENTS
We would like to thank Mike Pope, Jean Hsieh, and Bob Lindsay for the helpful and constructive reviews of this manuscript. We are most grateful to Burlington Resources and the Petroleum Research Fund (ACS PRF 35663-AC8) for their financial support of this research and to Marathon Oil Company for their generosity with access to core and log data. The first author would also like to thank both of his coauthors for their help and guidance in this project.
ABSTRACT
Exploration-scale cross
sections
of the Mississippian Madison Formation of Wyoming and Montana show variations in the distribution of dolomite and porous dolomite in a sequence-stratigraphic hierarchy along a 500-km dip transect. These variations in the distribution of dolomite across the ramp have significant implications for exploration and dolomitization models. This study also shows how understanding of rock and porosity types and vertical and lateral reservoir distribution and compartmentalization can be better understood through outcrop studies.
The Madison Formation is composed of a fivefold hierarchy of sequences and cycles. The entire Madison Formation comprises a single, unconformity-bounded second-order supersequence that consists internally of two composite sequences. The composite sequences are composed of two to four third-order sequences, with a total of as much as six sequences. The third-order sequences are composed of two orders of higher-frequency cycles. Most of the porous dolomite occurs in the transgressive portion of the second-order supersequence.
Downdip on the ramp, fabric-selective dolomitization is dominant, as more than 90% of the mud-dominated strata and less than 5% of the grain-dominated strata were dolomitized. The only porous dolomite occurs in laterally extensive mud-dominated strata in the transgressive portions of the two composite sequences. Moving updip, fabric-selective dolomitization is still common, but porosity occurs in the transgressive portions of progressively higher-frequency sequences and cycles. In the middle part of the ramp, all rock types in the transgressive systems tract (TST) of the second-order supersequence are pervasively dolomitized, and porosity occurs throughout the interval. Farther updip, the amount of dolomite and porous dolomite decreases upward in the TST of the supersequence, and rock fabric played only a small role, as 70% of the grainstones and only 50% of the mudstones were dolomitized.
Any models for dolomitization and porosity development should explain the variations in dolomite and porous dolomite distribution across the ramp. Exploration concepts for Madison dolomitized reservoirs and dolomitized reservoirs worldwide should include possible variations in dolomite distribution in sequences across carbonate ramps and platforms.
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