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.