Langhorne B. Smith¹, Gregor P. Eberli², Mark D. Sonnenfeld³

(1) New York State Museum, Albany, NY
(2) University of Miami, Miami, FL
(3) Consultant, Morrison, CO

ABSTRACT: The Sequence Stratigraphic and Paleogeographic Distribution of Reservoir-Quality Dolomite, Madison Formation, Wyoming and Montana

Exploration-scale cross-sections of the Mississippian Madison Formation of Wyoming and Montana show that porous early dolomite is most commonly found in the transgressive portions of a hierarchy of sequences and cycles and that pervasive dolomitization of all rock types only occurs in the middle part of the ramp.

The Madison Formation is composed of a second order supersequence that consists internally of two composite sequences and numerous third-order sequences and higher frequency cycles. Most of the porous dolomite occurs within the transgressive portion of the second-order supersequence.

Downdip on the ramp, fabric selective dolomitization was 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, however, 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 within the transgressive portions of progressively higher frequency sequences and cycles. In the middle part of the ramp all rock types in the TST of the second-order supersequence were pervasively dolomitized and porosity occurs throughout the interval. Further updip, the amount of dolomite and porous dolomite decreases upward within the TST of the supersequence, and rock fabric played little role as 70% of the grainstones and only 50% of the mudstones were dolomitized.

Any models for dolomitization and porosity development should explain the abundance of porous dolomite in the transgressive portions of sequences and the pervasive dolomitization of the middle part of the ramp.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado