Langhorne B. Smith1,
Gregor P. Eberli1,
(1) University of Miami, Miami, FL
(2) Universität Hannover, Hannover, Germany
Abstract: Sequence Stratigraphic Distribution of Reservoir-Quality Dolomite, Madison Formation (Mississippian), Wyoming and Montana
Integrated outcrop and subsurface data suggest that reservoir-quality dolomite is concentrated in the transgressive portion of a second-order super-sequence and the early and middle highstand systems tracts of third- and fourth-order sequences in the Mississippian Madison Formation of Wyoming and Montana.
The Madison is composed of a single unconformity-bounded second-order super-sequence which is composed of up to six third-order sequences (I-VI). The majority of the reservoir-quality dolomite occurs within the bottom three third-order sequences (I-III) which are in the transgressive portion of the second-order super-sequence.
With the exception of Sequence III, which is mud-dominated throughout, the sequences are mud-dominated in the TST, grainstone dominated in the early and middle HST and mud-dominated in the late HST. Muddy carbonates above and below all of the sequence boundaries are all dolomitized and typically have low porosity.
The highest porosity occurs in the lower and middle portions of the HSTs whether they are composed of dolomitized grainstone (Sequences I and II) or dolomitized mudstone and wackestone (Sequence III). This suggests that it is the position within the sequence rather than the rock type that controlled porosity development and preservation.
Early dolomitization in the Madison Formation probably occurred during second, third- and fourth-order transgressions. Rising sea level likely provided the pump for reflux dolomitization of underlying carbonates. Low-porosity strata above and below the third- and fourth-order sequence boundaries are "over-dolomitized" because they had a greater volume of seawater pass through them than the high-porosity strata in the lower and middle highstand systems tracts. The sequence stratigraphic distribution of reservoir-quality dolomite in the Madison may serve as a model for dolomitized reservoirs world-wide.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana