WESTPHAL, HILDEGARD¹, GREGOR P. EBERLI¹, and G. MICHAEL GRAMMER^2
1University of Miami, RSMAS, Miami, FL
²Texaco EPTD, Houston, TX.

Abstract: Reservoir Compartments as a Result of Facies-Sensitive Dolomitization - the Mississippian Madison Formation

Carbonate formations containing hydrocarbons pose unique problems to exploration and production for their complex variations in lithology and diagenetic history that result in heterogeneous reservoir properties. Optimizing field development in such an environment requires a level of reservoir characterization which adequately describes the flow networks both laterally and vertically. In the Mississippian Madison Formation reservoir quality and heterogeneity of flow units are the result of a combination of (1) stacking of reservoir facies in cyclic manner and (2) diagenetic overprint that largely follows the depositional units.

The Madison Formation in the Wind River area (Wyoming) consists of a 90 m thick succession composed of four stacked depositional sequences that are all cyclic. The diagenetic overprint on the depositional units is important for the reservoir formation on a large scale. The two basal sequences of the Madison Formation are dolomitized and show excellent reservoir quality, because most of the dolomite in the Madison Formation is porosity enhancing. The upper two sequences which are not dolomitized do not contribute considerably to the reservoir, despite the fact that they contain coarse-grained facies with potential reservoir facies.

The compartmentalization of the reservoir into smaller-scale units is also controlled by the high-frequency depositional cyclicity and the probably near contemporaneous dolomitization. The distribution of the dolomite is roughly confined to the individual cycles and their facies. In vertical dimension the compartmentalization is determined by the thickness of the individual cycles, and laterally be the extension of facies belts within each cycle. Dolomitization and porosity development is partly controlled by facies distribution. For example, dolomitization of the abundant grainstones results in intercrystalline porosity with high permeability, transforming a favorable reservoir facies into an even better one. Non- or vaguely dolomitized beds result in compartment boundaries within the larger reservoir unit. Cycles and fundamental flow units, however, might be connected to form larger flow units, where dolomitization is pervasive.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas