A "Petrofacies" Based Approach to the Characterization and Modeling of a Niagaran Pinnacle Reef, Ray Reef Field, Michigan Basin

Jill Haynie
University of Colorado Department of Geological Sciences Boulder, Colorado; [email protected]

This study establishes a set of petrophysically motivated facies (or "petrofacies") that relate rock fabric to petrophysical properties within a pinnacle reef reservoir of the Michigan Basin (Ray Reef), and examines their application in the creation of a geologically realistic three dimensional petrophysical model. Comparisons between petrofacies-conditioned and non-facies-conditioned models show that petrofacies conditioning yields a lower overall pore volume and a porosity distribution that more accurately reflects the stratigraphic framework of the reef.

Niagaran pinnacle reefs of the Michigan Basin consist of dolomitized coral-stromatoporoid buildups; rock fabric in these reefs is the product of depositional texture and subsequent alteration. The petrofacies introduced in this study are based primarily on pore type and size - components of rock fabric which should exert some control upon porosity and permeability development and distribution. As such, these facies are distinct from - though related to - depositional facies. These petrofacies, identified in core, show moderate correlation with petrophysical trends established from core data and wireline logs.

Although the Niagaran reef trend of the Michigan Basin has been extensively drilled, a detailed geologic framework was never established. This study introduces a stratigraphic framework for the Ray Reef reservoir based on log and core picks for bioherm, reef, and overlying anhydrite and restricted carbonate units.

Three dimensional models created using this framework investigate the merits of a petrofacies based approach to reservoir characterization. Results show that the use of petrofacies conditioning captures subtle yet important spatial trends in porosity and permeability distribution.

AAPG Search and Discovery Article #90083 © 2008 AAPG Foundation Grants in Aid