Modeling Pinnacle Reefs and Their Associated Surfaces

David E. Hamilton, Skye Henize

Although computer mapping programs are becoming widely available, simple to use, and powerful, the blind application of these programs often produces poor results. By considering the environment of deposition, postdepositional events, and scale of map when computer mapping, a better result can be achieved. To demonstrate the importance of these issues, data from Antrim and Kalkaska Counties, Michigan, were used to model pinnacle reefs in the Northern trend of the Michigan basin. The data consisted of 640 wells containing tops, facies, and show information for six units: Niagaran Gray, Niagaran Brown, Salina A1 evaporite, Salina A1 carbonate, Salina A2 evaporite, and Saline A2 carbonate.

Building grids and contour maps of the Niagaran Brown (pinnacle) surface, using standard algorithms, produced a surface that either projected below or above the interreef surface. By using a mix of standard algorithms, the pinnacles and interreef surface were acceptably modeled. The Salina evaporites and carbonates are usually modeled using an isochore gridding approach. However, using this approach did not produce surfaces that reflected the geologist's interpretation. Simple modifications of the isochore approach allowed more accurate representation of the geologic interpretation. The evaporite surface models were built assuming they paralleled a paleowater surface. The carbonate surface models were built assuming they draped the surface existing at the time of deposition.

Large- and small-scale maps were built for these data. Facies and show information were also mapped. Combined maps of several variables were constructed and evaluated for potential pinnacle locations.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.