Depositional Patterns in the Trenton and Black River Formations Revealed by Well Log Tomography and K-Bentonite Time Planes, Michigan Basin and Beyond

Wylie, Albert S., Jr. and James R. Wood
Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI

Well log tomography (WLT) is a new visualization technique that generates quasi-time slices through log curves. WLT was used to create a series of slices through the Michigan Basin using log curves from 400 wells penetrating the Trenton and Black River Formations. The gamma ray curve amplitude slices show delta-like patterns in the Black River, Trenton, and Collingwood Member (source rock) of the Trenton in the northern basin. The patterns indicate submarine deltas or debris flows were entering the basin from the north during the middle Ordovician while carbonate platform deposition was occurring to the south. These patterns clearly show that the Trenton bathymetric basin center was located in the northern portion of the present day Michigan Basin as suggested by Wilson and Sengupta (1985).

Ordovician K-bentonite layers are important time-lines that can be used as approximate chronostratigraphic surfaces to flatten the log curves for amplitude slicing. It was found that where one well-defined bentonite was used as a datum, WLT helped distinguish the presence and continuity of other less well-defined ash layers (based upon gamma ray amplitudes). In some cases, these other ash layers were encountered simultaneously in all wells indicating that rates of deposition and compaction were similar between the ash layers. Given sufficient well control, the absence of bentonites/ashes or a change in thickness between bentonites/ashes could be used as an exploration tool or indicator of proximity to potential dolomitized fracture trends that may have disrupted or weakened the normal gamma ray signal of the bentonites/ashes.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004