Strategies for Trenton Exploration in the Illinois Basin

Leetaru, H. E. and A. L. Luther
Illinois State Geological Survey, Champaign, IL

The Ordovician age Trenton Limestone (Galena Group) has produced 13.5 million barrels of oil and accounts for 0.5 percent of the oil production within Illinois; however, this play is under explored and has a high potential for significant new oil reserves. Structure maps of the top of the Trenton Limestone show no distinctive trend for which to develop exploration leads. Trend surface analysis, a numerical mapping method of removing the effects of regional dip, does show a good relationship between oil production and third order residual high areas.

Structural mapping of oil fields within Illinois shows that shallow horizons do not necessarily reflect the deeper structure. The apex and trend of the anticlines may be displaced by as much as one mile when comparing deeper and shallower mapping horizons. The result of this structural displacement is that wells drilled to the Trenton based on shallow (Pennsylvanian and Mississippian) strata may not be located on the crest of the anticline. The presence of MVT (Mississippi Valley-Type) ore deposits in Trenton equivalent strata along the northwest and southwest flank of the Illinois Basin suggests that traps similar to the Albion-Scipio trend of Michigan may also be present in Illinois. Porosity and mineralization occur in both these areas because hydrothermal fluids migrating along faults have diagenetically altered the host strata.

Exploration strategies for the Trenton need to evaluate different paradigms such as the use of trend surface analysis for structural mapping, taking into account structural displacement when locating Trenton wells, and using geologic models based on equivalent MVT deposits along the flank of the basin.

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