Abstract: Mississippian St. Louis Limestone: A New Source-Rock Play in the Illinois Basin?

Jennie L. Ridgley, Vito F. Nuccio

The carbonate-evaporite sequence, as much as 73 meters thick, in the lower part of the Mississippian St. Louis Limestone of the Illinois basin in south-central Indiana can be divided into as many as 14 fourth-order transgressive-regressive sedimentary cycles. Organic-rich strata within each of these cycles may be good to excellent hydrocarbon source rocks. The most organic-rich strata have laminated fabrics of primary or secondary origin. These strata were deposited both during peak transgression (maximum flooding) or in restricted lagoonal environments associated with gypsum/anhydrite precipitation when clastic input decreased and anoxic conditions were prevalent. The organic-rich intervals range from a few centimeters to as much as a meter in thickness; their cumulative thickness ma represent as much as 5-20 percent of the lower St. Louis.

Limited biomarker data from bitumen extract from cores of the lower St. Louis show a pristine/phytene ratio of less than one and a Tm/Ts ratio much greater than one; these observations are consistent with a carbonate source rock. Rock-Eval pyrolysis analysis indicate that selected samples in core from several parts of the basin contain from 0.03 to 15.72 percent total organic carbon. Plots of hydrogen versus oxygen indices indicate type III and mixed type II-III organic matter. The hydrogen indices are less than 300, which suggests that the rocks are gas/oil or gas prone. A number of samples have a genetic potential [(S1 + S2)/g of samples] greater than 2.0, production indices (PI) greater than 0.08, and T_max values up to 440°C, indicating good to excellent source-rock potential. The distribution of PI and T_max from this study indicate a decrease in thermal maturity from south to north in the basin. This is consistent with other regional thermal maturity indicators, such as R_o values in shales of the Devonian-Mississippian New Albany Shale and Pennsylvanian coals.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994