ABSTRACT: Lopatin Analysis of Maturation and Petroleum Generation in the Illinois Basin

BYRNES, ALAN P., Loveland, CO, and ROBERT M. CLUFF, The Discovery Group Inc., Denver, CO

A modified Lopatin approach was used to evaluate the present-day maturity of Paleozoic source rock units across the Illinois basin, timing of hydrocarbon generation, regional porosity trends, and basin paleostructure. Ten cases were modeled at 98 locations to test assumed paleogeothermal gradients, post-Pennsylvanian overburden thickness, and rates of erosional stripping. Lopatin predicted maturities for the Herrin (#6) Coal and the New Albany Shale are in good agreement ((plus/minus) 0.02% Ro) with measured maturities 500-2,000 ft of post-Middle Pennsylvanian strata were deposited and later eroded between the Permian and mid-Cretaceous and if paleogeothermal gradients were within a few degrees C/km of present-day gradients.

Predicted mean reflectance levels are 1.0-4.0% Ro, at the base of the Potsdam Supergroup, 0.7-3.5% Ro at the base of the Knox Supergroup, and 0.6-1.3% Ro at the base of the Maquoketa Shale, excluding a small high maturity area in southeastern Illinois. The Knox and Potsdam section attained oil generation 475-300 Ma, while the Maquoketa and the younger New Albany Shale reached the oil window much later at 300-250 Ma. Because most of the significant structures in the Illinois basin formed post-300 Ma, any pre-Maquoketa source rocks were already within the gas zone and may have been largely spent by the time the known structures formed. Any Knox or deeper traps in the basin will probably be restricted to structures formed prior to 300 Ma or to stratigraphic traps and most likely will con ain hydrocarbons that were emplaced as liquids but have subsequently been cracked to gas.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)