Lopatin Analysis of Maturation and Petroleum Generation in the Illinois Basin

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

A modified Lopatin approach was used to evaluate the present-day maturity of Paleozoic source rock units across the Illinois basin, timing of generation, regional porosity trends, and basin paleostructure during major generative events. Ten cases were modeled at 100 locations to test assumed paleogeothermal gradients, post-Pennsylvanian overburden thicknesses, and rates of erosional stripping. Lopatin predicted maturities for the Herrin (Number 6) Coal and the New Albany Shale are in good agreement (+/-0.02% Ro) with measured maturities if 500-3000 ft of post-Middle Pennsylvanian strata were deposited and subsequently 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 range from 1.0 to 4.0% Ro at the base of the Potsdam Megagroup, 0.7 to 3.5% at the base of the Knox Megagroup, and 0.6 to 1.3% at the base of the Maquoketa Shale, excluding only 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: 300-250 Ma. Because most significant structures in the basin formed after 300 Ma, any pre-Maquoketa source rocks were already within the gas zone and may have been largely spent by the time known structures formed. Any Knox or deeper traps in the basin will probably contain gas, be restricted to old structures (earlier than 300 Ma) or stratigraphic traps, and will hold pre-3 0 Ma generated hydrocarbons which subsequently cracked to gas.

AAPG Search and Discovery Article #91008©1991 AAPG Mid-Continent Section Meeting, Kansas Geological Society, Wichita Kansas, September 22-24, 1991 (2009)