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ABSTRACT: Effects of Porosity, Permeability, and Facies Changes on the St. Louis and McCloskey Formations in the Illinois Basin

STANLEY, ROBBIE J., and MICHAEL A. KRUGE, Southern Illinois University, Carbondale, IL, and GARY L. SALMON, Illinois State Geological Survey, Champaign, IL

Organic geochemical data on oils from the Mississippian St. Louis and McCloskey reservoirs show a correlation between variations in oil composition and porosity, permeability, and facies changes, based on a conventional core from a well in Lawrence County, Illinois. Organic molecular markers in residual oils from segments of the core, as determined by gas chromatography-mass spectrometry, are utilized in observing changes in the chemical composition throughout the vertical section. Lithologically, the core exhibits a shoaling-upward sequence going from a skeletal wackestone, to a skeletal packstone, to a skeletal grainstone.Highest yield of extractable oil was at the base of the core in the wackestone, which was the zone perforated for production in this well, and also in the grainsto e in the upper portion of the core. Molecular weight distribution of n-alkanes varied as a function of lithology. Ratios of C[30] hopanes to the

C[31] n-alkane were low at the base of the core and increased toward to top.

Ratios of dimethylphenanthrene to the C[20] n-alkane exhibited the opposite trend. Results from this study could aid in enhanced recovery by detailed subdivision of the reservoir into levels of petroleum production potential, and in understanding how an oil can change in composition as it moves through the pore network of the reservoir rock.


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