POROSITY OF DEVONIAN AND MISSISSIPPIAN NEW ALBANY SHALE ACROSS A MATURATION GRADIENT: INSIGHTS FROM ORGANIC PETROLOGY, GAS ADSORPTION, AND MERCURY INTRUSION

The evolution of porosity in shales with increasing maturity was examined in a suite of five New Albany Shale samples spanning a maturity range from immature (vitrinite reflectance R_o 0.35%) to post- mature (R_o 1.41%). Devonian to Early Mississippian New Albany Shale samples from the Illinois Basin used in this study contain marine Type II kerogen having total organic carbon contents from 1.2 to 13.0 wt. %. Organic petrology, CO₂ and N₂ low-pressure adsorption, and mercury intrusion capillary pressure techniques were used to quantify pore volumes, pore sizes, and pore size distribution.

For the suite of samples studied, increasing maturity is paralleled by large changes in the characteristics of porosity. Total porosity of 9.1 vol. % in the immature shale decreases to 1.5 vol. % in the late mature sample, whereas total pore volumes decrease from 0.0365 cm³/g to 0.0059 cm³/g in the same sequence. Reversing the trend at even higher maturity, the post-mature shale exhibits higher porosity and larger total pore volumes compared to the late mature sample. With increasing maturity, changes in total porosity and total pore volumes are accompanied by changes in pore size distributions and relative proportions of micropores, mesopores, and macropores. Porosity-related variances are related to differences in the amounts and character of the organic matter and mineralogical composition, but maturity appears to exert the dominant control upon the porosity characteristics. We conclude that organic matter transformation owing to hydrocarbon generation and migration out of the shale is a pivotal cause of the observed porosity differences.