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Characterization of the Pore System and its Storage Capacity in Devonian Black Shale of Appalachian Basin


Unlike porosity within the conventional rock matrix, the generation and primary migration of hydrocarbons in organic-rich shale leaves void space in organic matter, which is the porosity associated with organic matter commonly observed under scanning electron microscopy (SEM). In this study, Devonian black shale core samples were collected from four wells penetrating Mahantango and Marcellus shale units in Pennsylvania and West Virginia. Pyrolysis, organic petrology, XRD and low pressure N2 adsorption analysis were conducted to investigate the organic richness, thermal maturity, minerology and the properties of the pore system (including porosity, pore volume, specific surface area, and pore size distribution). The black shale samples have total organic carbon (TOC) values from 0.65% to 8.90% and vitrinite reflectance (Ro) values are in the range of 1.36% to 2.89%. SEM analysis indicates that the pore systems are made of both organic-matter pores and inorganic-matrix pores. Organic-matter pores are well developed and comprise the majority of the Marcellus Shale with pore sizes ranging from less than ten to hundreds of nanometers. Also, the pore micro-texture shows a stratigraphic distribution vertically controlled by the lithology and rock texture of individual thin beds. The total porosity estimated from BET test for the samples ranges from 2.44% to 10.94%. The BET specific surface area ranges from 4.10 m2 /g to 47.58 m2 /g and the micropore (pore width < 2nm) surface area estimated by the t-plot method ranges from 0 to 25.422 m2 /g. TOC values show positive correlations with the porosity, specific surface area, and micropore volume and surface area. Increasing thermal maturity correlates with a significant decrease of pore volume and surface area, primarily through diminishing or vanishing of micropores. The richness, depositional environment, and thermal maturity of organic matter in organic-rich Devonian shale can be effective parameters for evaluation of reservoir quality and upscaling the appraisal.