The pore structural evolution of the Marcellus and Mahantango shales, Appalachian Basin

Abstract

Abstract 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 microscope (SEM). In this study, Middle Devonian black shale core samples were collected from three wells penetrating the organic-rich Marcellus Shale and the organic-lean Mahantango Formation in Pennsylvania and West Virginia. Pyrolysis, ion milled SEM and low-pressure nitrogen adsorption analysis were conducted to investigate the organic richness and the properties of the pore system. Vitrinite reflectance (Ro) values in the range of 1.36%–2.89% represent a maturity spectrum covering the wet-gas to post-mature zones. In general, the pore system is composed of organic matter-hosted pores and mineral-hosted pores. However, the dominant pore types and pore sizes vary stratigraphically across lithology and abundance of organic matter. All the organic matter observed in this study shows an amorphous occurrence. Pore space between mineral grains (both silt-size and clay-size) can be filled by organic matter, which contains secondary porosity generated by thermal cracking of kerogen. Mineral-hosted pores are concentrated in organic-lean samples in which secondary organic matter could not fill most of the primary pore space. The destruction of primary mineral-hosted pores and the generation of secondary organic matter-hosted pores were observed. TOC values show positive correlations with the porosity, specific surface area, and the abundance of micropores. Increasing thermal maturity correlates with a significant decrease of pore volume and surface area, primarily through diminishing or vanishing of micropores. The richness and thermal maturity of organic matter in organic-rich Devonian shale can be effective parameters for evaluation of reservoir quality and upscaling the appraisal.