Compositional Control on Shale Pore Structure Characteristics across a Maturation Gradient: Insights from the Devonian New Albany Shale and Marcellus Shale in the Eastern United States

Abstract

The pore structure characteristics of shales are controlled by their mineralogical and organic matter (OM) compositions. However, the contributions by different components in shales at varying thermal maturities remain poorly understood. In this study, Devonian New Albany Shale and Marcellus Shale samples spanning a thermal maturity from marginally mature (vitrinite reflectance Ro 0.55%) to post-mature (Ro 2.41%) were selected to study the control of the composition on the pore structure properties of shales at different stages of thermal maturation. Scanning electron microscopy (SEM) imaging was used to examine the pore types in shales, and low-pressure N2 and CO2 adsorption analyses were used to quantitatively characterize the mesopore and micropore characteristics of bulk shales and major components in shales. The results show that matrix-associated pores including interparticle pores between silt-sized mineral grains, phyllosilicate framework pores, and intraparticle pores within mineral grains exist in all samples but become less common with increasing maturity, which is likely caused by the elevated compaction, cementation, and occlusion with bitumen. Secondary organic pores were not observed by SEM at marginal maturity but were detected in the condensate-wet gas and dry gas windows, with more organic pores in the dry gas window. At marginal maturity, OM has large amounts of mesopores and micropores, as demonstrated by low-pressure N2 and CO2 adsorption analyses of OM isolated from shales, even though no OM-hosted pores were observed by SEM. With increasing thermal maturity, the mesopore and micropore specific surface areas (SSAs) of OM increase and make greater contributions to the pore structure properties of bulk shales. The mesopore and micropore properties of shales are controlled by the OM content and maturity as well as by the clay mineral type and content, and they can be estimated from the contribution of each component at different stages of thermal maturation. Accurate evaluation of the pore volume and SSA of shales will have important implications for assessing gas adsorption and transport in shales.