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 R o 0.55%) to post-mature (R o 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.

Accessibility of Pores to Methane in New Albany Shale Samples of Varying Maturity Determined Using SANS and USANS

“…The fractal dimensions do not seem to be related to the TOC values of the samples either (Table 5). This is consistent with previous observations that the organic matter porosity is concentrated in the nanometer size region [19,48], meaning it should not affect the geometry of the pore-matrix interface at larger scales.…”

“…organic matter because of organic matter transformation and subsequent generati hydrocarbons. Specifically, in the marginally mature sample 1 (Ro 0.50%), even th there is a lot of organic matter (TOC 14.4%), this OM contains few pores [48,49] an accessible porosity. At the mid-mature stage (sample 2, Ro 0.70%), there was alr massive oil generation by amorphous organic matter followed by the migration o This generation of oil resulted in the formation of pores whose connectivity allowe migration from the New Albany Shale.…”

“…We suggest that the key to understanding differences in the accessible porosity is the evolution of pores in organic matter because of organic matter transformation and subsequent generation of hydrocarbons. Specifically, in the marginally mature sample 1 (R o 0.50%), even though there is a lot of organic matter (TOC 14.4%), this OM contains few pores [48,49] and no accessible porosity. At the mid-mature stage (sample 2, R o 0.70%), there was already massive oil generation by amorphous organic matter followed by the migration of oil.…”

Controlling Factor Analysis of Microstructural Property and Storage Capacity of Deep Longmaxi Formation Shale in Sichuan Basin

“…Shale gas is stored in and yields from the micro-nano scale pore network of shale, which is of high complexity and high heterogeneity. The storage and microstructural properties of the pore system in shale are crucial to the estimation of geological reserves and the gas exploitation process. , Numerous studies have been done on the storage and structural properties’ characterization for shallow shale, , and the shale composition is found to be important to the storage capacity (e.g., porosity and gas adsorption capacity). − As compared to the shallow shale, deep shale has undergone more intense compaction and thermal evolution, which are crucial to the pore evolution and fluid storage property. − However, the studies on storage property and its controlling factor analysis of deep shale are rarely carried out because the breakthrough of deep shale gas development in the Luzhou Block was achieved only two years ago (in 2019). , Therefore, quantitative characterization of the fluid storage and structural properties of deep shale and further clarifying their controlling factors are essential to the development of deep shale gas reservoirs in Sichuan Basin as the next step.…”

Pore Properties in Organic-Rich Shales Derived Using Multiple Fractal Determination Models Applied to Two Indian Permian Basins

“…To sustain the global energy supply chain, research in unconventional shale-reservoir characterization has drawn much attention. − A shale petroleum reservoir represents a self-contained source and store of gaseous and/or liquid petroleum. Significant quantities of petroleum products are preserved in organic-rich shales in their pore networks, intergranular spaces, adsorbed onto the surfaces of the organic matter and clay mineral surfaces, and dissolved in the kerogen and bitumen fractions. − The topology of the pores and their structural networks within such shales, especially their kerogens, tend to be complex and variable. These characteristics warrant thorough investigation because of their variety and their direct influence on petroleum resource storage and recovery.…”