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The Nature of Pore Structure in Middle Devonian Organic-Rich Black Shale From West Virginia and Pennsylvania

Abstract

Analyses of organic-rich mudstones from three wells that penetrated the Marcellus Shale in West Virginia and Pennsylvania were performed to evaluate the nature of pore structure (porosity and permeability) in both organic matter (OM) and inorganic matrix (IM). Samples include 32 core plugs with different levels of thermal maturity and TOC from Mahantango Formation and Marcellus Shale. For each sample, 10 to 15 SEM images were digitalized to quantify shape, size, and SEM-visible porosity. Also, X-ray Fluorescence (XRF) was conducted on every sample and converted to weight percent of quartz and feldspar, clay, and carbonate to study the heterogeneity and the influence of the inorganic matrix (IM) on pore structure and SEM-visible porosity. Pore types are categorized according to the shape and location. Inter-clay-particle (also referred to as phyllosilicate framework) pores and spongy pores are the most ubiquitous pore type in IM and OM respectively. Inter-clay-particle pores usually show triangular or elongate shapes. Their sizes are from tens to hundreds of nanometers. Spongy pores in OM are usually roundish, and show a relatively uniform size in a single OM particle. Their sizes range from ten (resolution-limitation) to several hundreds of nanometers. We defined OM degradation index (OMDI) to describe the extent of development of OM pores, which is porosity in organic matter divided by whole area OM covered and the voids in it. There is no systematic change in SEM-visible porosity as a function of the abundance of OM. However, there is a negative correlation between abundance of OM and OMDI. Samples with higher content of OM usually have large OM particles without pores, which can be explained by differences of maceral types. The ten nanometer-per-pixel resolution of the SEM could hinder recognition of smaller pores. The stratigraphic distribution of pore structure in the Mahantango, upper Marcellus, and lower Marcellus is strongly affected by heterogeneity of mineral composition. OMDI indicates a positive correlation between thermal maturity and development of OM pores in lower Marcellus, which hasn't been found in other formations. OM pores have been considered as a secondary pore type that formed during post-depositional process primarily affected by thermal maturity. However, the derivation and supply of different maceral types exert a significant control on development of porosity in organic-rich Marcellus Shale.