Porosity and Organic Content in the Tuscaloosa Marine Shale: Insights from Focused Ion Beam-Scanning Electron Microscopy Characterization

Abstract

This study utilizes a combination of 2D high-resolution microscopy images and 3D reconstructions to shed light on pore connectivity and the distribution of organic matter within three different lithotypes encountered in a well situated within the productive core area of the Tuscaloosa Marine Shale play. It distinguishes unique fabric patterns among a silt-rich mudstone, a calcite-rich mudstone, and a clayrich mudstone. The spatial distribution of minerals and organic materials within these three types of mudstones significantly impacts the properties of the pore network, revealing that interparticle porosity plays a pivotal role in sustaining oil flow, while the presence of kerogen can impede pore communication. The calcite-rich mudstone exhibits the highest organic material content at 5.30 vol %, with most of its pores filled with ductile kerogen. In addition, as organic matter matures toward the base of the shale formation, it gives rise to a sponge-like structure characterized by the presence of nanometer-sized pores. In contrast, the silt-rich mudstone sample features pores exceeding 100 nm in size and boasts a porosity of 3.43 vol %, while an increase in clay content within the other two mudstone samples results in a sheet-like pore structure with smaller pores. The analysis reveals two distinct pore populations, with the group exhibiting pore radii smaller than 20 nm being the predominant statistic. When examining pore connectivity in the z-direction, it becomes evident that oil flow within the Tuscaloosa Marine Shale relies on isolated submicrometer-sized pore clusters. However, for hydrocarbons to flow effectively into the wellbore, these clusters would need to establish connectivity with the matrix pore network. This study is significant as it is the first to explore the relationship between pore structures and organic matter networks across various lithotypes in the Tuscaloosa Marine Shale, utilizing 3D FIB-SEM characterization.