Relationships between Organic Structure Carbonization and Organic Pore Destruction at the Overmatured Stage: Implications for the Fate of Organic Pores in Marine Shales

Abstract

To reveal the relationships between kerogen structure variations and the evolution of organic pore (OP) characteristics in overmatured shales, we conducted total organic carbon (TOC) analysis, Raman spectroscopy, Fourier-transform infrared spectroscopy, gas (N2 and CO2) adsorption tests, and field-emission scanning electron microscopy on a high-maturity Silurian shale from South China, alongside its solid residues (kerogen) produced from semi-closed pyrolysis. The results showed that the TOC value of solid residues increased significantly during heating, and the chemical structure of organic matter (OM) in kerogen samples abruptly changed at 500 °C (3.49% R mc), which is interpreted as recording intense carbonization of OM. These variations in the OM structure lead to reductions in the pore volume and specific surface area of bulk-rock and kerogen samples at 450–500 °C. Oval, elliptical, and spongy OPs gradually became interconnected and coalesced into organic microcracks. The diameter of newly formed microcracks increased with the degree of graphitization, exceeding 200 nm at temperatures above 500 °C. During this process, the rock matrix changed from being relatively compact to being relatively loose. Cambrian shale located in South China has irregular and arcuate OP boundaries under normal geological conditions, and the high thermal maturity of this shale may indicate that the coupling of OM carbonization and mechanical compaction is responsible for its poor development of OPs. Here, we present a schematic model for pore evolution in type-I/II marine shales that encompasses the entire hydrocarbon generation stage, which is based on semi-closed pyrolysis analyses. The model is examined using the collected and compilated data for many type-I/II marine shales around the world. These data show that naturally matured marine shales acquire a maximum porosity at 2.5% < R o < 3.0%. This study provides an important guide for marine shale gas exploration and development in China.

Scale-Dependent Fractal Properties and Geological Factors for the Pore Structure in Shale: Insights from Field Emission Scanning Electron Microscopy and Fluid Intrusion

“…Shale is composed of various compositions such as OM, brittle minerals, and clay minerals, all of which closely affect pore structure at each scale. ,,,, As shown in Figure a, with the increase in the TOC content, the total PVs of shale samples initially show an increasing trend, followed by a slight decline (Figure a). Similar findings were reported by Milliken et al in the Marcellus shale samples, and they concluded that shale porosity does not consistently increase with rising TOC content; instead, it may decrease at high TOC levels.…”