Evolution of kerogen structure during the carbonization stage

Currently, certain unconventional gas is mainly extracted from high-maturity shale reservoirs, but limited attention has been given to the evolution of kerogen structure subsequent to the overmature, gas generative stage, corresponding to metagenesis and is termed the carbonization stage. In this study, we performed heating treatment on overmature kerogen samples to obtain samples in the carbonization stage and then conducted Raman, thermogravimetric-mass spectrometric analysis (TG-MS), X ray diffraction (XRD), and high resolution transmission electron microscope (HRTEM) experiments on these samples to investigate the changes in kerogen structure. Low pressure N₂ and CO₂ adsorption experiments also were performed to investigate the changes in pore structure. The results show that as the heat treatment temperature is raised to 1000 °C, the I_D/I_G ratio (D band intensity to G band intensity) experiences an increase, reaching 1.12. Additionally, the full width at half maximum (FWHM) of 002 peak consistently remains above 3.8°. This suggests that these samples were matured to an early meta-anthracite (meta-kerogen) stage, which is significantly distant from the graphite stage. During this particular stage, the most obvious changes in molecular structure are the enlargement of aromatic clusters and the decrease in hydrogen atoms, and thus, H₂ rather than methane is produced, as revealed by the results of TC-MS experiment. Following an initial increase, the pore volume and surface area of kerogen samples decrease gradually, reaching their maximum values at 700 °C. Kerogen subjected to a heating temperature of 1000 °C exhibits a greater pore volume in comparison to the initial overmature kerogen. Thus, this observation provides evidence that shale kerogen in carbonization stage, which is typically lying at significant depths, holds promise as a viable reservoir for shale gas extraction.