Thermal degradation of medium-low maturity shale under various organic matter compositions: Insights into reaction mechanism and kinetic modeling

The pyrolysis of medium-low maturity shale under various organic matter compositions involves complex thermal conversion of multiple organic components. Studying the reaction mechanisms and kinetics is crucial. Thermogravimetric and non-isothermal pyrolysis experiments were conducted on samples with different kerogen to heavy hydrocarbon mass ratios. Gaussian function deconvolution was applied to resolve independent reaction of kerogen and heavy hydrocarbons, elucidating their interaction mechanisms. Kinetic triplet of individual reactions was calculated by master plot and iso-conversional methods. A novel pyrolysis reaction network integrating true mechanism functions (F2 for kerogen, R3/D3 for heavy hydrocarbons, and F1 for pyrolysis oil) was proposed. A kinetic model encompassing kerogen, heavy hydrocarbon, and oil cracking reactions was established. It was found that high initial heavy hydrocarbon content delayed pyrolysis oil and gas secondary cracking. Kerogen pyrolysis concentrated between 350 and 520 °C and activation energy fluctuating around 220 kJ/mol. As initial heavy hydrocarbon content increased, kerogen pyrolysis was inhibited, shifting reaction window to higher temperatures. The peak temperature and activation energy of heavy hydrocarbon cracking began to be lower than that of kerogen. The proportion factor of heavy hydrocarbon generated by kerogen reactions decreased from 0.26 to 0.06, and heavy hydrocarbon cracking reactions generated less gas and more oil.