Study on pyrolysis characteristics and feasibility of autothermal pyrolysis for low-medium maturity shale oil from Xunyi

Abstract:

The Chang 7 Member of the Ordos Basin contains abundant low-to medium-maturity shale oil resources， with the Xunyi area exhibiting rich reserves and favorable potential for in-situ development. However， the thermal dynamic characteristics， reaction threshold temperature， and product evolution patterns of autothermal reactions in this reservoir remain unclear. To address this， low-medium maturity shale samples from the Chang 7 Member of the Yanchang Formation in the Xunyi area were investigated using thermogravimetry-Fourier transform infrared spectroscopy （TG-FTIR）， pyrolysis experiments， elemental analysis， SARA analysis， and gas chromatography-mass spectrometry （GC-MS）. The effects of pyrolysis atmosphere and preheating temperature on oxidation-assisted pyrolysis characteristics were systematically studied. Results show that an oxidative atmosphere significantly enhances organic matter reactivity， triggering a low-temperature oxidation-pyrolysis synergy that markedly reduces the initial decomposition temperature and promotes oxidative cracking of heavy components at high temperatures. The oxidative atmosphere exerts a chemical heat-promoting effect on the pyrolysis of low-medium maturity shale oil， with the apparent activation energy under air being 167 kJ/mol—about 30% lower than under nitrogen-indicating a distinct "pyrolysis-oxidation coupling" mechanism. The suitable preheating temperature range for oxidation-assisted pyrolysis is 230~390 ℃， which effectively activates the autothermal reaction and promotes hydrocarbon lightening. Excessive temperatures， however， lead to over-oxidation of some oil and gas， reducing oil yield and altering product composition. Thermodynamic analysis confirms that when the pyrolysis temperature is ≤530 ℃， the exothermic oxidation of residual carbon can maintain system energy balance， demonstrating the feasibility of autothermal in-situ conversion. This study clarifies the control ranges for key process parameters and provides theoretical support for the efficient in-situ development of low-medium maturity shale oil.