Activation energy and organic matter structure characteristics of shale kerogen and their significance for the in-situ conversion process of shale oil

•Reactions in natural and artificially simulated kerogen are similar at low maturity.•The activation energy is more susceptible to the maturity compared to TOC.•Activation energy increases sequentially for retained oil, shale and kerogen.•Aromatic functional groups coincide with reduction of aliphatic functional groups. Shale oil is a significant alternative energy, and in-situ conversion technology can achieve large-scale yield. The objective is to investigate the activation energy characteristics of kerogen at different maturities and total organic carbon contents. The study compared the activation energy of shale, kerogen and retained oil. The correlation between organic matter structure and in-situ conversion mechanism were also studied. In order to achieve the above objectives, the Chang 7 Formation in the Ordos Basin was used as an example. The methods include open and semi-open hydrocarbon generation thermal simulation experiment, Fourier transform infrared spectroscopy, solid-state nuclear magnetic resonance and other tests. The results indicate that the hydrocarbon generation in natural and artificially simulated kerogen samples are similar at low maturity. The proportions of low and main activation energy groups decrease with maturity, while those of high activation energy groups increase. TOC has an impact on activation energy but is not the primary factor. The proportion of low activation energy groups at different maturity stages is greater in shale than in kerogen. The activation energy of retained oil is smaller than that of shale. The activation energy increases sequentially for retained oil, shale and kerogen. The area of aliphatic carbon is significantly greater than that of aromatic carbon at low maturity. With increasing maturity, the areas of both gradually converge, and aromatic carbon surpasses aliphatic carbon by Ro around 1.1 %. Aromatic carbon and hydroxy/carboxylic carbon are mainly derived from the consumption and transformation of aliphatic carbon. The aliphatic carbon structure is mainly composed of methylene carbon, and aromatic structure coexists as single and multiple rings. There is no significant aromatic ring condensation in the maturity Ro = 0.5 ∼ 1.1 %. Different maturities affect the structure of kerogen, with TOC having a small impact. As the thermal maturity increases, the increase in the aromatic component leads to a gradual increase in the proportion of average and high activation energy groups. HI, H/C and fali e