Natural and experimental structural evolution of dispersed organic matter in mudstones: The Shimanto accretionary complex, southwest Japan

Structural changes induced by thermal maturation of dispersed organic matter (OM) in the Shimanto accretionary complex, southwest Japan, were investigated using micro-Fourier-transform infrared spectroscopy and micro-Raman spectroscopy. Natural dispersed OM exhibits systematic structural changes inferred from D1- and G-band FWHM values, Raman band separation (RBS), and intensity ratios of the D1- and G-bands (I-D1/I-G ratio) from diagenetic zone to anchizone (IC values: 0.75-0.30). Infrared spectra indicate a loss of aliphatic CHx, aromatic CHx, and oxygen-containing structures as temperature increases. These changes are consistent with discontinuities in thermal structures bounded by out-of-sequence thrusts. Kinetic pyrolysis experiments indicate that the I-D1/I-G ratio of synthesized OM has a power law relationship with heat treatment time. Kinetic models of temperature dependence were fitted using the I-D1/I-G ratio, and an effective activation energy of 106 +/- 17 kJ/mol was estimated using an Arrhenius equation. The activation energies estimated by power law rate and Avrami models have a least-square correlation coefficient of 0.93, indicating the temperature dependence of carbonization. The estimated effective activation energy is consistent with that of coal, lignin, cellulose, and hemicellulose during thermal degradation. On the other hand, RBS, and D1- and G-band FWHM values of OM display more complex changes with increasing heating temperature and time, and it is difficult to constrain rate parameters during pyrolysis experiments. Our data indicate that the I-D1/I-G ratio is controlled by a simple thermally activated process, whereas RBS and D1- and G-band FWHM values can be affected by lithostatic pressure, fluid activity, hydrogen index, and host lithology, as well as temperature. Structural evolution of dispersed OM in mudstones differs between natural and anhydrous closed experimental systems. Natural carbonization based on micro-Raman spectroscopy should be applied for a limited indicator of thermal maturation, especially for dispersed OM in diagenetic zone.