Characterisation of dispersed organic matter from lower Palaeozoic metasedimentary rocks by organic petrography, X-ray diffraction and micro-Raman spectroscopy analyses

Abstract

Dispersed organic matter (DOM) concentrates of C-rich rocks from areas with different metamorphic characteristics were studied by organic petrography, X-ray diffraction (XRD) and micro-Raman spectroscopy analysis. The concentrates contain several types of DOM with different morphologies, reflectance, X-ray diffraction and micro-Raman characteristics. Four different morphological types were identified in the two studied areas. The different types have different distributions and their reflectance is quite variable in the four types, with a dominance of the highest reflectance values in the DOM from the area with the highest metamorphic grade. XRD analyses of samples from the areas reveal the presence of fine graphite together with non-graphitised carbons. The Raman spectral profiles show the usual bands G (1582 cm −1) and D1 (1350 cm −1), on the first-order Raman spectrum, and S1 (2700 cm −1) on the second-order spectrum. Additional weaker bands, D2 (1620 cm −1), and more rarely D3 (1500 cm −1) and S2 (2900 cm −1), are present. These are characteristic for disordered carbons in the different types of DOM and in both studied areas. However, the Raman parameters (D1/G intensity area ratio and the frequency and width of G band) indicate variable degrees of organisation in all DOM types. The existence of different types of DOM with different degrees of ordering in the same lithologies and metamorphic grade seems to be related to different organic precursors, as they are graphitised to different extents under the same metamorphic conditions. However, in the same lithologies and metamorphic grade, the existence of various stages of graphitisation within the same type of DOM can only be explained though the interaction of DOM with the metamorphic fluids present in the rocks. The ordering graphitisation process may be due to the existence of metamorphic fluid circulation events with a variety of compositions.