Determination of chemical-structural changes in vitrinite accompanying luminescence alteration using C-NEXAFS analysis

Abstract

The phenomenon of luminescence alteration has been shown to correlate with the thermal maturity of Type III kerogens (vitrinites). In order to establish a chemical structural basis for this correlation, carbon near edge X-ray absorption fine structure (C-NEXAFS) spectroscopy is used to monitor the gain and loss of organic functionality in ultra-thin sections of vitrinite following time incremental exposure to blue light (390–490 nm) irradiation in air. These data are compared with luminescence alteration behavior measured at 600 nm. Three samples are studied; low maturity (% R 0=0.29), medium maturity (% R 0=0.73), and high maturity (% R 0=1.35) vitrinite. These exhibit “positive”, “dual”, and “negative” luminescence alteration, respectively. It has been previously established that the luminescence alteration of vitrinites is the result of photo–oxidation. C-NEXAFS data are used to identify the types of reactions and correlate the chemical structural changes with luminescence alteration behavior. The unaltered C-NEXAFS spectrum of each vitrinite is significantly different, reflecting the broad range in vitrinite maturity. The dominant reaction is the formation of COOH groups, through the attack of singlet oxygen on, predominantly, benzylic carbon. Carbonyl substituted aromatics are the dominant photo–oxidation product of the most mature vitrinite. The photo–chemical oxidation pathways and kinetics vary significantly between the three samples. Virtually all of the major spectral trends (excluding the formation of COOH groups) reverse, moving from low to high maturity, i.e. gains in absorption at a given energy at one maturity level are observed to be losses at a different maturity level. The spectral changes reveal that in the lower maturity samples aromatic acids, aliphatic ketones, and hydroxylated aromatic compounds are formed; aliphatic and aldehydic carbon are lost. In the more mature vitrinite, aryl–ketones and aromatic acids are formed, whereas polycyclic aromatic compounds are lost. Strong correlations exist between the development of “positive” alteration and the formation of COOH functionality. No obvious correlation could be made between the C-NEXAFS data and the “negative” luminescence alteration, suggesting that the lumophor participating in this reaction is below the detection limit of C-NEXAFS spectroscopy. The maturity of a given vitrinite sample, hence its molecular structure, strongly controls the specific reaction pathways as well as the total extent of reaction.