DETERMINATION OF CHEMICAL-STRUCTURAL CHANGES IN VITRINITE ACCOMPANYING LUMINESCENCE ALTERATION USING C-NEXAFS ANALYSIS

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 u sed to monitor the gain and loss of organic functionality in ultra-thi n sections of vitrinite following time incremental exposure to blue li ght (390-490 nm) irradiation in air. These data are compared with lumi nescence alteration behavior measured at 600 nm. Three samples are stu died; 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 ha s been previously established that the luminescence alteration of vitr inites is the result of photo-oxidation. C-NEXAFS data are used to ide ntify the types of reactions and correlate the chemical structural cha nges with luminescence alteration behavior. The unaltered C-NEXAFS spe ctrum of each vitrinite is significantly different, reflecting the bro ad range in vitrinite maturity. The dominant reaction is the formation of COOH groups, through the attack of singlet oxygen on, predominantl y, benzylic carbon. Carbonyl substituted aromatics are the dominant ph oto-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 for mation of COOH groups) reverse, moving from low to high maturity, i.e. gains in absorption at a given energy at one maturity level are obser ved to be losses at a different maturity level. The spectral changes r eveal that in the lower maturity samples aromatic acids, aliphatic ket ones, and hydroxylated aromatic compounds are formed; aliphatic and al dehydic carbon are lost. In the more mature vitrinite, aryl-ketones an d 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 corr elation could be made between the C-NEXAFS data and the ''negative'' l uminescence alteration, suggesting that the lumophor participating in this reaction is below the detection limit of C-NEXAFS spectroscopy. T he maturity of a given vitrinite sample, hence its molecular structure , strongly controls the specific reaction pathways as well as the tota l extent of reaction. (C) 1998 Elsevier Science Ltd. All rights reserv ed.