Characterization of pyrolysates from maceral components of Tarim coals in closed system experiments and implications to natural gas generation

The macerals vitrinite, exinite, fusinite, and semi-fusinite from low-maturity coal (VRr = 0.40%) of the Manjiaer sag, Tarim Basin were isolated and subjected to isothermal pyrolysis in a sealed stainless steel reactor at temperatures ranging from 250 to 550 degrees C in 50 degrees C intervals for a duration of 72 h. Gas yields, chemical and isotopic compositions vary significantly for the different coal macerals. Total gas yields (predominantly CH4 and CO2) from exinite and vitrinite are much higher than those from fusinite and semi-fusinite. Carbon dioxide generated from exinite is slightly enriched in C-13 (delta C-13(CO2) range: -19.7 parts per thousand to -12.2 parts per thousand) as compared to CO2 produced from vitrinite (delta C-13(CO2) range: -22.4 parts per thousand to -19.1 parts per thousand). Methane produced from exinite (delta C-13(CH4) range: -35.0 parts per thousand to -30.3 parts per thousand) tends to be isotopically lighter than methane generated from inertinite (fusinite and semi-fusinite; delta C-13(CH4) range: -33.7 parts per thousand to -25.2 parts per thousand). Interestingly, the delta C-13(CH4) values of all macerals show a similar evolution pattern with temperature - the initial gas is isotopically heavy, then becomes lighter at moderate temperatures, and finally becomes heavier again. This may be an indication of isotopic heterogeneity within the coal precursors. Hydrocarbon potential of coal closely depends on the maceral composition. Exinite shows the highest hydrocarbon generation potential followed by vitrinite, while inertinite has only a poor hydrocarbon potential. With increasing thermal stress. n-alkane parameters such as OEP and CPI decrease while the Sigma C21-/ Sigma C22+ ratio increases and the bimodal pattern of the n-alkane distribution changes to a unimodal pattern. The Pr/Ph ratio varies irregularly through catagenesis. (c) 2007 Elsevier Ltd. All rights reserved.