Factors controlling the origin of gas in Australian Bowen Basin coals

Open system pyrolysis (heating rate 10 degrees C/min) of coal maturity (vit rinite reflectance, VR) sequence (0.5%, 0.8% and 1.4% VR) demonstrates that there are two stages of thermogenic methane generation from Bowen Basin co als. The first and major stage shows a steady increase in methane generatio n maximising at 570 degrees C, corresponding to a VR of 2-2.5%. This is fol lowed by a less intense methane generation which has not as yet maximised b y 800 degrees C (equivalent to VR of 5%). Heavier (C2+) hydrocarbons are ge nerated up to 570 degrees C after which only the C-1 (CH4, CO and CO2) gase s are produced. The main phase of heavy hydrocarbon generation occurs betwe en 420 and 510 degrees C. Over this temperature range,methane generation ac counts for only a minor component, whereas the wet gases (C-2-C-5) are eith er in equal abundance or are more abundant by a factor of two than the liqu id hydrocarbons. The yields of non-hydrocarbon gases CO2 and CO are greater then methane during the early stages of gas generation from an immature co al, subordinate to methane during the main phase of methane generation afte r which they are again dominant. Compositional data for desorbed and produc ed coal seam gases from the Bowen show that CO2 and wet gases are a minor c omponent. This discrepancy between the proportion of wet gas components pro duced during open system pyrolysis and that observed in naturally matured c oals may be the result of preferential migration of wet gas components, by dilution of methane generated during secondary cracking of bitumen, or kine tic effects associated with different activations for production of individ ual hydrocarbon gases. Extrapolation of results of artificial pyrolysis of the main organic components in coal to geological significant heating rates suggests that isotopically light methane to delta(13)C of -50 parts per th ousand can be generated. Carbon isotope depletions in C-13 are further enha nced, however, as a result of trapping of gases over selected rank levels ( instantaneous generation) which is a probable explanation for the range of delta(13)C values we have recorded in methane desorbed from Bowen Basin coa ls (-51 +/- 9 parts per thousand). Pervasive carbonate-rich veins in Bowen Basin coals are the product of magmatism-related hydrothermal activity. Fur thermore, the pyrolysis results suggest an additional organic carbon source front CO2 released at any stage during the maturation history could "mix" in varying proportions with CO2 from the other sources. This interpretation is supported by C and O isotopic ratios, of carbonates that indicate mixin g between magmatic and meteoric fluids. Also, the steep slope of the C and O isotope correlation trend suggests that the carbonates were deposited ove r a very narrow temperature interval basin-wide, or at relatively high temp eratures (i.e., greater than 150 degrees C) where mineral-fluid oxygen isot ope fractionations are small. These temperatures are high enough for catage nic production of methane and higher hydrocarbons from the coal and coal-de rived bitumen. The results suggests th;lt a combination of thermogenic gene ration of methane and thermodynamic processes associated with CH4/CO2 equil ibria are the two most important factors that control the primary isotope a nd molecular composition of coal seam gases in the Bowen Basin. Biological process are regionally subordinate but may be locally significant. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.