Field Example of Catalytic Coalbed Methane in Cambay, India

Ramaswamy reports 3.5 to 6 cu m/tonne of abiogenic coalbed gas in low-rank coals in the Mehsana area of the North Cambay basin (Oil & Gas Journal, Sept. 16, 32-36, 2002).  Since coals normally begin generating thermal methane at vitrinite reflectance of ~ 0.6% and these coals were between 0.30 – 0.36%, he ruled out thermal cracking as the mode of origin.  It is indigenous rather than migrated because the Mehsana coals are sealed in Cambay shales, source rocks for oil rather than gas in the Mehsana area.  The carbon isotopic composition of methane and ethane indicated an early thermogenic origin, but so-called thermal gas generated by cracking is improbable at these levels of maturity.  

The author proposed catalysis by transition metals to explain low-temperature gas generation in bituminous coals, an idea consistent with experimental work by others in coalbed gas generation (Butala et al., Energy & Fuels 14, 235-259, 2000).  Metal analysis of the Mehsana coals showed high concentrations of iron (5 – 9%), a transition metal known to catalyze the conversion of bitumen to gas (Mango, Org. Geochem. 24, 977-984, 1996; Butala et al., AAPG annual convention abs., Vol 1, 1998, p. 102).

Ramaswamy suggests that catalytic coalbed gas could be widespread in other lignite basins.  For example, Indonesia’s vast Tertiary low-rank coals with anomalously high amounts of gas for coals with vitrinite reflectance of 0.3%.  Metal catalysis is probably a major pathway through which natural gas is formed according to Ramaswamy, who suggests analyzing rocks and coals for transition metals in oil and gas exploration.