Field Example of Shallow Catalytic Gas in Western Canada 

There are two sources of natural gas according to the current paradigm on the origin of gas: thermal and biogenic (Hunt, Petroleum Geochemistry & Geology, Freeman, NY, 1996).  Thermal gas is believed to come from thermal cracking, cracking of kerogen in source rocks and oil cracking in reservoir rocks.  In either case, it is a high-temperature reaction commencing ~ 130^oC in source rocks and ~150^oC in reservoir rocks.   When gas is found in reservoir rocks cooler than 100^oC, it is labeled biogenic, the presumed product of biological intervention.  Biogenic gas is easily distinguished from so-called thermal gas in its molecular composition (biogenic gas 99+ % methane and thermal gas is ~ 85% methane) and carbon isotopic composition.   

So it was a complete surprise to Rowe & Muehlenbachs exploring for gas in Western Canada to find non-biogenic gas in reservoir rocks that had never been warmer than 62^oC (Nature 388, 61, 1999).  The gas was genuinely ‘thermal’ in molecular and isotopic composition and indigenous since it was distinct from a deeper gas also carrying the thermal fingerprint but sufficiently altered by biodegradation to be easily distinguished from the shallower gas. 

Because thermal cracking is out of the question at these temperatures, and a biological source is equally so, the authors invoked catalysis to explain this apparent anomaly.   But it is an anomaly only in the context of the current paradigm.  If that paradigm is wrong, and all so-called thermal gas is in fact catalytic gas, then indigenous non-biogenic gas can exist anywhere in a basin, irrespective of temperature. 

In fact, there is compelling evidence supporting the catalytic paradigm.  First, oil cracking does not give a gas resembling natural gas.  It contains ~ 45% methane (% wt C1 – C4) while natural gas is consistently ~ 85% methane.  Second, the major hydrocarbons in wet gas, ethane and propane, are extraordinarily stable and cannot possibly crack to methane at typical basin temperatures over geologic time (Mango, Org. Geochem. 32, 1283, 2001).  In other words, gas with 45% methane cannot crack to gas with 85% methane under any conditions in petroleum basins. 

If thermal cracking cannot explain the composition of natural gas, can catalysis by low-valent transition metals (LVTM), the alternative paradigm, explain it?  The answer is yes, and it does so precisely.  Catalytic gas is identical to natural gas in molecular and isotopic composition (Mango et al., Nature 368, 536, 1994).  The Figure shows average values for 450 natural gases analyzed by the US Department of Interior superimposed over 5 catalytic gases from the decomposition of hydrocarbons over low-valent Ni and Co (200^oC).