Metathesis in the generation of low-temperature gas in marine shales

Thermal cracking has been accepted as the source of natural gas for decades [1,2]. Although alternatives have been proposed and deficiencies in thermal cracking theory cited [3], it has retained extraordinary allegiance over time. This was in spite of the fact that laboratory simulations had consistently failed to generate gas resembling natural gas [4-10]. Natural gas (C1-C4) contains about 80% wt methane while experimentally generated gas from thermal cracking was always depleted in methane, and remained so over prolonged periods of cracking [3]. Higher methane concentrations had been generated, but only at extraordinary temperatures (> 400°C) where ethane and propane decompose [11]. It has been argued that natural gas is generated depleted in methane, and becomes enriched in methane after generation by some unspecified fractionation [12,13]. But, there is no sign of the hypothetical heavy fraction in conventional reservoirs and no plausible explanation for its disappearance [3]. Thermal cracking has nevertheless been embraced as the primary source of natural gas and alternatives essentially dismissed as possible contributors [4-10].This changed with the recent disclosure of gas generation at temperatures 300° below thermal cracking temperatures [14]. It was catalytic gas generated from marine shales under anoxic conditions, natural catalysis carried from the subsurface requiring no artificial stimulation. Shales generated gas in aperiodic episodes at ambient temperatures under inert gas flow. When gas was retained in closed reactors, it reached metathetic equilibrium in methane, ethane, and propane, and became enriched in methane over time [15]. Natural gas was also shown to be constrained to equilibrium in molecular and isotopic compositions. Other reports have shown counter-intuitive effects in low-temperature gas generation over time [16]. Shales released increasing concentrations of lighter hydrocarbons over time, the exact opposite to desorption or other sim