Thermodynamic equilibrium is a fundamental characteristic of a catalytic reaction sustained over time. The proposal that natural gas is largely catalytic (Mango, 2000) would suggest that natural gas should be at or very close to thermodynamic equilibrium, particularly when gas resides in active reservoirs over geologic time. In the Gulf of Mexico, deltaic reservoir rocks with interbedded marine shales (Paine, et al. 1968) show high levels of catalytic activity in our laboratory experiments. If equally catalytic in the subsurface, their gas compositions should be near thermodynamic equilibrium K (T) for reservoir temperatures T according to:

([C₂]² / [C₁][C₃]) = K (T) (1)

This proved to be the case for the 64 gas deposits published in Paine, et al. (1968). Since the equilibrium compositions corresponded to reservoir temperatures, they suggest in situ catalytic oil-to-gas at reservoir temperatures. We estimate >> 10⁹ years to attain equilibrium thermally at these temperatures, and 6 years catalytically and therefore must rule out in-reservoir thermal cracking as the source of equilibrium.

There is now substantial evidence that non-biogenic gas is largely catalytic as opposed to thermogenic. Thermal cracking is inherently a low-methane process, typically yielding gas with between 30 and 60 % wt CH₄ (C1-C4), while natural gas is rarely found with this composition and more typically contains ~ 85% CH₄, the composition of catalytic gas generated in marine shales. One hypothesis for the discrepancy is that gas is somehow fractionated between source and reservoir (Price & Schoell, 1995; Snowdon, 2002). Although methane enrichment can be obtained through physical fractionation, thermodynamic equilibrium cannot. Because gas compositions from thermal cracking are far from equilibrium, it is impossible to explain deltaic gas compositions at equilibrium without invoking robust catalytic intervention.

References

Mango, Geochim. Cosmochim. Acta. 64, 1265-1277 (2000).

Paine et al.,Memoir 9, Vol I, AAPG (1968).

Price & Schoell, Nature 378, 368-371 (1995).

Snowdon, Org. Geochem. 32, 913-931 (2001).

End_of_Record - Last_Page 15---------------