The Origin of Natural Gas: A Paradigm Shift 

The current paradigm invokes thermal cracking to explain the evolution of oil to gas at basin temperatures > 150^oC (Hunt, 1996).  It is probably wrong for a number of reasons.  First, oil is too stable to crack at these temperatures (Domine et al., 2002), and second, when oil does crack, the thermal gas produced does not resemble natural gas (Mango, 2001).  Finally, wet gas is extraordinarily stabile (Laidler et al., 1961, 1962) and cannot possibly crack to dry gas (methane) over geologic time.  The thermal cracking paradigm, in other words, fails to explain the basic tenets of petroleum geochemistry: oil converts to wet gas and wet gas to dry gas progressively at reservoir temperatures above 150^oC (Hunt, 1996).   The paradigm is thus unstable, in danger of displacement by a new paradigm that better explains the evolution of oil to gas in sedimentary basins.    

If oil cracking were the source of so-called thermal gas, thermal kinetic models would have powerful predictive powers.  Predicting oil or gas would be a simple function of oil stability, reservoir temperature and residence time.  But thermal cracking models consistently fail to predict oil or gas above statistical chance.  Although they are sometimes used qualitatively in exploration, they are rarely critical factors in decision making where oil or gas has significant economic consequences. 

An alternative hypothesis was published a decade ago (Mango, 1992) suggesting low-valent transition metals (LVTM) as catalysts.  It was purely theoretical, however, and without empirical support at that time.  Since then, the evidence for catalysis has been mounting, particularly for LVTM: a Monterey source rock (Miocene, CA; 250 ppm Ni) converted hydrocarbons to gas catalytically under realistic basin conditions (Mango et al., 1994); pure zero-valent nickel exhibited identical activity (Mango, 1996); crude oil decomposed to gas over zero-valent Ni and Co (Mango and Hightower, 1997); catalytic gas proved indistinguishable from natural gas in molecular and isotopic composition (Mango and Elrod, 1999); zero-valent nickel and iron proved catalytic in the generation of coalbed gas (Medina et al., 2000); catalytic light hydrocarbons (C₆ to C₇) from oil decomposition over zero-valent Ni & Co proved compositionally the same as natural light hydrocarbons (Mango, 2000). 

Finally, Petroleum Habitats’ rock assay on outer-neritic shales, potential source rocks for oil and gas in deltaic basins, indicates LVTM in high concentrations (~ 20 ppm), thus robust catalytic activity under basin conditions.  Projected activities adequately explain for the first time, 1) the onset of gas generation in source rocks, 2) oil’s conversion to gas above 150^oC and, 3) the conversion of wet gas to dry gas within the same temperature window.  Rocks are catalytic in their natural state, as received in the laboratory.  Thermal cracking rates fall well below catalytic cracking rates at all temperatures up to 200^oC.  Residual oil in a maturing source rock, therefore, should convert to catalytic gas long before the rock reaches cracking temperatures.

The question of whether natural gas is thermal or catalytic may now be moot.  Rocks analyzed by us are catalytic.  This is a serious, perhaps even fatal blow to the already weakened paradigm.   Of the two theories, catalysis explains more, needs less in the way of auxiliary explanations, and now has compelling experimental support. 

References

Domine, F., Bounaceur, R., Scacchi, G., Marquaire, P., Dessort, D., Pradier, B., and Brevart, O. (2002)  Up to what temperature is petroleum stable?  New insights from 5200 free radical reaction model.  Org. Geochem.  33, 1487-1499.

Hunt, J. M., 1996. Petroleum Geochemistry and Geology, 2^nd ed., W. H. Freeman, New York., Chapters 7  & 12.

Laidler, K. J., Wojciechowske, B. W., 1961. Kinetics and mechanisms of the thermal decomposition of ethane. I. The uninhibited reaction. Proceeding of the Royal Society A260, 91-102.

Laidler, K. J., Sagert, N. H., Wojciechowske, B. W., 1962. Kinetics and mechanisms of the thermal decomposition of propane. Proceedings of the Royal Society A270, 242-253.

Mango, F. D. (1992) Transition metal catalysis in the generation of petroleum and natural gas.  . Cosmochim. Cosmochim. Acta 56, 553-555.

Mango, F. D., Hightower, J. W., and James, A. T. (1994) Role of transition metal catalysis in the formation of natural gas. Nature 368, 536.

Mango, F. D. (1996) Transition metal catalysis in the generation of natural gas. Org. Geochem. 24, 977-984.

Mango, F. D., and Hightower, J. (1997) The catalytic decomposition of petroleum into natural gas. Geochim. Cosmochim. Acta 61, 5347-5350.

Mango, F. D.,, and Elrod, L. W. (1999) The carbon isotopic composition of catalytic gas: A comparative analysis with natural gas. Geochim. Cosmochim. Acta 63, 1097-1106.

Mango, F. D. (2000) The origin of light hydrocarbons. Geochim. Cosmochim. Acta 64, 1265-1277.

Mango, F. D. (2001) Methane concentrations in natural gas: the genetic implications. Org. Geochem. 32, 1283-1287.

Medina, J. C. et al., (2000) Low temperatura iron- and nickel-catalyzed reactions leading to coalbed gas formation. Geochim. Cosmochim. Acta 64, 643-649.