The Origin of Natural Gas: Thermal or Catalytic?

Frank D. Mango

The idea that natural gas is thermolytic, coming from decomposing organic debris, has remained almost unchallenged for nearly half a century. But scientific evidence supporting this view has been elusive. For example, laboratory attempts to simulate the process have been largely unsuccessful; thermolysis consistently yields low concentrations of methane (10 to 50% of C₁-C₄) while natural gas is typically 90% methane.

A new hypothesis on the origin of natural gas accounts for high methane by invoking the catalytic action of transition metals in carbonaceous sedimentary rocks. In this view, n-alkenes (from decomposing kerogen) and hydrogen (from kerogen or water + minerals) are the catalytic substrates that generate light hydrocarbons. This concept was recently verified: olefins, hydrogen, and a metalliferous source rock catalytically generated gas which was identical to natural gas in molecular and isotopic composition.

We shall present evidence that the gas generated from a source rock and hydrogen, without the addition of olefin, is also catalytic. Thus, kerogen provides a continuous source of carbon substrate in the generation of gas. Pure transition metal complexes (e.g., porphyrins and acetylacetonates) are similarly catalytic in the conversion of hydrogen and olefins into gas. These results provide compelling evidence supporting the contention that the transition metals in source rocks are catalytic in the generation of natural gas.

The laboratory generation of natural gas under these mild conditions (~200°C) is, to our knowledge, unprecedented. That catalytic gas is compositionally identical to natural gas while thermolytic gas is not suggests that catalysis could be a significant pathway through which natural gas is formed in the earth. The implications to artificial maturation experiments and the possible roles of the various transition metals in natural gas generation will be discussed.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995