THE idea that natural gas is formed by thermal decomposition of sedimentary organic matter1,2 enjoys almost universal acceptance3-6. But pyrolysis experiments on organic matter7-13 have failed to reproduce the composition of natural gas (typically 90% methane). It has recently been suggested14 that natural gas may instead be generated catalytically : transition metals are often found in carbonaceous sedimentary rocks, and might promote the reaction between hydrogen and n-alkenes (which are themselves formed during thermal decomposition of kerogen) to give light hydrocarbons and natural gas. We report here experimental results that support this hypothesis. In particular, we find that n-alkenes, hydrogen and a carbonaceous sedimentary rock containing moderately high concentrations of transition metals react under mild conditions (approximately 200-degrees-C) to generate a light-hydrocarbon product indistinguishable from natural gas in both molecular and carbon isotope composition. Our results demonstrate that the reaction is indeed catalytic, and could alter the way in which we view the generation and distribution of oil and gas in the Earth.