Transition metal catalysis in natural gas generation: Evidence from nonhydrous pyrolysis experiment

Transition metal catalysis has been advocated as a critical mechanism for natural gas generation. But its presence and significance in geological settings have long been debated due to conflicting results of laboratory experiments and the lack of reliable field indicator. In this study, a low rank coal sample from the Shahezi Formation of the Songliao Basin was subjected to nonhydrous pyrolysis with and without transition metal. The chemical and isotopic compositions of gaseous products in experiments were analyzed to investigate the mechanism and potential indicator of transition metal catalysis in sedimentary basins. The chemical and carbon isotopic compositions of hydrocarbon gases generated in pyrolysis experiments revealed that transition metal could catalyze both decomposition of organic matters and methane synthesis such as Fischer-Tropsch Type reactions during maturation of source rocks under experimental conditions resembling geological settings. Catalytic methane has uniform δD values unrelated to source rock maturity, which is helpful in identification of catalytic natural gases in sedimentary basins. Transition metal catalysis is probably not a common mechanism for natural gas generation in sedimentary basins due to the positive correlation between δDCH4 of natural gases and maturities of source rocks observed in most natural cases. However, transition metal catalysis could well interpret the restricted range of δDCH4 and carbon isotopic reversal phenomenon of natural gases in the Yingcheng Formation of the Xujiaweizi fault-depression whose origin has long been debated. So we inferred that the transition metal catalysis may have occurred in deep strata of the Xujiaweizi fault-depression and played a significant role in natural gas generation. Our study revealed that restricted range of δDCH4 of natural gases with different maturities may serve as an effective indicator of transition metal catalysis in sedimentary basins. It provides a new perspective to investigate the significance and universality of transition metal catalysis in different geological settings.