Abstract 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.

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天然气生产中的过渡金属催化：来自无水热解实验的证据

摘要 过渡金属催化被认为是天然气生产的关键机制。但由于实验室实验的结果相互矛盾以及缺乏可靠的现场指标，它在地质环境中的存在和意义长期以来一直存在争议。在本研究中，对松辽盆地沙河子组低阶煤样品进行了含过渡金属和不含过渡金属的无水热解。分析实验中气态产物的化学和同位素组成，研究沉积盆地过渡金属催化的机理和潜在指标。热解实验中产生的烃类气体的化学和碳同位素组成表明，在类似地质环境的实验条件下，过渡金属可以在烃源岩成熟过程中催化有机物分解和甲烷合成，例如费托型反应。催化甲烷具有均匀的δD值，与烃源岩成熟度无关，有助于识别沉积盆地催化天然气。由于在大多数自然情况下观察到的天然气 δDCH4 与烃源岩成熟度之间存在正相关关系，过渡金属催化可能不是沉积盆地天然气生成的常见机制。然而，过渡金属催化可以很好地解释徐家围子断陷营城组天然气δDCH4的受限范围和碳同位素反转现象，其成因一直存在争议。因此我们推断过渡金属催化作用可能发生在徐家围子断陷深部，对天然气的生成起到了重要作用。我们的研究表明，不同成熟度的天然气δDCH4 的限制范围可以作为沉积盆地过渡金属催化的有效指标。它为研究过渡金属催化在不同地质环境中的意义和普遍性提供了新的视角。因此我们推断过渡金属催化作用可能发生在徐家围子断陷深部，对天然气的生成起到了重要作用。我们的研究表明，不同成熟度的天然气δDCH4 的限制范围可以作为沉积盆地过渡金属催化的有效指标。它为研究过渡金属催化在不同地质环境中的意义和普遍性提供了新的视角。因此我们推断过渡金属催化作用可能发生在徐家围子断陷深部，对天然气的生成起到了重要作用。我们的研究表明，不同成熟度的天然气δDCH4 的限制范围可以作为沉积盆地过渡金属催化的有效指标。它为研究过渡金属催化在不同地质环境中的意义和普遍性提供了新的视角。