甲烷裂解——中国南方寒武系页岩含气量低的一个重要原因

Methane cracking: An important factor to cause the low content of gas found in the Cambrian shale in South China

对中国南方页岩气地球化学特征统计分析结果表明,寒武系筇竹寺组页岩气除了具有与志留系龙马溪组页岩气相似的干燥系数极高(>98.9%)、碳同位素倒转的地球化学特征外,还具有N_(2)含量高,页岩含气量低的特征。在对目前各种高过成熟阶段天然气碳同位素倒转成因理论分析的基础上,提出甲烷裂解过程的聚合作用是高过成熟阶段天然气碳同位素倒转的重要原因。这一观点得到了CH_(4)+N_(2)混合气体及碳同位素正序天然气裂解模拟实验验证。不同条件下CH_(4)+N_(2)混合气体裂解模拟实验结果证明,甲烷大量裂解时氮气也不会分解,且页岩对甲烷的裂解具有催化作用。结合有机质结构及页岩储层导电性能随成熟度的演化、不同结构烃类分子裂解键能的对比,认为有机质在生气结束后(甲基完全脱落)的强烈石墨化作用会导致页岩电阻率明显降低,并伴随着甲烷的裂解。相关模拟实验结果表明有机质演化过程中氮气生成虽主要发生在烃类气体生成将近结束阶段之后(R_(0)>3.0%),但氮气生成量与有机质整个演化过程烃类气体的生成量相比非常少。寒武系页岩气地球化学特征、生烃模拟实验结果、甲烷与天然气的裂解模拟实验、寒武系页岩含气量与页岩储层物性及页岩成熟度综合研究结果表明,除了地质保存条件外,甲烷裂解应是寒武系(特别是R_(0)>3.0%区域)页岩气低产及高含N_(2)的另一个重要因素。

The statistical results of the geochemistry of Qiongzhusi shale gas found in south China show that the shale gas share the characteristics of low total gas content as well as high N2 percentage,besides the characteristics of high gas dryness(>98.9%)and carbon isotopic reversal which is the same as that of Longmaxi Formation shale gas.The authors proposed that methane polymerization happened at the early stage of methane cracking might be an important factor to cause the carbon isotopic reversal of natural gas from over mature source rocks based on the discussion in theory on the existing explains about carbon isotopic reversal of natural gas.The view of methane polymerization to cause the carbon isotopic reversal is supported firmly by the cracking experiments result of CH_(4)+N2 mixing gas and natural gas with normal isotopic distribution characteristic.The aforementioned experimental results also indicated that N2 could not decompose as methane cracking in geological setting and that shale had an acceleration on methane cracking.Based on the relative research on chemical structure evolution of organic matter and shales conductivity with maturity increasing as well as the dissociation energy of various chemical bond in different molecule and location of hydrocarbon,the authors proposed that the graphitization of organic matter closed to the exhaustion of hydrocarbon gas generation is attributed to the decreasing of shale conductivity,accompany with methane cracking.The relative experimental results show that N2 generation happen closed to the end of hydrocarbon gas generation(R_(0)>3.0%),but the yield of N2 generated from organic matter evolution is much lower than that of hydrocarbon gas.The comprehensive analysis about geochemistry of shale gas,hydrocarbon generation experiment,methane and natural cracking experiments and the relationship between the gas content and physical properties as well as the maturity of Qiongzhusi shales show that methane cracking is an important factor to cause the low content of gas in the Cambrian shale developed in South China,especially in the area with R_(0)>3.0%.