Abstract
In this study, a methane (CH4) cracking experiment in the temperature range of 425–800°C is presented. The experimental result shows that there are some alkane and alkene generation during CH4 cracking, in addition to hydrogen (H2). Moreover, the hydrocarbon gas displays carbon isotopic reversal ( δ 13 C 1 > δ 13 C 2 ) below 700°C, while solid carbon appears on the inner wall of the gold tube above 700°C. The variation in experimental products (including gas and solid carbon) with increasing temperature suggests that CH4 does not crack into carbon and H2 directly during its cracking, but first cracks into methyl (CH3⋅) and proton (H+) groups. CH3⋅ shares depleted 13C for preferential bond cleavage in 12C–H rather than 13C–H. CH3⋅ combination leads to depletion of 13C in heavy gas and further causes the carbon isotopic reversal ( δ 13 C 1 > δ 13 C 2 ) of hydrocarbon gas. Geological analysis of the experimental data indicates that the amount of heavy gas formed by the combination of CH3⋅ from CH4 early cracking and with depleted 13C is so little that can be masked by the bulk heavy gas from organic matter (OM) and with enriched 13C at R o < 2.5 % . Thus, natural gas shows normal isotope distribution ( δ 13 C 1 < δ 13 C 2 ) in this maturity stage. CH3⋅ combination (or CH4 polymerization) intensifies on exhaustion gas generation from OM in the maturity range of R o > 2.5 % . Therefore, the carbon isotopic reversal of natural gas appears at the overmature stage. CH4 polymerization is a possible mechanism for carbon isotopic reversal of overmature natural gas. The experimental results indicate that although CH4 might have start cracking at R o > 2.5 % , but it cracks substantially above 6.0% R o in actual geological settings.
Highlights
The notable development in natural gas exploration in the 21st century is the significant discovery of shale gas
Some researchers have suggested that the isotopic reversal of shale gas at overmature stages is caused by mixing of primary gas from kerogen cracking and secondary gas from the cracking of remained hydrocarbons [4,5,6], while others proposed that the redox reactions between water and CH4 at 250-300°C generate isotopically light carbon dioxide and hydrogen in the overmature phase, which further interacts to form isotopically light ethane (C2H6) and causes the isotopic reversal of shale gas [7, 8]
Our experimental result shows that heavy hydrocarbon gases are generated during
Summary
The notable development in natural gas exploration in the 21st century is the significant discovery of shale gas. Some new geochemical and geological characteristics of shale gas have been encountered during exploration, such as the isotopic rollover and/or isotopic reversal of shale gas at very high thermal maturity levels. These features have been observed in coal-derived tight gas [1,2,3]. Substantial research has been conducted to unravel the mechanism of isotopic rollover and reversal of overmature natural gas. Some researchers have suggested that the isotopic reversal of shale gas at overmature stages is caused by mixing of primary gas from kerogen cracking and secondary gas from the cracking of remained hydrocarbons [4,5,6], while others proposed that the redox reactions between water and CH4 at 250-300°C generate isotopically light carbon dioxide and hydrogen in the overmature phase, which further interacts to form isotopically light ethane (C2H6) and causes the isotopic reversal of shale gas [7, 8]
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