Carbon and hydrogen isotope fractionation for methane from non-isothermal pyrolysis of oil in anhydrous and hydrothermal conditions

Abstract

In this study, to ascertain the carbon and hydrogen isotope fractionation of oil cracking gas (secondary gas) in hydrothermal conditions, non-isothermal pyrolysis of oil with and without water was carried out by a gold-tube system. By determination of the yields of individual gas products, it is found that the presence of water enhanced the yields of hydrocarbon gases. However, kinetic calculations indicate that Ea for the generation of methane and C2–5 in pyrolysis in hydrothermal conditions are essentially identical with those in anhydrous pyrolysis. The yields of carbon dioxide (CO2) and alkene gases in pyrolysis in hydrothermal conditions are evidently higher than those in anhydrous pyrolysis. It is reasonable that water–hydrocarbon reactions occurred and contributed to the generation of secondary gas in hydrothermal conditions. Meanwhile, the presence of water resulted in a slight depletion of 13C for methane and an evident depletion of 13C for CO2. Thermodynamic calculations suggest that water–hydrocarbon reactions in non-isothermal pyrolysis are dominated by free radical mechanism rather than ionic mechanism. Moreover, δ2H values of methane are apparently different in pyrolysis involving water with different δ2H. This result demonstrates that water provided hydrogen for hydrocarbon gas generation. Finally, we established mathematical models based on isotope fractionation to quantitatively determine the contribution of water–hydrocarbon reactions for gas generation in both experimental and geological conditions.

Geological–Geochemical Models and Isotope Fractionation Laws and Control Factors of Thermogenic Coalbed Gas in Panxian, China

“…The earth’s interior contains many different types of gases with rich geological and geochemical information, such as natural gas, volcanic gas, and hot spring gas, and their carbon and hydrogen isotopes are the most important geochemical tracers. − …”

Carbon Isotope Reversal Gas Generated in the Hydrothermal Experiments Involving 1-MNa and MnCO3

“…The isotope fractionation during the water–hydrocarbon reaction is different from thermal cracking alone. Previous studies demonstrated that the water–hydrocarbon reaction has smaller carbon isotope fractionation factor than thermal cracking. , This means that the isotopic fractionation between alkane gas and its precursor from water–hydrocarbon reactions is greater, resulting in the overall lighter carbon isotopes of generated alkane gas.…”

“…According to He et al, the transfer of hydrogen from water to methane through water–hydrocarbon reactions will affect δ 2 H of methane. Before equilibrium for H transfer, such an effect is governed by the reaction extent . Generally, large H fractionation between H 2 O and CH 4 would generate 2 H-depleted CH 4 .…”

“…In addition, completely isotopic reversal natural gas (δ 13 C 1 > δ 13 C 2 > δ 13 C 3 ) has also been widely observed in shale gas (i.e., Fayetteville, Barnett, and Utica shale in the United States). ,, Dai et al analyzed the genesis of completely carbon isotope reversal gas and attributed it to the generation of extremely dry gas at over-maturation and high temperatures. Unfortunately, anhydrous pyrolysis of OM or oil preferentially generated 13 C isotope normal gas (δ 13 C 1 < δ 13 C 2 < δ 13 C 3 ) even at high-over artificial maturity. , …”

“…Numerous hydrous experiments at elevated temperatures have been conducted to explore the influence of water on hydrocarbon generation. These studies have demonstrated that the presence of water shows evident effects on the yields and chemical compositions of pyrolyzed products. ,, It is generally accepted that hydrogen can transfer from water to hydrocarbons and residual OM via water–organic reactions in hydrothermal experiments. ,− Considering the potential H source from water, geochemists have proposed that gas potential at high maturity in deep formations may be much higher. , Lewan observed that the yield of oil or liquid products from low mature OM or kerogen in hydrous pyrolysis experiments is about 20–30% higher than that in anhydrous pyrolysis . He et al also observed an evident increase in hydrocarbon gas yield in the presence of water in the pyrolysis involving n -C 16 .…”