Carbon and hydrogen isotopic variations in gold tube gas pyrolysates from an Athabasca oil sand

Abstract

Closed system gold tube pyrolysis experiments were conducted on an oil sand bitumen from the Athabasca region in western Canada to investigate gas isotope variation with heating temperatures. Experiments were carried out at 350–650 °C and a pressure of 50 MPa, with heating rates of 2 °C/h and 20 °C/h. The progressive cracking of bitumen and liquid oil lead to continuous release of methane with increasing pyrolysis temperatures, while the yields of wet gas components increase initially at the lower temperature range and then decrease drastically at high temperatures. A general trend of 13C- and 2H-enrichment in the gas products is seen with increasing pyrolysis temperatures due to thermal cracking of various hydrocarbon and non-hydrocarbon components. Isotopic variations can be quite extensive, commonly occurring for both hydrocarbon gases and CO2 as pyrolysis progresses. A carbon isotopic rollover (isotope of gas component shifting from 13C enrichment to 12C-enrichment with increasing temperature) occurs for bulk hydrocarbon gas components at low temperature (< 450 °C), while rollover only occurs for ethane and propane at high temperature (> 550 °C), where partial reversal (lower carbon numbered gas component becomes more 13C-enriched relative to higher carbon numbered ones) also exists. The isotopic values of CO2 show a wide range of variation with the highest δ13C value seen at the lowest temperature while the lowest δ13C value occurs at 500 °C and 550 °C for 2 °C/h and 20 °C/h, respectively. Relative 13C-enrichment occurs only in the high temperature range. The unusual carbon isotope ratios of CO2 in pyrolysates is likely related to biodegradation influence on the precursors, but further investigation is still called for. Hydrogen isotopic values of hydrocarbon gases show even more dramatic variation than that seen for carbon. While methane is progressively 2H-enriched to 650 °C, ethane and propane are relatively 2H-enriched when the pyrolysis temperature is < 500 °C, while the trend is inverted at higher temperatures. Data presented here clearly deviate from the trajectories expected by simple equilibrium or Rayleigh-type kinetic cracking models. Dynamic generation, destruction and mixing processes govern the final isotopic signature in an associated gas product. Isotopic rollover and reversal seem to be a common phenomenon during thermal evolution where heterogeneous precursors are available. While the reaction system of pyrolysis at high temperature differs completely from source rock evolution at low temperature and some artifacts are inevitably involved, the isotope behavior observed here may provide some insights into the complexity in natural cases and help our understanding of the mechanisms of isotopic evolution during thermal maturation.