Reaction kinetics and n-alkane product profiles from the thermal degradation of 13C-labeled n-C25 in two dissimilar oils as determined by SIM/GC/MS

Select ion monitoring/gas chromatography/mass spectrometry (SIM/GC/MS) in conjunction with 13C-labeling has been utilized in order to monitor the thermal degradation of 13C-labeled n-C25 mixed with a typical marine derived crude oil from the Arabian peninsula and an oil derived from terrestrial source rocks in the Ardjuna Basin. Pyrolyses were carried out in anhydrous closed gold vessels at times ranging from 1–360 h under isothermal conditions (350–425°C) at a pressure of 120 bar and the degradation of the labeled n-C25 followed first order kinetics in the two oil systems. For each temperature regime, the rate of 13C-labeled n-C25 decomposition was suppressed in comparison to the neat system with the Arabian Light marine oil exerting a greater influence on the n-C25 thermal stability. It was determined that the activation energies for the n-C25 compared favorably to the neat pyrolysis experiments [Ea(marine oil) = 67.5 kcal/mol, Ea(terrestrial oil) = 67.9 kcal/mol and Ea(neat) = 68.2 kcal/mol]. However, the pre-exponential factor obtained for the n-C25 decomposition varied by as much as a factor of five in the two oils in comparison to the neat pyrolysis experiments. An extrapolation of these kinetic parameters to geologic conditions shows that the n-C25 will be more thermally stable in high temperature reservoirs than previously anticipated. Using a similar procedure as was used on the quantification of the 13C-labeled n-C25, n-alkanes, the major products stemming from the degradation of the 13C-labeled n-C25, containing enrichments in 13C were quantified by SIM/GC/MS, as well. In monitoring and quantifying the overall enrichments of individual n-alkanes in the oils, it was shown that the mechanism for the thermal degradation of 13C-labeled n-C25 is similar between the neat system and the Ardjuna oil, but differs somewhat in the Arabian Light oil.