Predicting the composition of natural gas generated by the Duvernay Formation (Western Canada Sedimentary Basin) using a compositional kinetic approach

The formation and composition of natural gas generated from the Duvernay Formation was simulated by heating immature kerogens in a closed system (MSSV pyrolysis) at three different heating rates (0.1, 0.7 and 5.0 °C/min). It was not possible to evaluate reasonable kinetic parameters for the formation of single compounds using accepted protocols, because even at the highest pyrolysis temperatures there is still methane formation, while ethane and propane formations show a significant heating rate dependency of their maximum yields. To circumvent these obstacles a new method was developed utilising the following observations:(1) bulk gas fraction C1−5 does not show any heating rate dependency of its maximum yield,(2) omitting either ethane or propane, the generation of both of which are heating rate dependent, from the total gas fraction (which gives the C1−5 − C2 as well as C1−5 − C3) resulted in yields which were not heating rate-dependent.Thus, the kinetic evaluation for these fractions is valid and could be used to predict the formation of the bulk fractions for geological heating rates. By calculating the difference of C1−5 and C2−5, C1−5 − C2 as well as C1−5 − C3, it was possible to predict the temperatures of methane, ethane and propane formation under geological conditions. Methane was predicted to represent 60–70 mol% of the natural gas formed from the Duvernay Formation with an onset temperature of 120 °C for a geological heating rate of 3 °C/my. Ethane and propane were predicted to represent 20 and 10 mol% of the Duvernay gas, respectively. The onset of ethane and propane formations was predicted to take place at 130 °C. Interestingly, using this concept the composition of gas formed from the Duvernay Formation can be predicted very well and fits to reservoired Duvernay gas in petroleum accumulations along the Rimbey–Leduc–Meadowbrook Reef System, which was formed during the main phase of petroleum formation.