The Upper Devonian Duvernay is a prolific source rock in the Western Canada Sedimentary Basin (WCSB). In the Kaybob area, the Duvernay Fm. can be described as an ultra-low permeability, self-sourced, mudstone unconventional reservoir exploited for light hydrocarbons and condensate. Regional carbon isotope analyses of C2 to C5 hydrocarbons from the Duvernay indicate that the system was "closed" and not over mature. Therefore, carbon isotope thermometry could provide key insights of liquids fraction distribution, especially considering the Duvernay within the "oil window" in thermal maturity. In this study, we tested how variation of isotopically determined maturity within the Kaybob field relates to condensate production in the wells.
Production gases were sampled from wells at the Kaybob-Duvernay (KD) field targeting the Duvernay Fm. The carbon isotope ratios of methane, ethane, propane, iso-butane, n-butane, iso-pentane, and n-pentane were analyzed at the University of Alberta using a Finnigan MAT 252 GC-C CF-IRMS system. Carbon isotope compositions were reported as δ13C values in ‰ (per mil) using the V-PDB reference standard. Reproducibility of δ13C values for methane was ± 0.5‰ and ± 0.2‰ for the C2+ alkanes (ethane, propane, and butane). Chevron Canada Limited and KUFPEC Canada Inc. provided current well production data and reservoir properties based on static modelling. Geochemical-reservoir well data correlations were developed to further predict fluid trends.
All the gas samples showed a good correlation among their constituent components. 85% of the gas samples showed almost perfect correlations when plotted on a natural gas or Chung plot (δ13C vs. 1/n) indicative of "closed" system behavior. There is a very strong correlation between δ13C values of propane and ethane reflecting significant thermal maturity variation within our study area. Ethane and propane carbon isotope fingerprint showed good spatial and thermal trends. These gas carbon isotope markers also showed good correlation with production variables, such as: (i) condensate gas ratio (CGR); (ii) cumulative condensate; and (iii) cumulative gas. These correlations are key to calibrate the current KD field thermal maturity model and potentially support the prediction of hydrocarbon fraction generation in undeveloped acreages.
Our study provides both the theoretical insights associated with condensate and light hydrocarbon generation in an unconventional low permeability reservoir, as well as a practical tool to support the KD field asset development strategies. The systematic gas isotopic evaluation per well led to identify "Closed vs Open System" behavior, which coupled with the regional mapped isotope trends enabled the identification of thermal maturity trends, and possible source variations. This study allowed us to improve the understanding of the optimal thermal maturity window associated with higher liquids generation based on geochemistry and production data integration. Our preliminary results suggest that this isotope analysis approach can be used to support production forecasting and field optimization.
