The influence of thermochemical sulphate reduction on hydrocarbon composition in Nisku reservoirs, Brazeau river area, Alberta, Canada

The Upper Devonian Nisku Formation reservoirs of the Brazeau river area of west-central Alberta, Canada, produce oil and sweet and sour gas condensate. Generally, oil pools are located updip in the study area and sour (6–31% H2S) gas condensate downdip. H2S in the study area is formed by thermochemical sulphate reduction (TSR). All liquid hydrocarbons probably have one source, with the Duvernay Formation being the most likely candidate. Assessing the relative maturity of the oils and condensates is difficult because of the wide variation in thermal maturity and the effects of TSR, but the ratio of pristane/n-heptadecane does appear to decrease with increasing maturity for this sample set. With increasing TSR, the following changes were noted: decrease in the saturate/aromatic hydrocarbon ratio; increase in the relative abundance of organo-sulphur compounds (e.g. benzothiophenes); δ34S values of liquid hydrocarbon samples approached the values for anhydrite of the Nisku Formation in the study area; and an increase in δ13C of the saturate fraction. H2S concentrations in hydrodynamically “isolated” pools provide a good estimate of the extent of TSR in these reservoirs. However, other pools have anomalously high H2S concentrations for their depth, suggesting that the H2S was generated at greater depths and migrated updip into these pools.