Chalmers, Gareth1, Bustin, Amanda2 and Bustin, Marc2
1 University of the Sunshine Coast, Sippy Downs, Australia, 2 University of British Columbia, Vancouver, Canada
The distribution and origin of hydrogen sulphide (H2S) within the Triassic Montney Formation of the western Canadian sedimentary basin (WCSB) were investigated in British Columbia and Alberta, Canada. Hydrogen sulphide is a toxic gas that can be co-produced with hydrocarbons and impacts well economics and the environment. Even small amounts of H2S can impact hydrocarbon operations by depositing ‘elemental sulphur’ within pipelines and compressors as observed in Australia and overseas.
This study has mapped the H2S concentration in the Upper, Middle and Lower sections of the Montney Formation as operators are drilling multi-directional well pads within three zones of a 200 m reservoir. The Montney Formation has tested or produced H2S gas at concentrations between 0.001% and 22%. The stratigraphic and lateral variation in the H2S concentration can be inexplicable.
Sulphur available to generate H2S includes sulphide oxidation, decomposition of well-completions surfactants, bacterial sulphide reduction, kerogen cracking or fluid migration of sulphate ions from sulphur-rich evaporites. The isotopic ratios of sulphur and oxygen will depend on the source and the formation pathway of the H2S gas and these ratios can be used to help model H2S gas generation. Samples were collected from the Triassic Charlie Lake, Doig, Montney formations and the Devonian Nisku, Elk Point and Muskeg formations within areas of sour wells. Organic matter, sulphate and sulphide minerals were isolated using chemical and physical mineral separation techniques. These samples were analysed for sulphur and oxygen isotopes at the Ján Veizer Stable Isotope Laboratory, University of Ottawa (Ontario, Canada). Sulphur and oxygen isotopic ratios from sulphate minerals within the Montney Formation and the Charlie Lake Formation have a range between 9.0 to 18.0 ‰ V-CDT and -5.0 to 19.0 ‰ V-SMOW, respectively. These isotopic ratios differ from the sulphur and oxygen isotopic ratios from sulphate minerals sampled from Devonian rock sources which vary between 18.0 to 30.0 ‰ V-CDT and 12.0 to 30 ‰ V-SMOW, respectively. The sulphur isotopic ratio measured from H2S gas of producing Montney Formation wells varies between 9.3 and 20.9 ‰ V-CDT.
Preliminary results from isotopic analyses suggest that the sulphur that generated H2S in the Montney Formation is from Triassic sulphates or a mixture of Triassic and Devonian sources and not solely from Devonian rocks as first expected. It is postulated that the sulphate ions have migrated through localised fractures into the Montney Formation and then the sulphate is used to generate H2S. Another possibility is the H2S gas formed in the Charlie Lake Formation and/or Devonian rocks and then migrated into the Montney Formation. Textural relationships between the reservoir rock and the sulphate minerals is currently being examined which will provide key data for creating a H2S generation model for the Montney Formation.
Gareth Chalmers has used a multidisciplinary approach to investigate coal seam gas and shale reservoirs since 2002 at University of British Columbia, Canada. He also worked at Shell to develop the Duvernay shale play. Gareth now lectures at the University of the Sunshine Coast and is researching Australian gas reservoirs.