Sour gas is natural gas that contains measurable amounts of H2S. Even in small amounts, H2S can turn ‘sweet’ natural gas into ‘sour gas’ – a colourless, flammable gas that smells like rotten eggs and can be deadly if inhaled. If sour gas is present in the subsurface, it creates health, environmental and economic risks while drilling, producing, or treating the gas.
The WCSB’s Montney, Doig, and Duvernay formations are important areas for natural gas activity, yet the distribution of sour gas within these formations is highly variable, complex, and poorly understood.
A better understanding of the distribution and composition of gas liquids in Northeast BC’s unconventional reservoirs can help operators more effectively and efficiently develop the resource.
This project fits under Geoscience BC’s Strategic Objective of ‘Advancing Science and Innovative Technologies’. Specifically, it was designed to:
- Gather gas analysis, isotopic data and intellectual contributions from industry partners;
- Generate new analyses of produced fluids and rock;
- Deliver detailed maps of produced gases and isotopic analyses together with predictive maps of H2S distribution and abundance; and
- Deliver reservoir production models to help plan resource exploration and drilling programs.
The predictive maps of the distribution of H2S-bearing gas generated by this study will help natural gas operators plan safer and more cost-efficient programs. They will also provide regulators with data to evaluate and mitigate the risks and hazards associated with sour gas. Communities and First Nations will be able to use the data to better understand risks and hazards.
The WCSB is a ‘wedge’ of sedimentary rocks that extends from BC’s Northeast Region, across most of Alberta and into southwest Saskatchewan. It is divided into regional basins, sub-basins, and rock formations. The Montney, Doig, and Duvernay shale formations examined in this project are located in the northeast region of B.C.
What Was Found
Basin modelling, including mapping of natural gas isotopes, has demonstrated the complex nature of the Montney and Doig formation hydrocarbon systems in the project area. Workflows have been used to generate hydrocarbon liquids and H2S properties and distribution maps.
The project provides the workflow that is needed to understand complex interactions between basin evolution, regional structural activity, sedimentation patterns, total organic carbon content and type, maturation trends, as well as in-situ versus migrated hydrocarbon compositional trends, to aid the prediction of the expected hydrocarbon composition in the study area. Workflows are recommended for operators to generate more detailed mapping of natural gas liquids and H2S, particularly utilizing 3-D seismic data where available.
The Montney Formation was shown to have massive anhydrite which is likely from infilled fractures. It is this anhydrite that is believed to be the source of the sulphur that reacts with hydrocarbons in place to form H2S in very discreet zones. Isotopic analysis shows that the sulphur isotopic range in the sour gas is the same as the sulphur in Triassic anhydrite. The research team therefore propose that the Triassic Charlie Lake Formation is the likely source of sulphates in the Montney, due to its close proximity and high concentration of anhydrite. The model that the researchers present suggests that sulphate-rich fluids have migrated from the Charlie Lake Formation through faults and fractures prior to hydrocarbon generation in the Montney Formation. This anhydrite has also been deposited within the nearby Halfway and Doig formations.
The sulphate migration created discrete, lateral sulphate-rich zones (in fractures) that, when in contact with hydrocarbons, generated H2S. That gas has strong sorption affinity: once formed it will bind to the organic matter surface and not migrate further. This model fits the observation of laterally discrete zones of H2S gas seen in the region.
The researchers also noted that some wells closer to the WCSB deformation front had slightly heavier isotopic sulphur signatures, indicating a possible deeper source from Devonian strata. The researchers recommend that operators develop their own H2S concentration and sulphur isotope maps together with maps of the Charlie Lake Formation distribution, sulphate-ion concentration in Montney pore water, and structural elements, to better understand H2S distribution in their development areas.