With depressed AECO gas prices, Duvernay operators are targeting areas of oil and liquids-rich gas to enhance economic returns on their wells. The Duvernay is unusual as it is difficult to predict expected gas-oil ratios (GORs), oil gravities, and the transition from gas to oil. The oil window of the Duvernay has only recently seen a significant increase in development focus.
Understanding liquids distribution (oil and condensate) in the Duvernay is essential to forecast commercial viability, development planning, and for reserves and resource evaluations. As GLJ has a large client base in all producing areas of the Duvernay, we need to better understand liquids to gas ratios. Mapping of thermal maturity does not result in a direct indication of liquids to gas ratio, but it is a useful predictor. By using only publicly available data in this presentation, GLJ has married thermal maturity to known production to develop a map of expected liquids yields. The complicating factor is that thermal maturity in the Duvernay does not track with burial depth in all areas.
Figure 1: GLJ's clients land base as of September 2018. Yellow polygons represent lands GLJ has completed evaluations in the Duvernay. These evaluations consist of yearly reserve and resource reporting, bank financing reports, acquisition, and divestiture evaluation reports and support, as well as local studies.
Some shale plays, the Eagle Ford being a common example, show thermal maturity windows track very closely to burial depth. At a play-wide scale, the Duvernay’s burial history strongly controls the expected hydrocarbon type (with notable exceptions). However, there are local influences in the areas that are under development, that can cause unexpected thermal maturity to be encountered for a given burial depth. In the Duvernay, Tmax data has been used to define areas of suitable maturity for exploration and as a guide to expected liquids to gas ratio. Tmax data is a direct measure of thermal maturity. However, there is ambiguity and error associated with Tmax data due to: age of data, methodology at different labs, variation in kerogen type, sample type, sample availability, analysis quality, interpretation of the S2 peak, among others.
Figure 2: Cross Plot of Depth vs Liquids to Gas Ratio and Cross Plot of Tmax vs Liquids to Gas Ratio. There is more of an apparent trend with Tmax data than depth, but, there is considerable scatter in the data. For example, with a Tmax value of approximately 445, wells have resulted in liquids to gas ratios of 0 to 1500bbls/mmcf. These plots suggest there is ambiguity in the data, and/or other influences impacting thermal maturity.
Figure 3: Petroleum generation of organic matter (Reid and Milici 2008, modified from Dow and O’Conner, 1982). The S2 peak during Rock-Eval pyrolysis to calculate Tmax is an indicator of the maturity of the shale. Tmax has limitations to predict liquids yields as different types of kerogen result in a range of Tmax for each hydrocarbon generating phase.
Recently there have been wells drilled in areas of lower maturity, as defined by Tmax data, but initial production data looks promising. Also, by just using Tmax data alone to predict liquids to gas ratio, it does not work well. Instead of relying solely on Tmax data, we used a combination of data to try and examine the spatial changes in liquids yields. These data sources were empirically combined and correlated with known production. This correlation may then be useful in trying to predict liquid yields away from areas of known production.
The data we examined, mapped and correlated with production data include:
- True Vertical Depth: A possible indicator of thermal maturity. Increased maturity with increased depth.
- Rock-Eval Pyrolysis (Tmax & Hydrogen Index): Measurements of thermal maturity and type of organic matter.
- Geothermal Gradient: The increase in temperature with increasing depth. Areas with a higher geothermal gradient are prone to have a higher thermal maturity.
- Magnetic Surveys: Aerial magnetic surveys can display basement structures which may be a conduit for heat to ‘cook’ the Duvernay. An example is the Snowbird Tectonic Zone.
- Other Geological Factors: Thickness of Majeau Lake, underlying carbonate platforms, etc., which may act as thermal conductors or insulators below the Duvernay.
- Duvernay Well Production: Production data is the final key. Maturity trends from the other sources are correlated back to liquids to gas ratios from production data.
Figure 4: Geological mapping of various parameters to help predict thermal maturity and liquids trends in the Duvernay. The trends of these maps were added together to predict variations in thermal maturity. (Tmax and Hydrogen Index from raw public data, Mag survey from AGS SR72, 2005)
The different sources of data were empirically combined to create a thermal maturity trend. This trend was then compared to liquids to gas ratios from producing wells. The combined thermal maturity trend analysis resulted in a better, more refined correlation with production data than just Tmax trends. The combined trend was then normalized to the liquids to gas ratios of producing wells, which resulted in a map with usable units (bbls/mmcf). By using the liquids to gas ratios from production data and thermal maturity mapping, it allows better prediction of liquids to gas ratio away from current production in Duvernay.
Figure 5: Liquids to Gas Ratio Map (bbls/mmcf) of the Duvernay Formation from combined thermal maturity trend analysis and public production data. Red colours depict dry gas, yellows, and oranges depict liquids-rich gas, and green is oil.
Mapping the liquids to gas ratio in the Duvernay has allowed for a better understanding of the transition from dry gas to oil. But, there are still limitations and questions to be answered, particularly with the transition from the productive oil window to immaturity. By understanding the productive liquids window of the Duvernay, it may allow for a large swath of future development which has been previously overlooked.
GLJ’s Chad Lemke will be presenting a talk on the Duvernay at the CSUR’s Duvernay Advanced Technology & Core Workshop on November 15, 2018, entitled: Setting the Stage: An Overview of the Duvernay Sub-Plays
If you have any questions on the Duvernay or are interested in the above methodology, one of our Duvernay team members Chad Lemke, P.Eng., Warren Bindon, M.Sc. P.Geo., or John Hirschmiller, P.Geo., would be happy to speak with you.
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