Hydrocarbon Column Heights – Part 2

by Kenneth C. Hood, Guest Contributor

Part 1 of this discussion illustrated the impact of the approach used to represent candidate hydrocarbon column height distributions and the importance of checking for geologic plausibility. Ideally, this checking should take place prior to any assurance review with an assurance team member who specializes in aspects of seal integrity, but many times that checking is done at the assurance review. The reason the checking is important is because of the dramatic influence the column height can have on prospect metrics. That is the focus of Part 2 of this blog on column height.

impact of column height on opportunity evaluation

The selected column height (or contact depth) distribution can dramatically impact the volumetric outcomes for an opportunity, and thus on the economic chance of success. The top of the figure 2 (with the depth y axis) illustrates seven alternative column height scenarios (shown as bar frequency and cumulative line frequency displays) applied to the same 500 m relief trap (with a schematic cross section shown on the far left adjacent to the depth axis). The bar chart immediately below shows for each scenario the percent of geologic successes that are economic failures.

other considerations

Often alternative scenarios will be required for a column height distribution. As an example, it may be necessary to use different column height distributions for oil, multiple-phase, or gas accumulations. Prospects for which a seismic-based Direct Hydrocarbon Indicator (DHI) constrains the potential range of contact depths provide a special case. The seismically constrained contact can be represented as a separate scenario, or as part of a weighted contact distribution. The weight associated with that outcome should be correlated to the DHI chance of validity (the higher the chance of validity, the more weighting can be assigned).

Because of the complexities associated with column height distributions, it is best to avoid using a single deterministic analysis to evaluate the hydrocarbon potential for an opportunity. Using the filled-to-spill case is generally too optimistic for representing opportunities. Generally, a full probabilistic analysis must be created to determine the mean or median column height that would be used in a single representative case.

key take away messages

1. Consider using shared column height distributions for families of related or similar prospects where appropriate.
   • Empirically based distributions based on analog fields and discoveries
   • Model-based distributions based on regional seal characteristic
2. Build analyses with separate contact and spill distributions.
   • Define contact (column height) distribution based on seal capacity
   • Truncate with spill distribution based on prospect-specific geometric controls (e.g., synclinal spill, reservoir juxtaposition)
3. Follow the appropriate column height guidance for DHI prospects.
4. Robust representation of Column Height distribution will frequently require multiple scenario analyses
   • Distinct distribution by commodity type (oil, gas, dual phase)
   • Distinct distribution by seal capacity
   • Follow a simplified, “merge late” rather than “merge early” philosophy where possible (multiple, simple cases rather than fewer, complex cases)
   • Simplifications are acceptable for screening analyses
   • Automation makes this easier

acknowledgements

I thank ExxonMobil for releasing this material. Many colleagues have contributed to this work.

reference

Hood, K., 2019, Hydrocarbon Column Height, Presentation at the 2019 Rose & Associates Risk Coordinators Workshop # 17, Houston, Texas.

About the Author

Ken Hood holds a Ph.D. in Geology from The University of Kansas. Ken retired in 2020 after 31 years with ExxonMobil. Much of his career was spent working in assessment and assurance of conventional and unconventional resources at play and prospect scales.