Rattle big black bones in the danger zone
There’s a rumblin’ groan down below
There’s a big dark town, it’s a place I’ve found
There’s a world going on underground
-Underground (Tom Waits)
The titular Petro Ninja – Enlighten Geoscience Well of the Week is 100/11-31-002-22W4/00. Why is this abandoned Wabamun exploration well being shown such attention? It isn’t because of this well’s stupendous cumulative production or the massive field it discovered, but for much less initially obvious reasons.
The 11-31 well is of interest because it is well beyond the Triangle Zone, approximately 20 km up-dip from the Cordilleran Deformation Front. Nonetheless, this well has a duplex structure in the Exshaw/Wabamun interval over 2,300 metres into the subsurface (see Figure 1). In isolation, this well is an interesting anomaly. Something you might come across, say, “Hmmmm. That is odd, “and move on.
But this is not an isolated occurrence. Similar structures have been documented in the Upper Cretaceous by authors including Skuce et al. (1992) and Hart et al. (1992). We could discuss some in the lower part of the NEBC Montney but we will remain focused on southern AB for now.
And as the salesman saying goes: “Wait! There’s more!” Warren and Cooper (2017) highlighted an interesting outcrop along the Oldman River more than 70 km up-dip of the Deformation Front. Figure 2 displays a photo montage of the subject outcrop. Their talk abstract concludes with the following observation on the reservoir implications: “Extensional faulting has influenced reservoir and seal facies distribution, and late thick-skinned uplift has influenced hydrocarbon migration and trapping. This deformational history also has influenced development and distribution of fractures critical for enhanced permeability.”
But there is even more! Over the course of describing kilometres of core from hundreds of wells, Graham Davies has observed structural fabrics (including pencil cleavage, limestone ridge and groove structures and polished slip faces) along the Deep Basin trend.
These foregoing observations, in my opinion, reinforce the need to understand these structural features at a variety of scales. Figure 3 maps out the Monarch outcrop, the Skuce et al. duplex fairway and the Davies (2016) structural fabrics relative to the 11-31 well.
Understanding the structural setting of the subsurface extends beyond those features that can be readily identified through seismic. While we are understandably drawn to the structures themselves, the structural profile from Warren and Cooper (personal communication) helps visualize the long eastward trending sub-horizontal thrust that would be indiscernible if you weren’t looking for it. And we haven’t even touched on how the Vulcan Low might have altered these structures.
Given the implications for fractured reservoirs, pressure compartmentalization, migration, induced seismicity, casing deformation, and other important subsurface factors, these horizontal faults are worthy of further attention.
The “Plains” indeed. To paraphrase Mr. Waits, there is a lot going on underground.
This article benefitted from discussions with Mark Cooper, Graham Davies, and Amy Fox. Any errors or omissions are, however, my sole responsibility.
Davies, G., 2016. Slip faces and Cleavage in the Montney and Other Unconventional Reservoirs in the WCSB: Structural, Geomechanical and Drilling/Fracking Implications. CSPG International Core Conference/AAPG Annual Convention & Exhibition 2016.
Hart, B. S., Marfurt, K. J., Varban, B. L. and Plint, A. G., 2007. Blind thrusts and fault-related folds in the Upper Cretaceous Alberta Group, deep basin, west-central Alberta: implications for fractured reservoirs. Bulletin of Canadian Petroleum Geology, v. 55, no. 2, p. 125–137. https://doi.org/10.2113/gscpgbull.55.2.125
Skuce, A. G., Goody, N. P., and Maloney, J., 1992. Passive-roof duplexes under the Rocky Mountain Foreland Basin, Alberta, Canada. American Association of Petroleum Geologists, Bulletin, v. 76, p. 67-80.
Warren, M. J. and Cooper, M., 2017. Deformation history in the southern Alberta foreland basin and petroleum system implications. GeoConvention 2017, Calgary, AB. May 15 – 19. [https://geoconvention.com/wp-content/uploads/abstracts/2017/268_GC2017_Deformation_history_in_southern_AB_foreland_basin.pdf] accessed 2023-04-04