In my kick-off to this series on the Devonian beneath the oil sands, I introduced the interval and explored its relevance to the development of the bitumen resource held in the sands of the McMurray. I follow up with a more in-depth look at the geology and hydrology.
The lunch special
The understanding of geology in the area is anchored by information obtained by wells drilled over sixty-plus years. The thousands of wells drilled in the Athabasca area have painted a detailed and very complicated picture of the post-Devonian sediments. However, these wells rarely penetrate the Devonian more than a handful of meters so their ability to characterize the deeper strata is limited, though some keen insights into the Devonian have been gleaned from these data by Dr Paul L. Broughton. Still, the understanding of the older strata is based on relatively few penetrations.
Based on the available well control, the Devonian in the Athabasca area can be simplified as an evaporite sandwiched between carbonates. Although, someone has eaten some of the filling (halite dissolution) and taken bites of the bread from the top (sub-Cretaceous Unconformity). As illustrated in the cross-section below, the McMurray sediments are deposited on a tilted, partially eaten lunch (last sandwich analogy, I promise).
Build a Devonian framework
Let’s build the Devonian stratigraphic column for the Athabasca area up from the bottom (in 150 words or less). The granite basement was flooded by marine water during the Middle Devonian, depositing the silty shale to argillaceous carbonate of the Contact Rapids Formation. Deposition continued with shallow-marine carbonates of the Keg River, which eventually included reef mounds in some locations. After this period of carbonate deposition, the marine environment became restricted, initiating the deposition of several evaporitic strata, including the Prairie Evaporites. The extensive platform carbonates of the Beaverhill Lake Group were deposited on top of the evaporites during Middle and Upper Devonian time. Additional Devonian strata were deposited on top of the Beaverhill Lake Group, but they were subsequently eroded along with some of the Beaverhill Lake, forming the sub-Cretaceous Unconformity. All this and more can be gleaned from the Chapters 10 and 11 of the Alberta Geologic Survey’s Geological Atlas of the Western Canadian Sedimentary Basin and a 2013 AGS Open File Report by Schneider et al.
A general understanding of the stratigraphic column is a start, but because the dissolution removal of the Prairie Evaporite is so important, some knowledge about the hydrogeology as required as well (and hey, if you can’t differentiate between aquiclude, aquitard, and aquifer, take a moment to read up). The Devonian in the Athabasca region was broken into several hydraulic units by Hackbarth and Nastasa (1979) and then refined by Bachu et al. (1993). The hydraulic system they describe is relatively simple: the evaporites of the Middle Devonian act as an aquiclude between the D-1 and D-2 aquifers.
The hydraulic characterization described so far applies to the western portion of the area, where the Prairie Evaporite is intact. Hackbarth and Nastasa go on to explain that to the east, the Prairie Evaporite is not an aquiclude because it has been compromised through one or more of faulting, brecciation, and removal. Without an aquiclude, the Devonian behaves as a single aquifer.
Hackbarth and Nastasa used borehole measurements to determine fluid flow in the aquifers. I added their flow data to the E-W cross-section below. The most striking thing is that the flow in the Devonian is toward the Athabasca River. The river is a pressure sink into which the aquifers flow.
The most striking thing is that the flow in the Devonian is toward the Athabasca River.
The water flowing through the aquifers in the image has different sources. The water flowing through the Devonian from the east is meteoric (young, fresh water). Some of the rain water absorbed by the ground in the east makes it into the aquifer and flows toward the Athabasca river. The water flowing through the D-1 aquifer (the deeper one) from the west is old and salty, sourced from the rocks buried deep in the Western Canadian Basin. The water flowing through the D-2 aquifer (the shallower one) from the west is a combination of old and new, a mix of basin waters and fresh water that makes its way down through the Lower Cretaceous sediments. The differences of water chemistry in these aquifers will come into play when we look at the dissolution of the Prairie Evaporite.
It’s a start
With this very quick look at the Devonian hydrogeology, you can start to think about some of the issues I mentioned in the first post of this series. If your mine experienced an unexpected influx of saline water, where would you think it was coming from? Should the design of your SAGD plant depend on which aquifer you source your water from? Does injected water stay near an injection site? In all likelihood, you have more questions now than when you started. That’s a good thing! I’ve provided links to all the publications I cited so you can begin your own explorations on this topic.
Now that we’ve built up this subsurface framework, we’re ready to look at the dissolution mechanisms responsible for removal of the Prairie Evaporite. Stay tuned for that in the next post in this series.
Bachu, S., Underschultz, J.R., Hitchon, B., and Cotterill, D. (1993). Regional-Scale Subsurface Hydrogeology in Northeast Alberta. Alberta Research Council Bulletin 60.
Carrigy, M.A., Kramers, J.W. (1973). Guide to the Athabasca Oil Sands Area. Oil Sands Symposium, 1973, Canadian Society of Petroleum Geologists.
Drees, N.C. Meijer (1994). Chapter 10 – Devonian Elk Point Group of the Western Canada Sedimentary Basin; in Geological Atlas of the Western Canadian Sedimentary Basin, G.D. Mossop and I. Shetsen (comp.), Canadian Society of Petroleum Geologists and Alberta Research Council. http://ags.aer.ca/reports/atlas-of-the-western-canada-sedimentary-basin.htm, accessed March 2016.
Hackbarth, D. A., and Nastasa, N. (1979). The Hydrogeology of the Athabasca Oil Sands Area, Alberta. Alberta Research Council Bulletin 38.
Oldale H. S.,Munday R.J. (1994). Chapter 11 – Devonian Beaverhill Lake Group of the Western Canada Sedimentary Basin; in Geological Atlas of the Western Canadian Sedimentary Basin, G.D. Mossop and I. Shetsen (comp.), Canadian Society of Petroleum Geologists and Alberta Research Council. http://ags.aer.ca/reports/atlas-of-the-western-canada-sedimentary-basin.htm, accessed March 2016.
Schneider, C.L., Grobe, M. and Hein, F.J. (2013): Outcrops of the La Loche, Contact Rapids, and Keg River formations (Elk Point Group, Devonian) on the Clearwater River: Alberta (NTS 74D/9) and Saskatchewan (NTS 74C/12); Energy Resources Conservation Board, ERCB/AGS Open File Report 2012-20, 36 p.
Feature image (pipes) from public domain.
Bibliography updated April 22, 2016