Under the Sandbox

kicking off a series on the Devonian beneath the oil sands


Almost all of Canadian oil reserves are held as bitumen in Alberta (97% according to CAPP’s Statistical Handbook). Extraction of this heavy oil, especially the resource in the McMurray formation in the Athabasca region, has received more than its share of media attention as its development has been politicized.

I spent the most recent part of my geoscience career studying the McMurray bitumen play. Politics aside, the development of the Athabasca oil sands is an intriguing subject because the complexity of the reservoir is not well understood and can have a dramatic affect on resource production.

The depositional complexity of the McMurray has a strong influence on in-situ production, especially Steam Assisted Gravity Drainage (SAGD). To understand the complexity of the reservoir, the company I worked for acquired 2D and 3D seismic data. Though there were many wells in the area, seismic data was seen as necessary to guide well correlations and perhaps predict the reservoir properties between wells.

As the team Geophysicist, I got to delve into the seismic. Cracking into new seismic is pretty special. It feels like opening a gift. For the most part, you find what you’re expecting, but you’ll always discover something surprising. The McMurray is certainly interesting on seismic, as I knew it would be. The surprise for me was the dramatically structured Devonian strata immediately beneath the Athabasca oil sands. The bright, beautiful  patterns caught my attention (beauty is subjective, right?).

My colleague was familiar with the pre-McMurray interval, owing mostly to a mining-oriented graduate geology thesis. Through conversations with her, as well as some literature search, I began to track down the genesis of the fascinating geology. The McMurray was deposited on top of a vast carbonate surface (the sub-Cretaceous unconformity surface) (Alberta Geologic Survey Atlas, Chapter 18Chapter 19). The carbonate sediments had been exposed for a long time and subjected to erosion and weathering; but more importantly for the deposition of the McMurray, the topography of the carbonate was strongly influenced by the solution removal of subsurface salt. Salt removal caused the carbonate to subside tens of meters (more than a hundred in places), leaving an extensive topographic low. The fluvial-estuarine system of the McMurray is mostly confined to this low topographic trend. Those interesting features I saw on the seismic were the result of karst deformation of the Devonian rocks beneath the McMurray oil sands.

simple section
A simple E-W cross-section through Fort McMurray. From Guide to the Athabasca Oil Sands Area.

Investigating the karst features became my pet project. When I could find time, I’d read a little or snag my friendly geologist to chat over a coffee and speculate about the mechanism that could remove all that salt. Solution removal implies that the water moving through the interval comes into contact with the salt and dissolves some and then continues on somewhere. It’s eventually redeposited by chemical reaction or precipitation, or held in solution. We speculated about water moving up, down, left, and right, but nothing we came up with really made sense. Our curiosity remained unsatisfied, but we had hit upon the idea that understanding the hydrogeology — particularly the hydrodynamics — would be key to understanding the Devonian structure.

Well-Deserved Attention

I was psyched when I read the technical program for the 2014 GeoConvention (the 2016 edition is coming soon – early bird registration ends February 3). Calgary’s annual geoscience convention featured whole sessions on the interval beneath the oil sands. Not only were other professionals interested, their abstracts showed they held a much more mature understanding of the Devonian. I attended as many of the talks as I could. The sessions didn’t disappoint: I learned a ton.

The significant and unexpected influx of saline water into a mine pit underscored a gap in understanding

Industry had become very interested in the hydrogeology of the Devonian in the oil sands area after a 2010 incident at Shell’s Muskeg River Mine (some details in this Edmonton Journal article on a news aggregator site).  The significant and unexpected influx of saline water into a mine pit underscored a gap in the understanding of the geology and hydrology beneath the McMurray. Several of the industry’s large players presented work at the conference on the geology under the McMurray (Shell, Nexen, Chevron, and Imperial), highlighting that this topic was an area of active investigation for a variety of reasons.

Flooding a mine with water that is not easily disposed of negatively affects the economics of a bitumen mining operation. In-situ bitumen producers were interested as well. How would McMurray SAGD production be affected by a hydraulic connection to a Devonian aquifer? Production modelling could factor it in if they understood the nature of the hydraulic connection. Would the chemistry of the Devonian water affect the productivity of a steam chamber? These issues appeared on the radar of conscious operators.

The evolution of understanding of the Devonian hydrogeology is also of interest to operators who target the interval for water source and/or disposal. Though bitumen producers are able to recycle most of the water needed for production, make-up water is required, perhaps sourced from the Devonian. Understanding the chemistry and deliverability of the aquifer over the lifetime of the project is critical. Non-recyclable wastewater must be disposed of, often using wastewater injection wells (however, at least one production facility uses the Zero Liquid Discharge approach, eliminating the need for disposal wells). Understanding how a disposal interval is plumbed is a prerequisite for water injection.

Devo Shelf
East bank of the Athabasca River opposite Fort MacKay. Drape in the Upper Devonian Waterways Formation resulting from the solution of the underlying salt beds. From Guide to the Athabasca Oil Sands Area.

What if salt dissolution persists today? New karst features could emerge. New sinkholes would be especially concerning for the cap rock integrity of a SAGD steam chamber, or to projects with key infrastructure built on the Devonian carbonates (Suncor’s bridge across the Athabasca River comes to mind). The likelihood of new sinkholes might be small — I haven’t heard of any causing problems for operators — but understanding the circumstances in which salt dissolution occurs today will inform where the risk of instability must be considered.

I recall seeing a map in a paper or at a conference which showed of a chain of small, circular lakes in the Athabasca area. They were presented as evidence that karstification could continue today. Sadly, I couldn’t find a reference or note in my files, but I did find the same or similar features on Google Maps. Rumour-mongering? Maybe. I couldn’t resist.

sinkhole lakes.png
Speculation: chain of possible sinkhole lakes near Bitumont, Alberta. Google Maps.

Devonian: The Series

As a kick-off to my series of articles on the Devonian beneath the oil sands, I wanted to show why the interval is of interest. More than just my natural curiosity, there are economic and environmental reasons to pursue the understanding of the Devonian. Following posts will take a closer look at some of the previous work on the interval, then bring some novel thinking to light.

For what other reasons are people interested in this interval?

Feature image (sandbox) from public domain.

AGS links updated February 29, 2016

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