Recent governments in Canada have not been particularly supportive of the oil industry, which mainly concentrates in Alberta. Some people are even actively contemplating separation from the Canadian federation and going it alone. Yet, it must still be concluded that despite the narrative and adverse legislation coming from Ottawa, oil production has increased steadily from the North American country.
There is one play that is responsible for this steady production growth. The Athabasca Oil Sands province of northeast Alberta. Situated in the most distant and shallow realms of the Rocky Mountains foreland Basin, the oil sands province hosts one of the largest oil resources in the world. And where open-pit extraction dominated at earlier stages, now it is predominantly through in-situ steam-assisted gravity drainage (SAGD).
How did the oil get where it is currently produced from? And why is it being found at such shallow depths? In this article, we go back to the basics; Graham Spray from Agat Laboratories took the time to walk me through a presentation that he gave for a group of engineers in Fort McMurray, the town that can be considered the heartland of the Athabasca oil sand region.
The basics
First of all, the area in which the oil sands are found is about the same size as England. It is considered that this vast space hosts about 1,700 to 2,000 billion barrels of oil, of which 10 % is currently booked as reserves. The implication of these numbers is that 98 % of Canada’s oil reserves are in the oil sands. And it is all very shallow; the deepest deposits are about 600-800 m, whilst about 10 % of the total area is within mineable depths.
“It is the mines that attract the headlines,” says Graham, “but the vast majority of the resource, especially those at depths greater than 60 m, will be or are being extracted through in situ recovery solutions, which means drilling steam injection and oil production wells.”
Loose sands
The reservoir sands in which the oil is found are unconsolidated. This doesn’t mean that they are young, though; the age is Lower Cretaceous. However, the sands never experienced significant burial, and combined with a mature mineral assemblage, of which 90 % is quartz, cementation is not a particular concern in this play.
“This lack of lithic fragments has another important advantage when it comes to steam-assisted production,” says Graham, “because the mineralogical changes induced by the steam injection do not take place. The upper part of the oil-bearing succession in the area do consist of more immature sands, compromising an effective drainage of the oil.”
The sands of the McMurray Formation, in which most of the oil is found, was deposited by a Mississippi scale river system that drained the North American craton from south to north. “Only a small part of the sand fraction is from the Rocky Mountains that now form such a prominent feature to our west”, says Graham, who is calling in from Calgary. At the time of deposition of the McMurray sands, the Rocky Mountains were not a huge topographic feature yet, as this only started in Late Cretaceous times.
The oil in the McMurray sands, even though the visitor centre in Fort McMurray will tell you that it resulted from in-situ transformation of organic material, is actually from the proximal part of the foreland basin where Devonian (Exshaw) and Lower Jurassic (Nordegg) anoxic shales were buried as a result of mountain building and the subsequent formation of a foreland basin. The oil thus migrated from west to east.
Biodegradation and glaciation
Most operators would prefer finding light oil over heavy oil. But if the oil in the Athabasca province would not have experienced biodegradation when it finally arrived in the area where it is now being found, after a long lateral journey through permeable carrier beds, there would not be any oil to produce left. The reason for that is the glacial cycles that took place in the last two million years.
The sealing units that occur on top of the McMurray sands were put to the test during the repeated loading and unloading as a result of the advancing and retreating ice caps. In combination with the overburden being scraped away – around 500 m was removed in places – the seals would not have been able to withstand the capillary pressure exerted by light oil”, Grahan explained. “It was thanks to the fact that the oils had been biodegraded already, with lower mobility as a result, that the seal breaches did not result in the oil leaking away.”
NO-MAN’S LAND
There is currently a no-man’s land between the areas where surface mining takes place and the areas where SAGD drilling happens. These are the areas where the overburden is too thick for open pit mining, but too thin for drilling. The reason for the latter is that a successful steam injection project needs a sufficiently thick overburden to prevent the steam, which is injected under high pressure, from making its way to surface in an uncontrolled way. That’s why other technologies are trialled in these areas, such as “microwaving” the oil.
The future
“Given the socio-political situation as it is at the moment, I think it is unlikely that more mines will open,” says Graham. This is the reason why we will probably see the gap widen between oil produced through SAGD and mining – at the moment, they are approximately on par with each other.
The technological advancement leading to operators being able to pump more oil out of the ground is increasing rapidly. “Twenty years ago, when I started, it was quite common for operators to achieve a recovery factor of around 20 % using SAGD,” says Graham. “Then I worked for a company that aimed at 60 % recovery, whilst a couple of years ago, a 90 % recovery was not exceptional, even reaching levels of 98 % in the very core of the reservoir.”
What causes this step-change in oil recovery? I ask. “I think a lot is due to well placement techniques,” continues Graham, “but there is a level of underestimation as well. We have a tendency of being wishy washy about things as geologists, but the reality is that the technique is just very effective. When we cut a core from a reservoir that has been stripped off the oil, it looks like white beach sand,” he says.
It looks like Canadian McMurray oil is here to stay for a while.
