The McMurray Formation is a package of Cretaceous rocks in northern Alberta and adjacent areas of Canada that was laid down in early Cretaceous time, so I should have talked about it earlier this month, but I didn’t.
The basin that the McMurray sediments accumulated in was an early aspect of the Cretaceous Interior Seaway that reached its height in Late Cretaceous time. The sea invaded from the north, the Arctic, so the transgression of the sea affected Canada earlier than it did the western United States. The McMurray environment was similar to that of the shores of the later Cretaceous Interior Seaway – rivers, deltas, estuaries, and coastal plains. Settings like that typically give rise to interbedded and alternating sands in river channels and beaches, silts in the more distant parts of deltas and estuaries, and muds in overbank deposits, flood plains, and in the parts of the coastal sea even more distant from shore.
What makes the McMurray special is that it holds one of the largest accumulations of hydrocarbons in the world – the Athabasca Tar Sands. You sometimes hear this called oil sands, but it really isn’t oil in the sense of liquid oil. It’s solid bitumen, tar, oil that has lost the volatile components that help make oil liquid. It’s called the Athabasca Tar Sands because the McMurray formation crops out in the drainage basin of the Athabasca River.
|Map by Norman Einstein, released to public domain.|
This stuff, because it is solid, is a lot harder to produce. About half of production is mined, dug out of the outcrop, and the rest is produced using steam injection and other methods to essentially melt the material in the subsurface to liquefy it. All methods combined yield about 2.3 million barrels of oil per day – more than half of Canada’s total oil output. Canada is the #5 oil producer in the world, after the U.S., Russia, Saudi Arabia, and China, and Canada is the leading source of U.S. imports, at more than 3.5 million barrels per day – more than all OPEC nations combined. Canadian imports amount to more than one-third of all US imports, which totaled about 9.2 million barrels a day in September 2014, and Canadian imports made for about 19% of total US consumption, which is just over 19 million barrels a day.
Canada projects that it may be able to double its production of oil from tar sands by 2020.
Leaving apart the controversies over this production, I’m going to focus on the origin of the deposit. I used to think it was more or less just an oil reservoir at the surface with the volatiles gone, but it’s more complicated than that.
First, there’s the question of the source rocks for the bitumen. The most common speculations for source rocks include marine Devonian carbonates or Triassic-Jurassic shales, or both. Those rocks are known to be the sources of conventional oil in the Western Canada Sedimentary Basin, but they may be inadequate to generate the volumes we see in the tar sands. It’s been suggested that they formed in place, from organic matter washed into the McMurray formation along with the sands and shales, or that it was derived from non-marine Jurassic and Lower Cretaceous coals.
There’s also argument about the migration history – how the bitumen got there – ranging from in-place formation to migration from deep sources. The process was complicated by dissolution and collapse of salt-bearing horizons in the Devonian. Their collapse changed the topography of the surface the McMurray sands were deposited upon, so that some places had thicker sands than others, and were more favorable for the oil and bitumen to accumulate.
Agreement is not unanimous, but I think there’s at least a slight preference for the Jurassic Fernie formation, a black shale, as the ultimate source of the organic matter. It would have migrated in a fairly standard way, and then it would have to be degraded, driving off the volatiles to leave the heavy, viscous tarry stuff behind in the sand reservoir. The standard view of degradation of oil to make bitumen is that it’s driven mostly by bacterial action – microbes consume the lighter, more volatile compounds in the oil and leave the tar behind. That certainly happens, but it begs the question of why did it happen on such a massive scale here in Canada. Even the weight of glacial ice on the reservoir may have been a factor in the degradation of the oil from a fluid to the bitumen, or from solid coal to tarry stuff.
There are lots of other tar sands and accumulations of heavy oil around the world, but the one in Canada and the one in Venezuela – which may be even slightly more voluminous than the Canadian deposits – are by far the largest.
—Richard I. Gibson
Geologic features of tar sands
Geology of the Oil Sands (PDF)
Origin of the tar sands
Western Canada Basin oil – Jurassic sources
Map by Norman Einstein, released to public domain.