"Subprime carbon assets" was Al Gore's choice expression in An Inconvenient Truth regarding Tar sands. But about them one thing is clear: there is lots and lots of energy in them thar Tar sands, subprime though they may be.
Tar sands in Canada and Venezeula are not equivalent to oil, as Chris Vernon explains: "A barrel of tar-sand oil reserves isn’t the same as a barrel of conventional oil reserves since it can’t support the same level of flow rates. We currently have around 1000 billion barrels of conventional oil reserves and these support a flow rate of around 31 billion barrels per year (84-85mbpd). If these 3500Gb were equivalent we could expect today’s tar sands and oil shale to support flow rates as high as 109 billion barrels per year (300mbpd). It is clearly impossible to draw 300mbpd from these reserves so they aren’t equivalent to conventional reserves from a peak oil point of view."
A debunker of Peak Oil notes that "In a way, tar sands are the best oil source. You can't pump them very fast, so they have two virtues: they will slow down our consumption, and they will last a very long time."
The biggest defect of tar sands and other unconventional sources is that they require for their exploitation the use of a tremendous amount of energy. Their energy return on investment is miserably low by comparison with oil itself. On every score, their environmental impact is grim.
If you take seriously the case for global warming and all that follows, the Tar sands option is the kiss of death; by comparison coal looks terrific. Both, however, would do less harm if a reliable way were found to capture and sequester carbon emissions--another question surrounded by considerable uncertainty.
The existence of these vast oilsand reserves is an important fact. It constitutes a potential corrective to the Peak Oil view and introduces another complication in calculating reserves. Reserve calculations based on these "unconventional" sources lead experts such as Peter Odell to conclude that carbon fuels will dominate the 21st century.
One possibility has to be allowed for: Developing the tar sands in Canada and Venezuela, to sustain our dependence on energy, will seem to many a tempting route, the prospect of climate change be damned.
It's just like the smoker thinking about quitting who is always weighing present need against an uncertain great hereafter, except that "humanity" has no center of decision. Even if half want to quit smoking, what will drag the other half along?