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First, the "tens of thousand" more heavy oil than lightr crude is false. To get this figure, you probably included all organic content in the lithosphere, as if there was a remote chance to gzet anything usable from a 5 km deep shale with 1% of kerogen. You got confused "light oil", and "conventionnal oil". what you categorized as "low-grade oil" is, for a large part, not oil at all (kerogen). 

Second, there is a important notion you ignored  net energy yeld. The vast majority of low grade fossil "fuels", like deep oil shale, would consume more energy to extract than they would give. Already, the Athabasca oil sands have a self-consumption of about 30% - that could be reduced to some 25% with advanced technology. The small-sclae oil shale projects that were teste had a near-zero net energy yields - except in estonia, but the shales they used were among the richest known.

Third, extraction rate is as important as reserves. Non conventionnal oil (biodegraded oil, bitumen, oil heavier than water...) is very difficult to take out. Kerogen from shale is even worst. Even if these ressources are in the trillions of barrels, they won't provide much help if we can only get a few million barrels out per day.

Fourth, non-conventionnal has an high environmental cost. CO2 emeissions on a full-cycle basis (from well to end use) are much higher. There are other problems like the destruction of ecosystems (for open-pit production of both oil shale of oil sands), water use (especially for oil shale), water pollution, disposal of wastes (for oil shales)... These environmental costs would become catastrophic is non-conventional production was to be upscaled to the same range of output than current conventional production.