[Eric in the Ozarks]

The coarse reject material (shale after oil removed) becoming 55 percent larger was something no one expected. If Exxon had a couple of mining people on the payroll they could have predicted much of this.
I was mining coal in the early 80s. Drilling and blasting the overburden, stacking via dragline and then dozing with a D9L would result in a higher topography, even if 6 feet of coal had been removed.
This was hard to explain to the uninitiated.
If we mined into a hillside and the cover got deeper as we advanced, we could be out of fill at the top of the hill...

[IronJack]

I thought the reason Exxon gave for the pullout was that the kettle extraction process then being considered was just too expensive and the yield too small.

Frankly, I'm not sure the public was ever given a clear reason. Exxon just locked the gates and walked away, taking losses that had to run in the billions.