[thackney]

a big recovery plant there back in the 1970s but the project fizzled for technological reasons

Not technological reasons, 100% due to economics. At that time, cheaper oil was available to meet demand, once the OPEC embargo had ended.

[Bernard Marx]

Point taken. But the "technological" issue was producing shale oil at a price competitive with foreign oil. That's still a fly in the ointment, especially with new competition from fracking.

The "in situ" retorting process will probably always be more expensive than other oil-extraction methods and I think environmental concerns will prevent strip-mining for oil shale