I've read in various places that the economics of shale petroleum extraction are prohibitive. On the other hand, I've read that "fracking" is economically feasible, but that it leaves an unstable geology behind. Any thoughts on these issues?
Fracking has been a common practice in Texas for sixty years.
Whatever you're reading that mentions "unstable geology" is a piece of environmental propaganda BS.
Sorry, there's no other way to put it.
I think you may be confusing oil sands (petroleum) with shale gas. This is the latter. No oil to be extracted.
"On the other hand, I've read that "fracking" is economically feasible, but that it leaves an unstable geology behind. Any thoughts on these issues?"
The amount of rock that is "fractured" is miniscule compared to the total shale mass. Probable effect = zero.