If you can use that energy to mine bitcoin, why not use it to power a data center or put it into the grid? Theres nothing unique about crypto here, its just one potential way to use recaptured efficiency, and not necessarily the highest ROI method either. Especially for oil companies, where this is outside their core competency and not something they would likely want complicating their balance sheet with all the mark to market requirements and all that would make holding crypto a particular sort of pain