Why can’t be paltry?
I thought as:-
Most productive and less ___ go in support
Hence less (unimportant or paltry)
The question is hinting that as the good ol’ oil reservoirs are drying, the task of finding new oil reservoirs even given the high cost and not a good location will seem less burdensome and more acceptable.
You need a word that goes in with less and acceptable. Also, if you fill in paltry it will say that the job at present is worthless and now it will become less worthless and somewhat acceptable . But the clue is high cost and potential risk of drilling at marginal location (this is a hard task )–> so , when we fill the blank with onerous we meant to say that it will be less difficult to deal with and will become acceptable.