You will be surprised how quickly nature deals with spilt oil, in the 80's we used to go to a stunning Spanish beach just up from Gibraltor, passing oil tankers used to clean their tanks and discharge them, the outcome was our sandy beaches were a mess of tar balls. Usually it took only weeks to completely grind down the tar with the waves and sand and it was surprising how quickly it dissappeared from the water line and that further up the beach seems to break down with the sun and elements. Our kids became realy adept at removing it from their feet with the best tar remover of all, olive oil.

Obviously quantities are greater here but I wouldn't be to dispondent about long term spoiling of your waters once the well is capped.