Great Idea, unless you end up upside down...
Say one buys 500 gallons at $2.00/gl. That works out great if the price climbs above that during the time ones 'reserve' is being drawn down. It sucks if the price drops below. It wouldn't be so bad if there was no time limit on how long one had to use it up... In the example above, one then could just pay the lower price at the pump, and hold back their 'reserve' $2/gl fuel until retail prices climb above it)
Same thing happens with home heating fuel. Some folks agree to pre-buy a quantity of fuel during the summer (when prices tend to be lower) When winter comes, they are contractually obliged to buy a set quantity of fuel at that price, even if it's selling for a lot less at the time it's delivered. Good luck getting out of a contract like that.
So unless there is no time limit on how long one can retain the option, it's a lot like gambling IMHO.