[AndyJackson]

The seller cannot put off the delivery. If he sells a July contract, he has to deliver between August 1st and August 31st.

But the seller can buy back the contract and sell a new one. Since the long side only index speculator doesn't want delivery they are both happy.

[Toddsterpatriot]

But the seller can buy back the contract and sell a new one.

Why would a guy with an oil well want to buy back a contract? Make up your mind, you said "likely a supplier who is quite happy to deliver you the barrel at the contracted price".

If he's happy to deliver, he'll be selling, not buying, contracts.