Where a natural reservoir from which there derives gas to which section 1 above applies is used for the storage of gas produced elsewhere which is acquired by F1any person, gas levy F2shall be payable by that person for any year only on the amount by which the number of therms represented by the gas supplied to F3that person from that reservoir during the year exceeds the number of therms represented by the gas put into the reservoir during the year.

If in any year the number of therms represented by the gas put into such a reservoir has exceeded the number of therms represented by the gas supplied to F4the person concerned from the reservoir, the amount of the excess shall be carried forward and, for the purpose of computing gas levy for a subsequent year, may be set off against the number of therms represented by the gas supplied to F5that person from the reservoir in that year.