All royalty required to be paid hereunder, when oil is to be delivered, at the option of the Department, at the mouth of the wells into tanks provided by it, or into the pipe line with which the wells may be connected, to its credit; and for royalty, when gas, including casinghead gas or other gaseous substance, the lessee shall account to the Department for the market value thereof at the well for all that may be sold by lessee or used by lessee in the manufacture of gasoline or other product therefrom. At the election of the Department, in lieu of delivering the royalty, when oil, in kind, the lessee shall purchase the same at its market value at the well when produced.
5 ILCS 615/23