Ill. Admin. Code tit. 86, § 475.120
a) All oil and gas removed from the premises where severed is subject to tax imposed by the Tax Act unless exempt under the terms of the Tax Act [35 ILCS 450/2-15]. The following severance and production of oil is not exempt from the tax imposed by the Tax Act:
b) Oil produced from a well whose average daily production is 15 barrels or less for the 12-month period immediately preceding the month of production is exempt from the tax imposed by the Tax Act [35 ILCS 450/2-15(b)].
3) When a well or production unit qualifies for the exemption based on the average daily production, the operator is responsible for notifying the purchaser of the exemption. The operator has 30 days from the end of the 12-month period to notify the first purchaser that a well or production unit no longer qualifies for the exemption. The first purchaser has 30 days to make the change to its records after it receives notice from the operator. The exemption begins on the first of a month. Once a well or production unit qualifies for the exemption, the operator annually shall determine whether oil production from a well or production unit remains exempt and shall notify the purchaser when the exempt status of a well or production unit changes.
c) The following severance and production of gas shall be exempt from the tax imposed by the Tax Act:
3) gas lawfully vented or flared; and