26 C.F.R. § 1.613A-5
The election under section 613A(c)(4) is an annual election which the taxpayer may make by claiming percentage depletion deductions for the taxable year based upon such election. The election may be made, on an original or amended tax return or a claim for credit or refund, at any time prior to the expiration of the statutory period (including any extensions thereof) for the filing of a claim for credit or refund by the taxpayer. The election may be changed by the taxpayer by filing an amended return or a claim for credit or refund. The election allows the taxpayer to treat as his depletable natural gas quantity an amount equal to 6,000 cubic feet multiplied by the number of barrels of the taxpayer's depletable oil quantity to which the election applies. The election applies to secondary or tertiary production, as well as primary production, but in determining the taxpayer's depletable natural gas quantity with respect to secondary or tertiary production the taxpayer's depletable oil quantity shall be determined without regard to section 613A(c)(3)(A)(ii) with respect to production from secondary or tertiary processes.
26 C.F.R. §1.613A-5