Mont. Code Ann. § 77-3-433

The royalty on gas, including casinghead gas and all gaseous substances not sold or used off the premises, must be at the rate of $400 per lease each year or the amount of the annual rental provided in the lease, whichever is the greater, payable on or before the annual anniversary date of the lease. As long as the leased lands contain a well capable of production in paying quantities and the requisite payment is made, the lease must be considered as a producing lease under the lease terms.