Mont. Code Ann. § 77-3-422

If oil or gas is not being produced from the leased premises at the expiration of the primary term of the lease but the owner of the lease is then engaged in drilling on the premises for oil or gas, then the lease continues in force so long as such drilling operations are being diligently prosecuted. If oil or gas is recovered from any such well drilled or being drilled at or after the expiration of the primary term of the lease, the lease continues in force so long as oil or gas in paying quantities is produced from the leased premises.