In every oil and gas lease granted pursuant hereto, there shall be reserved to the lessor a royalty of not less than twelve and one-half percent of all oil, gas, hydrocarbons and all other petroleum products produced and saved from the lands covered thereby and not used in connection with the development and operation of the leased premises, or twelve and one-half percent of the market value thereof at the leased premises. Such leases shall provide that the interest so reserved shall be produced free of cost to lessor and that an accounting shall be made therefor on a monthly basis.