Tex. Nat. Res. Code § 87.171

Any natural gas, including casinghead gas, produced from any gas well or oil well in this state, containing less than one and one-half gallons of propane and heavier hydrocarbons per 1,000 cubic feet, as determined by fractional analysis made of the gas, may be used for the manufacture of carbon black in a plant producing an average recovery of at least one and one-half pounds of carbon black for each 1,000 cubic feet of gas consumed.