Oil well and natural gas well operations; exploration for crude petroleum and natural gas and drilling, completing and equipping wells when not performed on a fee or contract basis; operation of separators, emulsion breakers, desilting equipment, topping and all other activities involved in making oil and gas marketable up to the point of shipment from the producing area. Also included are the mining of oil shale and oil sands and the extraction of oil therefrom; and the production of liquid hydrocarbons from oil and gas field gases. Recovery of liquefied petroleum gases incident to petroleum refining or to the manufacturing of chemicals is classified in the appropriate group of division 35 (Manufacture of chemicals and of chemical petroleum, coal, rubber and plastic products). The independent operation of oil and gas pipe lines is classified in group 7155 (Pipeline transport).