"PETROLEUM" derived from the Latin words petra (rock) and oleum (oil), is natural organic material composed principally of Hydrocarbons which occur in gaseous or liquid state in geological traps; the liquid part after being freed from dissolved gas, is commonly referred to as crude oil. From the outset it must be stresses that crude oil is not a uniform substance: its appearance and characteristics vary from oilfield to oilfield, and even from well to well in the same outfield. Thus each separate accumulation of petroleum is a unique mixture, not matched exactly in composition by any other sample; and it may not even be homogeneous, since it can contain colloidal suspension besides dissolved gases and solids. The types of carbon compound (organic compound) present in the crude oil are defined and exemplified. Readers are reminded that organic compounds may be classified according to the basic scheme set out below. All the classes shown are represented in Petroleum. Refining crude oil is the art of choosing the crude oils so far as choice may exist and of using the flexibility of the refinery process available so as to make the product required  for the market in the way showing the best financial return for the cost expended. Integrated refinery - BLOCK Diagram