The van Krevelen diagram indicates the carbon-hydrogen-oxygen composition of organic matter. It is a plot of the atomic ratio of oxygen to carbon (ranging from 0 to 0.3) on the abscissa vs. Atomic ratio of hydrogen to carbon on the ordinate (ranging from 0 to about 1.75). Different kinds of organic matter are more likely to produce oil during thermocatalytic maturation than other types: Algae, type 1, H-rich, O-poor, tend to make good oils. Trees and other woody vegetation are made of type 3 organic matter, O-rich, H-poor, and are sources of coal and gas.
It tells you the depth at which that particular type of hydrocarbon was formed, thus making it easy to track hydrocarbons.