Van Krevelen diagrams are a graphical-statistical method that cross-plots the hydrogen:carbon (hydrogen index) in function of the oxygen:carbon (oxygen index) atomic ratios of carbon compounds. Different kinds of organic matter

Organic matter

Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...

 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

Coal

Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

 and gas

Gas

Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

. This diagram is mostly used for differentiating different types of kerogen

Kerogen

Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...

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