In this part, the carbon cycle is explained. First, the concepts of organic carbon, inorganic carbon and phytoplankton are explained since these are crucial for understanding the carbon cycle. After that the carbon cycle and moreover mass balances that go together with the carbon cycle are presented as well. || The development of Flora and Fauna is explained by interpreting the partial pressure of several gases in the atmosphere. || The composition of different biomolecules and their abundance in different organisms is explained. || Preservation of organic matter is first explained. It can be seen that source rocks are the main preservation zones, which indicates the low efficiency of the preservation process. The concepts of Total Organic Carbon (TOC) and kerogen as a content of that in source rocks are eludicated. || The conversion of kerogen in source rocks to hydrocarbons like oil and natural gas are explaind. The "van Krevelen" diagram is used to illustrate this. Several types of maturation indicators, used to evaluate potential source rocks, are discussed.