Oil, natural gas, and oilfield water are all stored in the pores of underground rocks. Influenced by their sources and geological factors, their chemical compositions and physical properties vary from one region to another. Understanding the basic properties of oil, gas, and water is of great significance for clarifying the formation, migration, and accumulation patterns of oil and gas, and for developing rational exploitation and processing plans. This chapter mainly introduces the chemical composition, physical properties, and classification of petroleum, natural gas, and oilfield water. It also briefly discusses the stable isotopes in oil and gas, particularly the compositional characteristics of stable carbon isotopes.