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The composition and nature of oil
Crude oil, commonly referred to as petroleum, is a dark brown, viscous, and flammable liquid extracted from deep underground reservoirs. It is formed over millions of years from the remains of ancient marine organisms that have undergone complex chemical transformations under high pressure and temperature. As a fossil fuel, it shares similarities with coal in its origin and energy content. The physical properties of crude oil vary significantly depending on its source. Its density typically ranges between 0.8 and 1.0 g/cm³, while its viscosity can differ widely. The freezing point can range from as high as 30°C to as low as -60°C, and the boiling point can extend from room temperature up to over 500°C. Crude oil is generally insoluble in water but can form emulsions when mixed with it.
Chemically, crude oil is primarily composed of carbon (83–87%) and hydrogen (11–14%), with smaller amounts of sulfur (0.06–0.8%), nitrogen (0.02–1.7%), oxygen (0.08–1.82%), and trace metals such as nickel, vanadium, and iron. Hydrocarbons—compounds made of only carbon and hydrogen—make up about 95–99% of the oil. However, compounds containing sulfur, nitrogen, and oxygen are considered impurities, as they can negatively affect the quality of refined products. These substances are usually removed during the refining process to improve performance and reduce environmental impact.
The composition of hydrocarbons varies depending on the source of the oil. They are generally classified into three main groups: alkanes, cycloalkanes, and aromatic hydrocarbons. Oils dominated by alkanes are known as paraffinic crude, while those rich in cycloalkanes and aromatics are called naphthenic or aromatic crudes. Intermediate types fall between these categories. In China, most crude oils are characterized by high wax content, a relatively high pour point, low sulfur levels, moderate nickel and nitrogen content, and very low vanadium concentrations. Most crude oils in the country contain less gasoline fraction, with residual oil making up about one-third of the total. Different types of crude oil require specific processing methods due to their varying compositions, and each should be utilized according to its best potential. Daqing crude oil, for example, is known for its high wax content, elevated pour point, and low sulfur content, making it a classic example of a low-sulfur paraffinic crude.