Oil, one of the main objects of geological exploration, is a thick, dark brown liquid that has been called "the blood of industry." Parts of the upper crust contain oil reserves. The main component is a mixture of various alkanes, cycloalkanes and aromatics. There are two theories about the formation mechanism of petroleum: biological deposition of petroleum and petrochemical industry. The former is widely accepted. It is believed that petroleum is formed by organisms in ancient oceans or lakes after long-term evolution, and it is a biological deposit of petroleum, which is non-renewable. The latter argues that oil is produced from carbon in the earth's crust itself, has nothing to do with biology, and is renewable. Petroleum is primarily used as fuel oil and gasoline, and is also a raw material for many chemical products, such as solutions, fertilizers, pesticides, and plastics.