ABSTRACT
The chapter describes the origin and formation of hydrocarbons and concludes with a classification scheme useful to understand their evolution and bulk properties. The source of crude oil is organic matter preserved in sedimentary rocks deposited on the continent or bottom of the ocean, or in calcium precipitated out of water in the form of lakes or ocean. The most important factor in the formation of crude oil from organic matter in sedimentary rocks is temperature. After the oil and gas migrate into the trap they separate according to density with gas on top, crude oil in the middle, and salt water on the bottom. Nearly two-thirds of oil and gas reserves thusfar identified were generated between 90 to 169 million years ago. Depending on the type of kerogen, further increases in temperature, pressure, and time may yield oil, wet gas, or dry gas. The three main types of hydrocarbons found in crude oil are paraffins, naphthenes, and aromatics.
