OVERVIEW

Petroleum exploration is largely concerned with the search for oil and gas, two of the chemically and physically diverse group of compounds termed the hydrocarbons. Physically, hydrocarbons grade from gases, via liquids and plastic substances, to solids. The hydrocarbon gases include dry gas (methane) and the wet gases (ethane, propane, butane, etc.). Condensates are hydrocarbons that are gaseous in the subsurface, but condense to liquid when they are cooled at the surface. Liquid hydrocarbons are termed oil, crude oil, or just crude, to differentiate them from refined petroleum products. The plastic hydrocarbons include asphalt and related substances. Solid hydrocarbons include coal and kerogen. Gas hydrates are ice crystals with peculiarly structured atomic lattices, which contain molecules of methane and other gases. This chapter describes the physical and chemical properties of natural gas, oil, and the gas hydrates; it is a necessary prerequisite to Chapter 5, which deals with petroleum generation and migration. The plastic and solid hydrocarbons are discussed in Chapter 9, which covers the tar sands and oil shales.

The earth s atmosphere is composed of natural gas. In the oil industry, however, natural gas is defined as a mixture of hydrocarbons and varying quantities of nonhydrocarbons that exists either in the gaseous phase or in solution with crude oil in natural underground reservoirs. The foregoing is the definition adopted by the American Petroleum Institute (API), the American Association of Petroleum Geologists (AAPG), and the Society of Petroleum Engineers (SPE). The same authorities subclassify natural gas...