Petroleum
Authors: Barker Colin , Robbins Winston K. , Hsu Chang Samuel , Drew Lawrence J. ,   Updated by Staff
Published: May 2005

The origin of petroleum involves generation in source rocks, migration to a reservoir, and the possibility of further modification of composition after accumulation. Accumulation of oil and gas in natural reservoirs begins with sediments incorporating organic matter as these are deposited. Upon increasing depth of burial, rising temperature converts the complex insoluble organic material called kerogen into a lower molecular weight, solvent‐extractable material called bitumen. Generation of bitumen occurs at lower temperatures in older rocks. Part of the bitumen may migrate and accumulate in reservoirs as a commercial accumulation of petroleum. The type of organic matter in the source rock exercises initial control on whether oil or gas is generated, but considerable modification of petroleum composition can occur after it accumulates in the reservoir. Rising temperature owing to increasing burial depth, water flowing past an oil reservoir, and bacteria can all play a role.

The composition of petroleum conveniently described in terms of refinery distillation varies greatly from one source to another. The naphtha, middle‐distillate, vacuum gas oil, and vacuum resid fractions increase in complexity and molecular weight. The elemental and molecular composition of these fractions show a general shift from simple saturated hydrocarbons in the naphtha to more complex aromatics and eventually sulfur‐, nitrogen‐, and oxygen‐containing hydrocarbons in the heavier distillates. The nondistillable vacuum resid includes still larger molecules that appear to contain several groups of the benzologues of smaller molecular structures linked together plus the largest portion of the S, N, O, and metal compounds. Complete compositional analysis of petroleum is possible only for low boiling fractions. For high boiling fractions, composition is usually determined by compound types and carbon number ranges. For nonvolatile fractions, only average structure can be obtained. Elaborate separation into well‐defined fractions can aid in compositional analysis by sophisticated analytical techniques.

Activities in the petroleum industry range from exploration, to oil and gas recovery or production, refining, distribution, and marketing. The nomenclature used has been derived from the disciplines of geology, petroleum engineering, and chemistry. This, combined with the complex nature of oil and many refined products, makes for a terminology that is often not systematic and has many poorly defined terms. An outline of the more widely used terms from exploration, production, and refining is included.

The petroleum resources considered here are those liquid crude oils that can be produced by conventional production technologies and, to some extent, those unconventional sources of crude oils that can be captured and converted into conventional sources in the near term by advancing production technologies. Most of the large volume of these crude oils that are consumed in the world are produced from only a small fraction of the oil fields that have been discovered. By far the largest concentration of these reserves is located in Saudi Arabia, the United Arab Emirates, and Kuwait. More than one‐half of the world’s proved reserves are contained in 51 out of the more than 20,000 fields that have been discovered. World and U.S. reserves, production, and consumption of crude oil are given.

The world is not running out of crude oil in the ground. Important technological and economic problems, however, need to be solved so that these resources can be converted to usable reserves.