Oil and natural gas are produced by the same geological process: anaerobicdecay of organic matter deep under the Earth's surface. As a consequence, oil and natural gas are often found together. In common usage, deposits rich in oil are known as oil fields, and deposits rich in natural gas are called natural gas fields.
In general, organic sediments buried in depths of 1,000 m to 6,000 m (at temperatures of 60 °C to 150 °C) generate oil, while sediments buried deeper and at higher temperatures generate natural gas. The deeper the source, the "drier" the gas (that is, the smaller the proportion of condensates in the gas). Because both oil and natural gas are lighter than water, they tend to rise from their sources until they either seep to the surface or are trapped by a non-permeable layer of rock. They can be extracted from the trap by drilling.
The largest natural gas field is South Pars/Asalouyeh gas field, which is shared between Iran and Qatar. The second largest natural gas field is located in Novy Urengoy, Russia.
The drillship Discoverer Enterprise is shown at work during exploratory phase of a new offshore field. The Offshore Support Vessel Toisa Perseus is shown in the background, illustrating part of the complex logistics of offshore oil and gas exploration and production.
Like oil, natural gas is often found underwater in offshore gas fields such as the North Sea, Corrib Gas Field off Ireland, and the Scotian Shelf near Sable Island. The technology utilized to extract and transport offshore natural gas is different from land-based fields in that a few, very large, rigs are usually used, due to the cost and logistical difficulties in working over water.
Rising gas prices have encouraged drillers to revisit fields that, until now, were not considered economically viable. For example, McMoran Exploration has passed a drilling depth of over 32,000 feet (the deepest test well in the history of gas production) at the Blackbeard site in the Gulf of Mexico.[1]Exxon Mobil's drill rig had reached 30,000 feet by 2006 without finding gas; Exxon Mobil abandoned the site.
Hyne, Norman J. (2001). Nontechnical guide to petroleum geology, exploration, drilling, and production (2nd ed.). Tulsa, Oklahoma: PennWell Corporation. ISBN 0-87814-823-X
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