page_title: Petroleum reservoir
section_title: Gas field
hierarchical_section_title: Petroleum reservoir / Gas field
caption_attribution_description: English: Natural gas facility on the Vučkovec Gas Field, Medjimurie County, Croatia Hrvatski: Postrojenje na eksploatacijskom polju prirodnog plina Vučkovec, Međimurska županija, Hrvatska
context_page_description: A petroleum reservoir or oil and gas reservoir is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. Petroleum reservoirs are broadly classified as conventional and unconventional reservoirs. In conventional reservoirs, the naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability, while in unconventional reservoirs, the rocks have high porosity and low permeability, which keeps the hydrocarbons trapped in place, therefore not requiring a cap rock. Reservoirs are found using hydrocarbon exploration methods.
context_section_description: Natural gas originates by the same geological thermal cracking process that converts kerogen to petroleum.  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 stratigraphic trap.  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 the Urengoy gas field, and the third largest is the Yamburg gas field, both in Russia.
  Like oil, natural gas is often found underwater in offshore gas fields such as the North Sea, Corrib Gas Field off Ireland, and near Sable Island. The technology to extract and transport offshore natural gas is different from land-based fields. It uses a few, very large offshore drilling rigs, due to the cost and logistical difficulties in working over water.
  Rising gas prices in the early 21st century encouraged drillers to revisit fields that previously were not considered economically viable. For example, in 2008, McMoran Exploration passed a drilling depth of over 32,000 feet (9754 m) (the deepest test well in the history of gas production) at the Blackbeard site in the Gulf of Mexico.  Exxon Mobil's drill rig there had reached 30,000 feet by 2006, without finding gas, before it abandoned the site.
caption: Vučkovec Gas Field facility, Croatia
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