Natural Gas Formation

Natural gas is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, but commonly includes varying amounts of other higher alkanes and even a lesser percentage of carbon dioxide, nitrogen, and hydrogen sulfide.[1] Natural gas is an energy source often used for heating, cooking, and electricity generation. It is also used as fuel for vehicles and as a chemical feedstock in the manufacture of plastics and other commercially important organic chemicals.

Natural gas is found in deep underground natural rock formations or associated with other hydrocarbon reservoirs in coal beds and as methane clathrates. Petroleum is also another resource found in proximity to and with natural gas. Most natural gas was created over time by two mechanisms: biogenic and thermogenic. Biogenic gas is created by methanogenic organisms in marshes, bogs, landfills, and shallow sediments. Deeper in the earth, at greater temperature and pressure, thermogenic gas is created from buried organic material.[2][3]

Before natural gas can be used as a fuel, it must undergo processing to remove impurities, including water, to meet the specifications of marketable natural gas. The by-products of processing include ethane, propane, butanes, pentanes, and higher molecular weight hydrocarbons, hydrogen sulfide (which may be converted into pure sulfur), carbon dioxide, water vapor, and sometimes helium and nitrogen.

Natural gas is often informally referred to simply as gas, especially when compared to other energy sources such as oil or coal. But not to be confused with gasoline especially in North America, the term gasoline also is often shortened in colloquial usage to gas.

How Was Natural Gas Formed?

One theory is that natural gas was formed millions of years ago when plants and tiny sea animals were buried by sand and rock.  Layers of mud, sand, rock, plant, and animal matter continued to build up until the pressure and heat turned them into oil and natural gas.  Another theory proposes that the earth is made up of primordial materials that combined in space billions of years ago when the basic structure of the earth evolved.  The materials are still buried far below the earth's crust where they have been trapped for 4.5 billion years.
To locate natural gas, geologic mapping, surveys and aerial photographs are used. Recent technology is helping find natural gas more accurately: magnetic measurement (measure of the magnetic field of base rock to determine how much sediment is lying above it), satellite imagery (helps identify surface structures and patterns that aid in the search for probable underlying hydrocarbon deposits), gravity mapping (determines the thickness of the basin or sedimentary rock layer and helps identify base rock topography), and seismic sound wave reflection (measures the time to various rock units that reflect acoustic energy). Gas is recovered through wells on-shore or off-shore.



In the 19th century, natural gas was usually obtained as a by-product of producing oil, since the small, light gas carbon chains came out of solution as the extracted fluids underwent pressure reduction from the reservoir to the surface, similar to uncapping a bottle of soda where the carbon dioxide effervesces. Unwanted natural gas was a disposal problem in the active oil fields. If there was not a market for natural gas near the wellhead it was virtually valueless since it had to be piped to the end user.
In the 19th century and early 20th century, such unwanted gas was usually burned off at oil fields. Today, unwanted gas (or stranded gas without a market) associated with oil extraction often is returned to the reservoir with 'injection' wells while awaiting a possible future market or to repressurize the formation, which can enhance extraction rates from other wells. In regions with a high natural gas demand (such as the US), pipelines are constructed when it is economically feasible to transport gas from a wellsite to an end consumer.
Another possibility is to export natural gas as a liquid. Gas-to-liquids (GTL) is a developing technology that converts stranded natural gas into synthetic gasoline, diesel, or jet fuel through the Fischer-Tropsch process developed in Germany prior to World War II. Such fuel can be transported to users through conventional pipelines and tankers. Proponents claim that GTL burns cleaner than comparable petroleum fuels. Major international oil companies use sophisticated technology to produce GTL. A world-scale (140,000 barrels (22,000 m3) a day) GTL plant in Qatar went into production in 2011.
Natural gas can be "associated" (found in oil fields), or "non-associated" (isolated in natural gas fields), and is also found in coal beds (as coalbed methane). It sometimes contains a significant amount of ethane, propane, butane, and pentane—heavier hydrocarbons removed for commercial use prior to the methane being sold as a consumer fuel or chemical plant feedstock. Non-hydrocarbons such as carbon dioxide, nitrogen, helium (rarely), and hydrogen sulfide must also be removed before the natural gas can be transported.[4]
Natural gas extracted from oil wells is called casinghead gas or associated gas. The natural gas industry is extracting an increasing quantity of gas from challenging resource types: sour gas, tight gas, shale gas, and coalbed methane.
Iran has the world's largest reserves of natural gas (17.9% of the world's total).[5] It contains an estimated 1,187.3 trillion cubic feet (33,620 km3) (Tcf) in proven natural gas reserves.[6][7] With Gazprom, Russia is frequently the world's largest natural gas extractor. Major proven resources (in billion cubic meters) are world 187,300 (2013), Iran 33,600 (2013), Russia 32,900 (2013), Qatar 25,100 (2013), Turkmenistan 17,500 (2013) and the United States 8,500 (2013).
It is estimated that there are about 900 trillion cubic meters of "unconventional" gas such as shale gas, of which 180 trillion may be recoverable.[8] In turn, many studies from MIT, Black & Veatch and the DOE—see natural gas—will account for a larger portion of electricity generation and heat in the future.[9]
The world's largest gas field is Qatar's offshore North Field, estimated to have 25 trillion cubic meters[10] (9.0×1014cubic feet) of gas in place—enough to last more than 420 years[citation needed] at optimum extraction levels. The second largest natural gas field is the South Pars Gas Field in Iranian waters in the Persian Gulf. Located next to Qatar's North Field, it has an estimated reserve of 8 to 14 trillion cubic meters[11] (2.8×1014 to 5.0×1014 cubic feet) of gas.
Because natural gas is not a pure product, as the reservoir pressure drops when non-associated gas is extracted from a field under supercritical (pressure/temperature) conditions, the higher molecular weight components may partially condense upon isothermic depressurizing—an effect called retrograde condensation. The liquid thus formed may get trapped as the pores of the gas reservoir get deposited. One method to deal with this problem is to re-inject dried gas free of condensate to maintain the underground pressure and to allow re-evaporation and extraction of condensates. More frequently, the liquid condenses at the surface, and one of the tasks of the gas plant is to collect this condensate. The resulting liquid is called natural gas liquid (NGL) and has commercial value.