Natural gas before the natural gas can be transported. Natural gas extracted from oil wells is called casinghead gas (whether or not truly produced up the annulus and through a casinghead outlet) 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. There is some disagreement on which country has the largest proven gas reserves. Sources that consider that Russia has by far the largest proven reserves include the US CIA (47 600 km³), the US Energy Information Administration (47 800 km³), and OPEC (48
Natural gas reserves in Iran with India and China the main markets for Iran’s gas exports. A recent report (December 11, 2011) from Iran's Petroleum Minister indicates that a large natural gas reserve was discovered inside Iran's side of the Caspian Sea. These reserves are estimated at 50 trillion cubic feet (1.42 trillion cubic meter). Natural gas reserves in Iran According to the Iran Petroleum Ministry, the proved natural gas reserves of Iran are about or about 17.8% of world's total reserves, of which 33% are as associated gas and 67% is in non associated gas fields. It has the world's second largest reserves after