Investopedia explains 'Organization Of Petroleum Exporting Countries - OPEC'
OPEC membership is open to any country that is a substantial exporter of oil and that shares the ideals of the organization. As of 2011, OPEC had 12 member countries, including founder members Iran, Iraq, Kuwait and Venezuela.
OPEC's influence on the market has been widely critcized. Because its member countries hold the vast majority of crude oil reserves (about 80%) and nearly half of natural gas reserves in the world, the organization has considerable power in these markets.
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