The original OPEC members of the chart were Saudi Arabia, Venezuela and Kuwait, which founded the Organization of the Petroleum Exporting Countries along with Iran and Iraq on September 1960.
OPEC can have a great influence on the oil market, especially if it decides to reduce or increase its production level. 43% of world's oil production and 81% of world's oil reserves are in member countries.
OPEC also concentrates all the necessary capacity for oil production in the world, which, de facto, converts the Organization of the Petroleum Exporting Countries into the central bank of the oil market.