During the past 600 million years incompletely decayed plant and animal remains have become buried under thick layers of rock. It is believed that petroleum consists of the remains of these organisms but it is the small microscopic plankton organism remains that are largely responsible for the relatively high organic carbon content of fine-grained sediments like the Chattanooga shale which are the principle source rocks for petroleum. Among the leading producers of petroleum are Saudi Arabia, Russia, the United States (chiefly Texas, North Dakota, Alaska, and California), China, Iran, Canada, the United Arab Emirates, Mexico, Brazil, Kuwait, Iraq, Nigeria, Venezuela, and Norway. The largest proven reserves are in the Middle East.
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