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, California, Louisiana, Alaska,
Oklahoma, and Kansas), Iran, China, Norway, Mexico, Venezuela,
Iraq, Great Britain, the United Arab Emirates, Nigeria, and
Kuwait. The largest known reserves are in the Middle East.
