Visit the U.S. Energy Information Administration's Energy Explained [1]. Under “Oil and Petroleum Products,” read Where our Oil Comes From and all subpages.
Also from EIA, read this Frequently Asked Question [2]
Visit OPEC [3] (well, not really! But wouldn't that be fascinating?) Under "About Us," read "Brief History" and "Member Countries."
Links
[1] http://tonto.eia.doe.gov/energyexplained/index.cfm?page=oil_home
[2] http://www.eia.gov/tools/faqs/faq.cfm?id=727&t=6
[3] http://www.opec.org/opec_web/en/index.htm