Which is the biggest Petroleum Company in the world?

The largest petroleum company in the world is the Saudi Arabian Oil Company, not Shell, as the original answer stated. The top 10 petroleum companies in the world are as follows according to petrostrategies.org (http://www.petrostrategies.org/Links/Worlds_Largest_Oil_and_Gas_Companies_Sites.htm):

1. Saudi Arabian Oil Company (Saudi Arabia)
2. National Iranian Oil Company (Iran)
3. Qatar General Petroleum Corporation (Qatar)
4. Iraq National Oil Company (Iraq)
5. Petroleos de Venezuela.S.A. (Venezuela)
6. Abu Dhabi National Oil Company (UAE)
7. Kuwait Petroleum Corporation (Kuwait)
8. Nigerian National Petroleum Corporation (Nigeria)
9. National Oil Company (Libya)
10. Sonatrach (Algeria)

The largest petroleum company in the United States is Exxon/Mobile, which ranks seventeenth in the world.

Seems to be a pretty up to date list of Buyers, Sellers & Refiners below :

Abu Dhabi National Oil Company

BHP Billiton, Australia

BP, United Kingdom

Exxon Mobil

Chevron

ConocoPhillips

Citgo, Venezuela

Gazprom, Russia (Largest Per Barrell Refining in Russia)

Global Energy Swiss AG, ZUG, Swizterland

Gulf Oil, Luxembourg

Iraq National Oil Company

Kuwait Gulf Oil Company, Kuwait

Locstein Petroleum Commodities, Switzerland (Reported Multi Billion Dollar Buying Capabilities)

LUKoil, Russia

Nippon Oil, Japan

Oman Oil Company (OOC), Oman

Petrobras

Petrochina, China (Largest globally)

Petrolos De Venezuela

PETRONAS, Malaysia

Qatar Petroleum, Qatar

Questar, United States of America

Reliance Industries Limited, India

Repsol YPF, Spain ( Largest Spanish End User )

Rosneft, Russia (Second Largest in Russia )

Royal Dutch Shell, Netherlands

Sasol, South Africa

Saudi Aramco, Saudi Arabia (the largest in the world)

Shell Canada, Canada (subsidiary of Royal Dutch Shell)

Shell Oil Company, United States of America (subsidiary of Royal Dutch Shell

State Oil Company of Azerbaijan, SOCAR Azerbaijan

Sonatrach

Total, France