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):
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
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)
Nippon Oil, Japan
Oman Oil Company (OOC), Oman
Petrochina, China (Largest globally)
Petrolos De Venezuela
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
The biggest petroleum company in Italy is the Eni S.p.A. This is an oil and gas company.
The British Petroleum(BP) who are responsible for the oil spill but they are still the biggest company regarding the oil spills.
The biggest company in the world, by revenue, is Walmart. Walmart's revenue for the 2013 fiscal year was $476 billion. The second biggest company is Royal Dutch Shell, the third is Exxon Mobile Corporation, and the fourth is China National Petroleum Corporation.
Lots of companies drill for petroleum, It's one of the biggest industries in the world.
The Exxon oil company is the biggest petrochemical company in the world.
Studio Ghibli was ranked as the biggest anime company in the world.
Pfizer Corporation is the biggest pharmaceutical company in the world
bimbo bakeries USA is the biggest company in the world