There are three types of oil companies in the world today that supply crude oil to the global oil industry. Those three types of companies are described below:

International Oil Companies (IOCs)

International Oil Companies are privately owned companies that operate as free-market, profit-oriented organizations. Examples are ExxonMobil, Royal Dutch Shell, and BP Oil Company.

National Oil Companies (NOCs)

National Oil Companies operate as extensions of a national government. As such, they are not as profit-oriented as International Oil Companies and often have priorities geared to the benefit of citizens and organizations of the countries to which they belong. Examples are all the oil companies belonging to the Organization of Petroleum Exporting Countries (OPEC).

Autonomous National Oil Companies

Autonomous National Oil Companies are NOCs that operate as profit-oriented, corporate entities. They do, however, attend to the national interests of the governments to which they belong. Examples are Petrobras (Brazil) and Statoil (Norway).

The world's ten largest oil companies by reserves are all National Oil Companies (NOC). That is, they are owned by the countries in which they are headquartered. Following is a list of those top ten companies:

• Saudi Aramco
• NIOC
• Qatar Petroleum
• Iraq National Oil Company
• PDVSA
• ADNOC
• Pemex
• NNPC
• National Oil Company of Libya
• Sonatrach

The Organization of Petroleum Exporting Countries (OPEC) consists of the following nations:

• Iran
• Iraq
• Kuwait
• Saudi Arabia
• Qatar
• United Arab Emirates
• Algeria
• Angola
• Ecuador
• Equatorial Guinea
• Gabon
• Libya
• Nigeria
• Venezuela

Oil Refineries are huge industrial complexes with a wide variety of components, all of which are required for the difficult task of extracting, purifying, and storing the very wide variety of products that are derived from crude oil.