Following the defeat, and break-up of the Ottoman Empire after the war, shareholding in TPC became a major issue at the San Remo conference in 1920 (where the future of the non-Turkish areas of the Ottoman Empire was finally decided), as the war had demonstrated to the big powers the importance of having their own sources of oil. One of the original partners had been a German oil company, and the French had seized those shares as enemy property and demanded entrance into TPC through those. And both the Italian and United States governments demanded that their oil companies should be partners as well. After prolonged and sharp diplomatic exchanges, US oil companies were permitted to buy into the TPC, but it would take several years until the negotiations were completed.
The discovery hastened the negotiations over the composition of TPC, and in July 1928 the shareholders signed a formal agreement: the Anglo-Persian Oil Company (which in 1935 became the Anglo-Iranian Oil Company (AIOC) and in 1954 BP), Royal Dutch/Shell, the Compagnie française des pétroles (CFP, which in 1991 became Total), and the Near East Development Corporation (a consortium of five large US oil companies, among them Standard Oil) each received 23.7% of the shares, and Calouste Gulbenkian the remaining 5%. TPC was to be organized as a nonprofit company, registered in Britain, that produced crude oil for a fee for its parent companies, based on their shares. The company was only allowed to refine and sell to Iraq's internal market, in order to prevent any competition with the parent companies.
The big loser was Iraq. The San Remo conference had stipulated that Iraqis should be allowed 20% of the company if they wanted to invest in it, but the oil companies successfully resisted Iraqi efforts to participate, despite pressure by the British government to accept Iraqi shareholders. In 1929 the TPC was renamed the Iraq Petroleum Company (IPC).
In 1934, a pipeline was completed from Kirkuk to Al Hadithah, where it split into two branches: one going to Tripoli in Lebanon, and the other to Haifa, in what then was Palestine, and the same year the Kirkuk field was brought online. But not until 1938, nine years after the discovery, did IPC begin to export oil in significant quantities. The same year the company also got the concession rights to southern Iraq, and founded the Basrah Petroleum Company (BPC) as their wholly owned subsidiary to develop that region.
The Kirkuk production averaged 4 million tons per year until World War II, when restricted shipping in the Mediterranean forced down the production sharply.
During the 1940s and 1950s, the company also obtained concessions to explore for oil in Dubai and other Gulf states. It retained its monopoly of exploration and development in Iraq until 1961, when the revolutionary government of General Qassem nationalised 99.5 % of its concession areas in Iraq, leaving only the producing oilfields in the company's control. In 1971, the Iraqi government nationalised the remaining interests into the Iraq National Oil Company. This resulted in major increases in revenues for the Baath party government under Saddam Hussein to pursue massive infrastructure projects.
The Kirkuk field still forms the basis for northern Iraqi oil production. Kirkuk has over 10 billion barrels (1.6 km³) of remaining proven oil reserves. The Jambur, Bai Hassan, and Khabbaz fields are the only other currently producing oil fields in northern Iraq. While Iraq's northern oil industry remained relatively unscathed during the Iran-Iraq War, an estimated 60% of the facilities in southern and central Iraq were damaged in the Gulf War. Also, post-1991 fighting between Kurdish and Iraqi forces in northern Iraq resulted in temporary sabotage of the Kirkuk field's facilities. In 1996, production capacity in northern and central Iraq was estimated at between 0.7 to 1 million barrels (110,000 to 160,000 m³) per day, down from around 1.2 million barrels (190,000 m³) per day before the Gulf War.