Treaty of Lausanne in 1923 failed to solve the issue. For this reason, the question of Mosul was sent to the League of Nations. A committee travelled to the area before coming to a final decision: the territory south of the "Brussels line" belonged to Iraq. By the Treaty of Angora of 1926, Kirkuk became a part of the Kingdom of Iraq.
In 1927 a huge oil gusher was discovered at Baba Gurgur ("St. Blaze" or father blaze in Kurdish) near Kirkuk. The Kirkuk oil field was brought into use by the Iraq Petroleum Company (IPC) in 1934. The Company moved its headquarters from Tuz Khormatu to a camp on the outskirts of Kirkuk, which they named Arrapha after the ancient city. The Kirkuk field has remained the basis of northern Iraqi oil production with over ten billion barrels (1.6 km�) of proven remaining oil reserves as of 1998. After about seven decades of operation, Kirkuk still produces up to one million barrels a day, almost half of all Iraqi oil exports.
The exploitation of Kirkuk�s oil, which began around 1930, attracted both Arabs and Kurds to the city in search of work. Kirkuk, which had been a predominantly Turkmen city, gradually lost its uniquely Turkmen character. At the same time, large numbers of Kurds from the mountains were settling in the uninhabited but cultivable rural parts of the district of Kirkuk. The influx of Kurds into Kirkuk continued through the 1960s.
According to the 1957 census, Kirkuk was 40% Iraqi Turkmen, 35% Kurdish with Arabs less than 25% of its population.
Some analysts believe that poor reservoir-management practices during the Saddam Hussein years may have seriously, and even permanently, damaged Kirkuk's oil field. One example showed an estimated 1,500,000,000 barrels (240,000,000 m) of excess fuel oil being reinjected. Other problems include refinery residue and gas-stripped oil. Fuel oil reinjection has increased oil viscosity at Kirkuk making it more difficult and expensive to get the oil out of the
