The state revenue from the oil sector had increased as the result of the rising of a number of production sharing agreements with foreign oil companies. Two production-sharing agreements were signed with the Standard Oil Company of Ohio in January 1985 and Amoco in February 1986. In January 1989, another production sharing agreement was signed between Qatar and the French state-owned oil company Elf Aquitaine. In the middle of 1991, production of gas in the Qatar North Field, the world's largest single field of non-associated gas (proven gas reserves of around 250 trillion cubic feet and probable reserves of 500 trillion cubic feet), commenced. While the search for finding more oil deposits in Qatar continued, Qatar built an industrial base in order to reduce dependence on the oil sector.