Petroleum in commercial quantities was first discovered in the northeast in 1956. The most important oil fields are those of Suwaydiyah, Qaratshui, Rumayian, and Tayyem, near Dayr az–Zawr. The fields are a natural extension of the Iraqi fields of Mosul and Kirkuk. Petroleum became Syria's leading natural resource and chief export after 1974. Natural gas was discovered at the field of Jbessa in 1940.