China's Lukqin oil field, the world's lowest-altitude ultra-deep extra-heavy oil field located at the foot of the Flaming Mountains in Turpan, Xinjiang, has surpassed cumulative crude output of 10 million tonnes, China National Petroleum Corp (CNPC) said on June 29. Ultra-deep extra-heavy oil refers to petroleum resources buried thousands of meters underground with viscosity over 100 times that of conventional crude, resembling black mud with almost no fluidity at room temperature, making it extremely difficult to extract.