The Qaidam basin is a continental petroliferous basin located in northwest China. Its northern sector is an important hydrocarbon province, where one of the earliest oil discoveries in China was made. Although Mesozoic source rocks are understood to be important parts of Qaidam petroleum systems, the identification and distribution of Mesozoic source rocks in the subsurface are poorly understood. Middle Jurassic coal measures and associated shales have long been considered to be the primary Mesozoic source rocks for the northeastern Qaidam basin without any support from subsurface geochemical and geological data. However, new data have been gathered from earlier Mesozoic sediments that were penetrated by the Lengke-1 well, a deep scientific well with a total depth of 5200 m (17,060 ft). The well was drilled on the Lenghu structural belt in 1997 and documented other effective source rocks. Geochemical analyses indicate that Lower Jurassic mudstones are good to excellent source rocks for the commercial oil wells in the Lenghu area. The upper Middle Jurassic shales are very good source rocks for the commercial oil wells in the Yuka area. Two source rock intervals in the northeastern Qaidam basin allow for two petroleum systems to be distinguished, the Lower Jurassic–Lower Jurassic/Tertiary and Middle Jurassic–Upper Jurassic. The Lower Jurassic–Lower Jurassic/Tertiary petroleum system, with a geographical extent of 15,000 km2 (5800 mi2) and a cumulative amount of source rocks between 200 and 700 m (650 and 2300 ft), represents a favorable target for future exploration. The Middle Jurassic–Upper Jurassic petroleum system, however, may have a lower exploration potential because of a smaller area (1500 km2 [580 mi2]) and a thinner cumulative amount of source rocks (100–200 m [330–650 ft]).