In this paper, we discuss the organic geochemistry of source rocks in the Quidam Basin, NW China, using data generated during a recent four-year study. This study addressed some basic problems concerning petroleum systems in the basin, problems which persist despite some five decades of exploration. We show that three separate source rocks are present and these are distinct in terms of both age and depositional facies and also geographical distribution. The oldest source rocks are Lower Jurassic fresh-water lacustrine deposits which are generally confined to northern Qaidam. These are currently thermally mature or highly, mature and have generated the oils, condensates and gases which have been discovered in Jurassic and Tertiary, reservoirs in the North Qaidam area. By contrast, Tertian, source rocks it-ere deposited in saline lakes. Previous studies have shown that they are characterized by low TOC values (0.2-0.6%). However we found Tertiary, source rocks with TOC > 1.0%; these were deposited in hypersaline lakes around the Manya Depression in western Qaidam. These source rocks have probably generated the oils at fields recently, discovered in the western part of the basin. The youngest (gas-prone) source rocks in the basin are Quaternary. These were deposited in saline lakes in the east and central parts of the basin and are thermally, immature. However they, are thought to have generated the large volumes of biogenic gas which are present in Quaternary reservoirs.