Lower-middle Jurassic nonmarine source rocks and petroleum systems of the northern Qaidam basin, northwest China

The Qaidam basin is a nonmarine, petroliferous basin on the northeastern margin of the Tibet Plateau, Potential source rocks are reported from Tertiary saline lacustrine deposits and Jurassic freshwater lacustrine or terrestrial strata, The Jurassic source rock is similar, in terms of depositional system, Lithology, age, and organic geochemistry, to nonmarine Jurassic rocks that are thought to be potentially significant hydrocarbon sources in many basins of central Asia, including the Tarim, Junggar, and other important petroleum-producing basins. These Jurassic source rocks were deposited in freshwater lacustrine systems in intracontinental foreland-style basins during the Early and Middle Jurassic, These lacustrine strata are associated with coals and coaly mudstones with possible secondary importance as hydrocarbon source rocks. Oils from the northern Qaidam basin can be divided into two groups based on molecular indicators of depositional environment and age-diagnostic biomarkers: those derived from this Jurassic freshwater lacustrine source rock and those derived from a Tertiary hypersaline lacustrine source rock. The correlation of some Qaidam oils to a Jurassic source establishes a petroleum system involving these nonmarine Jurassic source rocks. Oils of this petroleum system have been produced from Tertiary and Mesozoic reservoirs in anticlinal and thrust-related traps in northeastern Qaidam, Maturation modeling of the Jurassic source rock indicates that these oils were generated and expelled during the Miocene-Pliocene in much of northern Qaidam, The geological context of the basin suggests that drainage within the basin primarily was vertical, probably mainly along faults, until evaporite or overpressured shale seals were encountered in the overlying Cenozoic section. Documentation of this petroleum system suggests other exploration targets remain underevaluated in northern and southwestern Qaidam; furthermore, the northern Qaidam petroleum system is a useful analog to interpret the role of similar Jurassic source rocks in other, sparsely documented, basins of central Asia.