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 ma rgin of the Tibet Plateau, Potential source rocks are reported from Tertiar y saline lacustrine deposits and Jurassic freshwater lacustrine or terrestr ial strata, The Jurassic source rock is similar, in terms of depositional s ystem, Lithology, age, and organic geochemistry, to nonmarine Jurassic rock s that are thought to be potentially significant hydrocarbon sources in man y 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 dur ing the Early and Middle Jurassic, These lacustrine strata are associated w ith coals and coaly mudstones with possible secondary importance as hydroca rbon source rocks. Oils from the northern Qaidam basin can be divided into two groups based on molecular indicators of depositional environment and age-diagnostic biomar kers: those derived from this Jurassic freshwater lacustrine source rock an d those derived from a Tertiary hypersaline lacustrine source rock. The cor relation of some Qaidam oils to a Jurassic source establishes a petroleum s ystem involving these nonmarine Jurassic source rocks. Oils of this petrole um system have been produced from Tertiary and Mesozoic reservoirs in antic linal and thrust-related traps in northeastern Qaidam, Maturation modeling of the Jurassic source rock indicates that these oils were generated and ex pelled during the Miocene-Pliocene in much of northern Qaidam, The geologic al context of the basin suggests that drainage within the basin primarily w as 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 r emain underevaluated in northern and southwestern Qaidam; furthermore, the northern Qaidam petroleum system is a useful analog to interpret the role o f similar Jurassic source rocks in other, sparsely documented, basins of ce ntral Asia.