CARBON ISOTOPIC STUDIES OF INDIVIDUAL N-ALKANES IN CRUDE OILS FROM QAIDAM BASIN

The carbon isotopes of individual n-alkanes in the crude oils from the Qaidam Basin were measured by the new isotopic analytical technique of GC-C-MS。Their composition features and distribution patterns of carbon isotopes were studied and the genesis of the crude oils were discussed. The results show that individual n-alkanes in the western Qaidam crude oils have heavier carbon isotopic compositions,δ~(13)C value ranging from -23.7‰ to -28.5‰. Distributional curves of n-alkane carbon isotopes for the western Qaidam crude oils exhibit the corrugated pattern. This indicates that the western Qaidam crude oils come from saline-hypersaline lacustrine environment. Carbon isotopic compositions of individual n-alkane in the northern Qaidam crude oils are from -25.3‰ to -33.4‰ and distributional curves of n-alkaner carbon isotopes exhibit the U-type or corrugated pattern.On the basis of compositional features and distributional patterns of individual n-alkane carbon isotopes, the northern Qaidam crude oils are thought to derive from coal seam, lacustrine sapropelic and mixed organic matter, respectively.