Abstract: Based on the stepwise separation and extraction on free oil, sealed oil, hydrocarbon in carbonate cement, bounded oil and inclusion hydrcarbon within quartz grain in six oil sand samples from Paleogene system and Neogene system in Gasikula oilfield of Qaidam basin, all these hydrocarbons are analyzed by GC-MS. The biomarkers of saturated hydrocarbon fractions in different states and their geochemical characteristics are studied. The result shows that the n-alkanes with single peak are dominant in the studied area with the range of carbon number from nC₁₃ to nC₄₂, I_CP is lower than 1.2, being of weak odd-carbon preference. The ratio of m(Pr)/m(Ph) is lower than 0.39, being of phytane preference, assuming strong reducing environment. The high concentration of gammacerane indicates saline lacustrine depositional environment of source rocks. These differences of hydrocarbons in different states reflect the saline evolution process of lacustrine. Also, the relative high concentration of C₂₇ regular steranes, hopanes and long-chain tricyclene from the oil sands suggest significant contributions of bacteria and algae to formation of hydrocarbons. The ratios of m (C₂₉ sterane ₂₀S)/m [C₂₉ sterane (20S+ 20R)] and m(C₂₉sterane ββ)/m(C₂₉ sterane ββ+C₂₉ sterane αα) suggest that the hydrocarbons in different states are crude oils generated from low mature and mature source rocks, and also reflect the different filling steps of sources into reservoirs.

Key words: Qaidam basin, Gasikula oilfield, Paleogene, Neogene, hydrocarbon, geochemical feature