Heavy oil in China is characterized by terrestrial genesis:low content of sulfur

traceelements and saturated hydrocarbon. Serious thickening resulted by biodegradation

waterwashing and free oxygen oxidization not only has effect on sterane and terpane compounds

but also on polycyclic aromatic compounds. The order of the effect on the abovementioned compounds is naphthalene series→phenanthrene series→chresene series→triaromatic steroid hydrocarbons. the thickening also causes obvious fractionation of stablecarbon isotopes in crude oil. Heayy oil pools are mainly caused by later tectonic movements. In the rising process of hydrocarbon basins (including faulting and folding)

paleopools might be destroyed

originally accumulated oil might migrate laterally or horizontally again. In this case

the oil might be biodesraded

oxidated and waterwashed and finallyform the heavy oil accumulation. The spatial distribution of heavy oil and air sands bearsclose relation to conventional oil.Heavy oil and tar sands are widely distributed in China

and the reserves are abundant.They play an important part in China’s oil resources.