Abstract: Applying the techniques of pyrolysis and thermal extraction, the variations of oil content and composition in the Triassic reservoir of Lunnan oilfield, the Tarim Basin. are studied in this paper. It has been found that the intervals with low porosity and permeability have a low content of oil, and less matured, and its distribution of n-alkane is predo minated by a front peak; while the intervals with high porosity and permeability contain more and matured oil with a predominance of higher carbon number n-alkanes; the petroliferousness and composition of oil from intervals with middle porosity and permeability range from the two above. The oil compositional heterogeneities of intervals with different porosities and permeabilities within a reservoir may reflect its accumulative history.