Abstract:

In order to restore the hydrocarbon charge history in the Tazhong area，an improved molecular composition of inclusion(MCI ) method is applied using oils from oil-hearing inclusions(FI-oils)and crude oils. The results show that the FI-oils and the crude oils in the Tazhong oil field originated from different source rock intervals and the crude oils are more mature than the FI-oils.  The FI-oils adsorbed and inclusion oils of the reservoir rocks generally have high concentrations of gammacerane and C₂₈ steranes，similar to the Cambrian-Lower Ordovician source rocks. In contrast，the crude oils generally have low concentrations of gammacerane and C₂₈ Steranes，similar to the Middle-Upper Ordovician source rocks.  The coexistence of 25-norhopane and the intact n-alkanes in all crude oils，and the higher concentration of 25-norhopanes in crude oils than in FI-oils，suggest that later non-biodegraded oil continue to charge and mix in the productive intervals during and/or after early biodegradation. Three episodes of oil charge from two source rock intervals can be restored in the Tazhong area.  The first and the second episodes of oil charge，origin of the Cambrian-Lower Ordovician source rocks，were determined in Late Silurian and Late Permian，followed by the third episode of oil charge derived from the Middle-Upper Ordovician source rocks in the Late Permian.

Key words: the Tazhong area, molecular composition of inclusion, hydrocarbon source, biodegradation, hydrocarbon accumulation process