On the basis of geological analysis, geochemical detection and simulation experiments, the hydrocarbon expulsion mechanisms of the Low Tertiary source rocks were studied in the Dongying Depression.The ratios of chloroform bitumem "A"/C increase with the depths of the source rocks of the middle and the lower parts of the Es3 Member, the upper part of the Es4 part. Compaction simulation of muddy sediment with oil and water presents that water is expelled preferentially, oil saturations in samples increase gradually. In addition, the rapidly decreasing stages of source rock porosities only match their early oil generation. So compaction is not the main mechanism of hydrocarbon expulsion of the Low Tertiary source rocks in the Dongying Depression.Bituminous veins and oil in fractures in source rocks were analyzed to shows that fractures are dominant paths for migration, which was also illustrated by rock fluorescence analysis after thermal simulation. From the point of view of the thick bituminous and carbonate veins, the fractures may not have opened and closed periodically. They keep opening for a long time. So the migration related to fractures is the main mechanism of hydrocarbon expulsion of the Low Tertiary source rocks in the Dongying Depression.The components of hydrocarbon also have close relations to migration. In thermal simulation of source rock cores, hydrocarbon generation and expulsion occurred simultaneously. Furthermore, hydrocarbons were greatly expelled when a lot of light components were generated. The chloroform bitumem "A" in source rocks greatly decreased in the meantime. The generation of light hydrocarbons not only enhance migration forces, but also decreases the viscosities of oil or the resistances of rocks to migration.
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