AN ATYPICAL LACUSTRINE SOURCE ROCK AND ITS PETROLEUM CHARACTERISTICS IN THE LISHUI SAG OF THE EAST CHINA SEA

The lacustrine mudstone of the Yueguifeng Formation of Paleocene is the predominant source rock in the Lishui Sag of the East China Sea. Both natural gas and condensate have been found there. However, it is rather different in hydrocarbon sources with the typical lacustrine rift basins in Eastern China where oil dominates. Comprehensive source rock evaluation on the Yueguifeng shale suggests that there occurs dead carbon to certain proportion, which cannot generate petroleum. In addition, the thermal evolution maturities are not in coupling with the requirement of the conversion index from organic matters to hydrocarbons. The organic matters in Yueguifeng Formation shale are mostly terrestrial in origin, and the percentages of lacustrine aquatic organisms varies. There occur some differences in geochemical characteristics between the Yueguifeng source rocks and the typical lacustrine source rocks. The type of lake-basin have certain control over source rock evolution. Based on the comparison of geochemical characteristics of Yueguifeng source rocks and the petroleum generated in the sag with those of the typical lacustrine source rock and petroleum generated in different lake-type basins, it is found that over compensate and balanced compensate lake basin dominated the Yueguifeng period. And the shales in Yueguifeng Formation are atypical lacustrine source rocks.