THE GEOCHEMICAL CHARACTERISTICS AND DISTRIBUTION OF SOURCE ROCKS OF THE BOZHONG SAG IN THE BOHAI BAY BASIN

Three source rock horizons occurred in the Bozhong Sag in Bohai Bay Basin: they are the 3rd Member and 1st Member of the Eocene Shahejie Formation and the Dongying Formation.The source rocks of the 3rd Member and the 1st Member of the Shahejie Formation are widely developed in the whole Bohai Bay Basin while the Dongying Formation only appeared in the Bozhong Sag.The source rock of the 3rd Member of the Shahejie Formation deposited in semi-deep to deep lacustrine environment has high amount of organic matter rich in hydrogen,oil-prone kerogen with excellent hydrocarbon potential.In the Bohai Bay Basin,the source rock of the 3rd Member of the Shahejie Formation is characteristic of mature to high mature,widely distributed,deeply buried with large thickness.The 1st Member of the Shahejie Formation formed in brackish to saline lakes with high organic matter content and contained type I and type II kerogens.Compared to source rock of the 3rd Member of the Shahejie Formation,the 1st Member of the Shahejie Formation is thin and only locally distributed.Constructive effects of neotectonism occurred in the Bozhong Sag and the Dongying Formation resulted in the deposition of thick sediments.The 3rd Member of the Dongying Formation deposited in semi-deep to deep lacustrine environment has high organic matter content and kerogen.In contrast to the 3rd Member of the Dongying Formation source rock,the low 2nd Member of the Dongying Formation has less organic matter content and contained poor organic matter.The source rocks of the 3rd to the Low 2nd Member of the Dongying Formation widely distribute in Bozhong Sag and are very thick and mainly mature at the peak time of hydrocarbon generation in the Bozhong,so they are the primary source rocks in this Sag.