A THESIS ABOUT FORMING CONDITIONS OF MARINE MIDDLE-UPPER ORDOVIAN SOURCE ROCKS IN CHINA

The wide area and extreme thick sediments of marine Lower Palaeozoic are well developed in northern and southern,and especially in the western part of China,Tarim Basin.The major petroleum source rock systems in the Tarim Basin,however,are disputed and debated for a long time.It is suggested in this paper that a global glaciating happened during the Middle Later Ordovician when would be favored for accumulating and preserving of sedimentary organic carbon,depending on the positive shifts of stable carbon and oxygen isotopic compositions within the marine carbonates taken from two typical outcrop and underground sections.According to comprehensive studies including contents of organic matter,maturity indexes and sedimentary facies,a macro evidence about the Middle Upper Ordovician as a major petroleum source rock in Tarim Basin is proposed here.Meanwhile,the research shows that the possibility and prerequisite of Middle Upper Ordovician as a major system of petroleum source rocks in the Paleozoic deposited and developed in Lower Yangtze and western margin of Huabei(North China)platform.