A NEWLY DISCOVERED SOURCE ROCK IN THE MID-LOWER ORDOVICIAN IN LOWER YANGTZE AREA

The Mid-lower Ordovician usually acts as reservoir for oil and gas accumulation in the Yangtze area, However, in the southern Anhui Province, there occurs a sequence of gray black to black siliceous mudstone and carbonaceous mudstone in the Ningguo Formation of Lower Ordovician. It consists of rich graptolite and is inferred as an open sea shelf or deep sea facies. The Hule Formation of Middle Ordovician which consists of thin-layered dark grey silty mudstone and thick-layered siliceous mudstone with rich graptolite, is also deposited in a shelf to deep basin environment. The two Formations are similar in depositional features and distribution patterns with rich graptolite fossil, but the Ningguo Formation is obviously thicker than the Hule Formation. Organic geochemistry of Ningguo Formation source rocks show that the average TOC is 1.24%. According to the upwelling features, and studies on lithology, lithofacies and organic geochemistry, it is concluded that the Middle-Lower Ordovician is likely to be a medium-good source rocks deposited by upwelling flow in the southern Anhui region. Since the South Yellow Sea Basin is the major part of the Lower Yangtze, it is suggested that the research of the Middle-Lower Ordovician source rocks be strengthened in the South Yellow Sea Basin.