CHARACTERISTICS OF MIDDLE PALAEOZOIC MARINE SOURCE ROCK IN THE NORTH MARGIN OF MIDDLE AND UPPER YANGTZE REGION

Organic geochemical researches on the Middle Palaeozoic marine profiles in the north margin of Middle and Upper Yangtze region show that the Middle Palaeozoic marine source rock mainly developed in Lower Cambrian,the bottom of Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation and Permian.The Lower Palaeozoic source rock is mainly argillaceous rock with high organic abundance and is considered as the best source rock.The Upper Palaeozoic source rock is given first place to carbonate rock while making argillaceous rock subsidiary.The former belongs under the relatively good source rock with organic carbon usually being less than 1%.The latter belongs in the best source rock with high organic abundance.Types of organic matter in Permian carbonate and argillaceous source rock are many and complex with type Ⅱ₁ being of primacy.The extent of thermal evolution of these three sets of source rock is generally high with equivalent vitrinite reflectance usually being more than 2.0%.Gas is dominant in Middle and Upper Yangtze region.Research on the organic geochemical features of soluble organic matter in source rock shows that the normal alkanes in Lower Palaeozoic source rock are of frantal monopeak while that in Permian source rock hawing three types.The ratios of C₂₁ tricyclic terpans to C₂₃ tricyclic terpans and C₂₇ regular steranes to C₂₈₊₂₉ regular steranes are low in Permian source rock while those in Lower Cambrian source rock being high.In Wufeng-Longmaxi Formation source rock,the above-mentioned ratios are between the both.