Source Types And Development Environments Of The Source Rocks In The 7th Member Of Yanchang Formation,Ordos Basin

The hydrocarbon source and development environment are important controlling factors of the high abundance source rocks' formation.Based on the basic geochemical characteristics of source rocks in the 7th member of the Yanchang Formation,their source types,source composition ratios and environmental factors such as paleo-climate,paleo-salinity,redox environment and nutrient conditions which affect the development of source rocks are studied in this paper according to the analysis of organic carbon content,rock pyrolysis,inorganic element tests and the mass spectrometry of saturated hydrocarbons and aromatics in the source rocks of the 7th member in the Yanchang Formation.The results show that source rocks in the 7th member of the Yanchang Formation in Ordos Basin are dominated by low-level aquatic organisms such as algae,but also have proportional land-source higher plant inputs.There are some differences in their source composition with different lithological characters.The oil shales have single-type source inputs and they are mainly composed of low-level aquatic organisms.Carbonaceous mudstones and dark mudstones have mixed-type source inputs.The proportion of terrestrial higher plant inputs in carbonaceous mudstones is significantly higher than that of dark mudstones and oil shales.The proportion of each source types in dark mudstones is between the oil shales and carbonaceous mudstones.Besides great source supply conditions,the source rocks in the 7th member of the Yanchang Formation in Ordos basin have a good development environment.The climate is warm and humid in the sedimentary period of the Yanchang Formation.At that time the Ordos Basin is an open freshwater lake with low salinity and weak alkaline.The lake is in a weak anoxic-anoxic environment and the local area is in a weak oxic environment.The formation of carbonaceous mudstone is related to the weak oxic environment and the oil shale are often formed in the strong anoxic environment.The lake of Ordos basin has a high abundance of P and Fe which provides good nutrient conditions for its source rocks' development.