Carbon Isotope Geochemistry Characteristics Of The Source Rocks And Their Products In The North Of Dongpu Depression

Dongpu depression is a typical salt rich sedimentary basin in China.The Cenozoic Paleozoic salt deposits in the north of the sag have a large thickness and many layers.It is rare in the Bohai bay basin,rich in oil and gas.The organic carbon isotope analysis of salt in northern area of Dongpu sag hydrocarbon source rock samples 116 block,the southern salt-free area sample 46,155 crude oil samples,180 samples of natural gas and hydrocarbon source rocks extract 19 for the sample.The experimental results show that the hydrocarbon source rocks in Dongpu depression are widely distributed,the range is-29.5‰ ~-22.0‰.The carbon isotope distribution range of crude oil is small,and the range is-28.6‰ ~-25.7‰.The carbon isotope distribution range of natural gas methane ranges from-48.3‰ to 24.9‰.The distribution range of ethane carbon isotope range from-25.5‰~-11.4‰.Propane carbon isotope distribution range from-32.6‰~-15.9‰.The carbon isotopic composition of hydrocarbon source rocks,crude oil and natural gas in Dongpu depression is significantly enriched with heavy carbon isotopes compared with that of ordinary freshwater lakes.The carbon isotopes of the hydrocarbon source rocks and crude oil samples in the south are slightly heavier than those in the north,and the carbon isotopic fractionation caused by the change of salinity of the sedimentary environment is not obvious.The carbon isotopic composition of hydrocarbon source rocks in Dongpu depression is mainly controlled by oxidation-reduction environment,climate and primary source.The carbon isotopes of crude oil and hydrocarbon source rocks have good inheritance.Most of the carbon isotopes of the extract are lighter than the source rock,which is consistent with the increase of the carbon isotope value because of the increase of the fraction polarity.There are also some sample extracts that are almost identical to the carbon isotopes of hydrocarbon source rocks or slightly reversed.Based on the systematic changes of the isotopes in crude oil,hydrocarbon source rocks and extraction components,the distribution range of carbon isotopes in the main source rocks of Dongpu depression is-24.7‰ ~-28.4‰,and the type is mainly mixed type.The reason for the large distribution range of natural gas a and ethane is due to the oil-type gas in dongpu depression and coal-type gas.Moreover,the degree of thermal evolution has a great influence on the carbon isotope fractionation of natural gas.In this paper,the thermal simulation experiment of gold tube of different types of hydrocarbon source rocks in Dongpu depression is discussed.Combining with hydrocarbon source rocks,crude oil and natural gas samples,the type of natural gas in Dongpu depression is determined.The oil and gas reservoir anatomy of five typical reservoirs in Dongpu depression has been established,and four oil and gas accumulation patterns have been established.