Organic Petrology Characteristics And High-quality Source Rock Development Model Of Paleogene Shahejie Formation In Dongpu Depression

The Dongpu sag is a well-known hydrocarbon-rich sag in eastern China.Predecessors have done a lot of research on the genesis of high-quality source rocks.Generally speaking,the formation of high-quality source rocks is mainly due to the high productivity of original organic matter and good preservation conditions.However,the development characteristics of high-quality source rocks in the third member of the Shahejie Formation of the Paleogene in the Dongpu Sag are extremely heterogeneous at the macro and micro levels,especially in the complex types of organic matter and the diverse types of microbial blooming combinations such as algae.And the frequent changes in preservation conditions,etc.,make the understanding of high-quality source rocks also have many doubts.This study conducted a systematic study on the types,assemblages and development environment of hydrocarbon-generating organisms in the organic-rich shale of Shahejie Formation in Dongpu Depression,and discussed the origin and development of highquality source rocks of Shahejie Formation in Dongpu Depression.mode.Through systematic collection of source rock samples of Paleogene Shahejie Formation from25 typical wells in Dongpu Sag,using organic petrology,element geochemistry,stable isotope geochemistry and other interdisciplinary methods,microscope observation and acid-treated cheese were carried out.Microscopic observation of root samples,scanning electron microscopy(SEM),electron probe(EMPA),transmission electron microscopy(TEM),total organic carbon TOC,carbon and oxygen isotopes,main and trace elements,etc.,analyzed and tested,clarifying the hydrocarbon-generating parent material of high-quality source rocks Source,reveals the hydrocarbon generation characteristics and differences of different types of source rocks,and discusses the development model of high-quality source rocks in the third member of Shahejie Formation.Organic petrology and micropaleontology studies show that the main hydrocarbon-generating organisms in the salt water area of the northern Dongpu Depression are mainly blooming cyanobacteria,dinoflagellate and sporopollen;the hydrocarbon-generating organisms in the southern freshwater area are mainly green algae and higher plants.Combination-based.Elemental geochemistry and isotope geochemistry research revealed that the salt water area in the northern part of the study area has high salinity,deep paleo-water depth conditions,arid paleo-climate,extremely high paleo-productivity,and a relatively oxygen-poor water environment;the southern freshwater area It has low paleo-salinity,shallow paleo-water depth conditions,wetter paleo-climate,medium paleo-productivity and more oxidized paleo-water conditions.It shows that there are huge differences in the palaeoenvironmental characteristics and parent material of high-quality source rocks in the south and north of the study area.The intermittent blooming of hydrocarbon-generating organisms is the main reason for the heterogeneity of the distribution of organic matter in the source rocks in the study area.Integrating the research results of geochemistry and hydrocarbon-generating organisms,a development model of high-quality source rocks of the third member of the Paleogene Shahejie Formation in the Dongpu Sag was established.The northern part of the study area was the model of "salinization period of deep water and narrow basins".Ancient lakes are mainly saline-super-salin water,with strong limitations and deep depths.The layering phenomenon of the water body is formed because of the salinity.The bottom water body is anoxic-reducing condition because the water body is stable and inactive.The storage conditions are superior.According to the analysis of major trace elements and the reduction of paleo-productivity,the surface layer of the water body was rich in oxygen during the sedimentation of the third member of Shahejie Formation,and a large amount of organic matter developed.After the organism died,the organic matter flocculated,adsorbed on the surface of iron manganese oxide at the redox interface,and settled to the bottom.The water body has the nature of hypoxia to reduction,suitable for preservation of organic matter.The high-quality source rock of the third member of Shahejie Formation in the south of Dongpu Sag is a model of "shallow water,wide basin,and deep anoxic deposition during flood period".The lake water body is fresh water-brackish water,the lake water is very limited and belongs to relatively open water area.Due to the large amount of nutrients brought by surface rivers and floods,the ancient productivity of the surface water body is high,but it lacks oxygen-deficient storage conditions and consumes organic matter in the biochemical stage.Many of them,only some organic matter derived from thick-walled green algae and higher plants are preserved,but weakly reduced water bodies can also develop locally to form high-quality source rocks.