Geochemical Characteristics And Organic Matter Enrichment Model Of Source Rocks Of The Dongying Formation In The Bozhong Sag

The Bohai Bay basin is an important petroliferous basin in Eastern China,and the Bozhong sag is one of the most important hydrocarbon-rich ones in the basin.In recent years,a series of large oil and gas fields such as Bozhong 19-6 and Bozhong 13-2 have been found in the exploration practice.More importantly,it is found that a considerable part of oil and gas at the near source slope of the sag comes from the source rocks of the Oligocene Dongying Formation.However,at present,the research on the quality and formation conditions of this set of source rocks is not deep enough.Clarifying its characteristics and development model is of great significance to promote conventional and unconventional oil and gas exploration in Bozhong sag.Based on the organic-inorganic geochemical experiments,whole rock clay and sporopollen analyses and tests of the source rock samples of the Dongying Formation in the Bozhong depression,this paper evaluated the quality of this set of source rocks,analyzed the paleosedimentary environment and paleoproductivity during its formation,and established the unique development model of the source rocks of the Dongying Formation.The results show that:1)the mudstone of the Dongying Formation（E₃d）,divided into three members including E₃d₃,E₃d₂and E₃d₁,is mainly composed of quartz and feldspar with their average content of 47%,the content of carbonate minerals is low,while the content of clay minerals is high,up to 32%.The lower part of the E₃d₂ and E₃d₃ belong to high-quality source rocks with high organic matter abundance（0.6-6.15%,with an average of 1.4%）.The type of organic matter is mainly typeⅡ₁-Ⅱ₂,reaching the stage of maturity-high maturity;The upper part of the the E₃d₂ and the E₃d₁ are of poor quality and belong to fair source rocks;The abundance of organic matter in the Shahejie formation（E₂s）,divided into three members including E₂s₃,E₂s₂,and E₂s₁,is higher than that in the Dongying Formation.The type of organic matter is mainly typeⅠ-Ⅱ₁,which also reaches the stage of maturity-high maturity;2)From the E₃d₃ to the E₃d₁,there is a warm and humid paleoclimate.The salinity and depth of the water body gradually becomes smaller.The water depth is from the E₃d₃（70.1 m）to the E₃d₂（41.7 m）and the E₃d₁（26.7 m）.The land-based invasion and water oxidation conditions show a gradually increasing trend.The paleoproductivity is as follows:E₃d₃（high:309.5-551.2g/cm³.a）>E₃d₂（middle:126.9-264.8 g/cm³.a）>E₃d₁（low:52.4 g/cm³.a）;The Shahejie formation also experiences a high water depth of 84.8m in the E₂s₃ to a low water depth of 31.2m in the E₂s_1-2.The E₂s₃ is formed in a weak redox freshwater environment under the control of warm and humid climate,and the paleoproductivity is 626.88-1059.7 g/cm³.a;The E₂s_1-2 is formed in the strong reducing saline water environment under the control of arid climate,and the paleoproductivity value is 368.3-521.8 g/cm³.a.The sedimentary environments of E₂s₃,the E₂s_1-2 and E₃d₃ belong to the eutrophic lake.3)The formation of high-quality source rocks in the Dongying Formation is controlled by multiple factors.Among them,high productivity and reduction preservation conditions dominate the enrichment of organic matter in the Dongying Formation,while paleoclimate,water depth,salinity and terrigenous input have limited influence on the enrichment of organic matter.The Dongying Formation formed a high-quality source rock development model under the control of warm and humid climate,weak redox conditions,fresh water brackish water,high water depth environment and high productivity（mixed supply of terrestrial higher plants and lower algae）.Deep-semi deep lake is the ideal sedimentary environment for the development of high-quality source rocks.