Geochemical Characteristics And Paleoenvironmental Analysis Of The Chang 7 Shale In Feng 75 Well Ordos Basin

The Ordos Basin is an important unconventional oil and gas producing area in China.The Chang 7 Member of the Yanchang Formation with semi-deep and deep lacustraine deposits is widely developed as one of the main source rocks of the basin.It is currently the focus of shale oil exploration area.The study of the geochemical characteristics of the Chang 7 shale and the analysis of the depositional environment plays an important role in the understanding of the organic matter enrichment and preservation conditions.However,previous studies did not analyze the paleoenvironment of the Chang 7 member in the northwest of the basin.In this paper,the core of well Feng 75 in Yanchi area has been taken and systematically analyzed in major,trace and rare earth elements,Rock-Eval pyrolysis,total organic carbon,extractable organic matter contenet,bulk composition,saturated hydrocarbon gas chromatography,carbon isotope.The paleoclimate,paleo-depositional environment and paleo-productivity during the deposition of Chang 7 period,as well as the organic matter abundance,type and thermal maturity of the Chang 7 member have been investigated.Analyze the hydrocarbon potential of the Chang 7 shale in Well Feng 75in Yanchi area.The element geochemical analysis results of Chang 7 shale show that,compared with the average value of major elements in the upper crust,Chang 7 shale is slightly depleted in Si O₂,Na₂O is obviously deficient,Al₂O₃,Mn O,and Ti₂O are slightly enriched,and rich in TFe₂O₃and P₂O₅.The content of major elements in each section of Chang 7 is relatively consistent,indicating that the supply of terrigenous materials was relatively stable when Chang 7 was deposited.Significantly high P₂O₅content,high loss on ignition and deposited tuff show that the Chang 7 sedimentary period is rich in nutrients and prosperous biological activities,which is the basis for the Chang 7 member to become a high-quality source rock.The Chang 7 shale is relatively rich in trace metal elements,especially Cu and Mo.It shows that the lake basin water bodies prospered during the Chang 7sedimentary period and the paleo productivity was high.The total amount of rare earth elements??REE?is significantly higher than the total amount of rare earth elements in the earth's crust,suggesting that the content of light rare earth elements is enriched in the Chang 7 member.In addition,the Chang 7 source rock has the typical characteristics of negative Eu anomalies.On the basis of trace elements and biomarkers ratios,the Chang 7 source rock at well Feng 75 in the Yanchi area was developed in a warm and humid climate,highly reducing depositional conditions in deep lakes,and high productivity of aquatic organisms,which facilitate the organic matter production and preservation.The Chang 7 source rock at well Feng 75 consists mainly of type II₁kerogen,enriched organic matter content with good source rock rank,and at mature level of thermal evolution.The Chang 7₃submember has the highest organic matter contentand high thickness,which has high potential to form large-scale shale oil.The Chang 7₂submember and the Chang 7₁submember also have high organic matter content,interbedded with silty sedimentary rocks and shale,which bears favorable conditions for the development of shale oil and tight oil.