ORBITAL-PACED OF ORGANIC MATTER ENRICHMENT DURING THE LATE PERMIAN GREENHOUSE PERIOD IN DALONG FORMATION SHALE
The western Hubei region(29°～32°N,108°～111°E)is located in the middle Yangtze Plate.Although the Dalong Formation shale from the Upper Permian in western Hubei is considered an important exploration layer for shale gas,the main controlling factors of organic matter enrichment are controversial,and the influencing factors of primary productivity are still unclear,which restricts the promotion of unconventional oil and gas exploration.Therefore,this paper aims to explore the sedimentary environment and genesis of siliceous shale,and the potential model of organic matter enrichment controlled by astronomical orbital forces is proposed through geochemical analysis and cyclostratigraphy of the Dalong Formation shale in Well-ED2(1204～1255 m)in the western Hubei.Geochemical analysis involved testing trace elements and total organic carbon content(TOC)in 35 samples from a 51 m interval of the Dalong Formation in Well-ED2(1204～1255 m),and a minimal amount of major elements.Trace elements include Ni,Zn,Cu,V,Mo,U,Mn,Co,and Cd.In the cyclostratigraphy part,natural gamma-ray logging(GR)was used as a paleoclimate proxy indicator,and time series analysis was conducted using the Acycle 2.6 software.Results are as follows:MoEF-UEF covariation diagrams and Mo-TOC water mass retention discriminant diagrams indicate that Western Hubei basin was a restricted basin environment during the sedimentation period,organic-rich shales were sedimented under anoxic-vulcanization conditions.Comparisons with sediments from the Peruvian upwelling region and correlation analysis between productivity indicator elements and TOC,it reveals that the organic matter enrichment in the Dalong Formation is primarily controlled by upwelling-driven primary productivity.Si-Al correlation analysis shows a positive significant correlation between Si and Al when Si content is below 15％,indicating a substantial influence of terrigenous silicon on the genesis of siliceous rocks in the western Hubei region at Si contents below 15％;conversely,the influence of terrigenous silicon is minimal when the Si content exceeds 15％.Furthermore,obvious orbital cycles have been recognized of the Upper Permian Dalong Formation shales in Western Hubei.COCO estimates indicate that the optimal sedimentation rate is approximately 1.6cm/ka.The filtering results show that 8 long eccentricity cycles(E1～E8)are recorded in Dalong Formation,with an estimated duration of about 3.14 Ma.By establishing the"floating"astronomical time scale for the Dalong Formation in Well-ED2 based on time"anchor points"and restoring the relative sea level change curve through the sedimentary noise model,it is compared with the three-level sequence divided by predecessors.The starting and ending times of Dalong Formation are 251.9 Ma and 255.04 Ma respectively.Finally,it is preliminarily concluded in this study that during the periods of maximum eccentricity in the Late Permian,an increase in the TOC values of the Dalong Formation shale in western Hubei was observed,indicating that long-term strong seasonal variations led to increased primary productivity,thereby promoting organic matter enrichment in the shale.By comparing the two established models of organic matter enrichment,the relationship between orbital cycles and organic matter enrichment in the Dalong Formation is further confirmed in the study.
cyclostratigraphyDalong Formationtime series analysisgeochemical characteristics
万方数据
维普
