A Study Of Milankovitch Cycle In The Lower Section Of Wufeng Formation-Longmaxi Formation In Northern Yunnan And Guizhou

The Dianqianbei region is a demonstration area for the shale gas exploration and development in southern sichuan province.With the exploration work in-depth,the fine stratigraphic division and correlation of the target strata from Wufeng formation to the lower segment of Longmaxi formation in the study area,and the establishment of high-precision and continuous astronomical dating scale become more and more important.These work can provide many support for exploration and development in the future.This paper mainly deals with the identification and analysis of Milankovitch cycles from the wufeng formation to the lower member of longmaxi formation of the three wells in the study area.The GR curve bight is selected as the palaeoclimatic replacement index.The selected data point interval was 0.125m.MATLAB software developed by MathWorks of the United States was used for the data preprocessing,including de-trending,de-outliers,de-noising and other steps to eliminate interference signals.The one-dimensional continuous wavelet transform is carried out on the processed data,and the wavelet transform result is extracted to establish the average value curve of the module,and the extreme value of the average value from the module is identified.The milankowicz cycle was identified depend on its main controlling factors comparing the subperiod of sedimentary cycle with the theoretical orbital period ratio(1:2:5:20).Three Wells in the study area are comprehensively analyzed from the perspectives of lithology and sedimentation,paleontology and paleoclimate,and TOC.The following conclusions can be drawn:The average formation duration of the wufeng formation to the lower member of longmaxi formation in Wells Al,A2 and A3 was 4.78Ma,4.73Ma and 4.48Ma,respectively.The sedimentary rates of section A and section B in the three Wells are quite different,which reflects the change of sedimentary environment in the study area from late ordovician to silurian.It is proved that geological events occurred at the boundary between ordovician and silurian were probably caused by changes in earth orbit periodic parameters(mainly eccentricity).The lithology of the target strata in the study area is coupled with the michelnian cycle,and the number of lithological cycles is very similar to that orbital cycle parameters.It shows that the core rhythm layer of the target stratum is affected by the orbit period of long eccentricity.Combined with biostratigraphic classification,it was found that the bottom of wufeng formation was rich in organisms,and 13 graptolith species were identified,corresponding to a low eccentricity and a warm climate.In the guanyinqiao layer on the upper part of the wufeng formation,8 species of graptolites were identified,and many organisms at the bottom of the wufeng formation disappeared,identifying the unique organism hernanttebe fauna in this section.The corresponding eccentricity increases,solar radiation decreases,and the climate is cold.Thirty-seven graptolith species were identified in the lower strata of longmaxi formation.The eccentricity value became smaller as a whole,and the eccentricity curve was relatively stable with little amplitude change.It was an interglacial period with a warm climate and good biological development.The eccentricity curve of the junction of longmaxi formation and wufeng formation has a large range,and the eccentricity of guanyinqiao layer reaches its peak,which corresponds to the period of glacial explosion.It shows that the glacier-organism mass extinction event at the end of ordovician was affected by the change of orbital period,and the change of orbital period affected the climate change,thus affecting the sedimentary environment.The TOC content at the bottom of wufeng formation is large,and its value increases gradually upward.At the guanyin bridge layer at the top of the wufeng formation,the TOC value reaches the peak,and then gradually decreases.In the lower part of longmaxi formation,the TOC value is relatively small and changes little,and the TOC curve is gentle.This has the same change rule with its long eccentricity period curve.We believe that changes in the earth's orbital period will lead to changes in the TOC content.