Milankovitch Cycle Of Wufeng-longmaxi Period And It's Influence On Organic Aggregation In The Northern Yunnan And Guizhou Area

Due to the rapid modernization of countries around the world,people's dependence on energy is increasing.China's demand for energy is also very urgent.The development and supply of conventional energy can no longer meet the rapid development of China,and the study of non-renewable resources around the world.It shows that shale gas is one of the most likely energy sources to complete the replacement of conventional oil and gas resources.However,the following problems are common in the research work on shale gas.Firstly,it is difficult to divide the high-resolution isochronous strata of shale.When the stratigraphic framework is divided by biostratigraphy,it is found that not all the intervals are well-preserved and indicative belts with fossils,and limited by the core coring data,most of the layers are difficult to carry out this work,which is a basic problem in the shale gas exploration and development process,which needs to be solved.Secondly,the exploration cost of shale gas is high and cannot be blindly explored and developed.However,China's research on the evolution and accumulation of organic-rich shale is still in its infancy,and these problems are all difficult caused by the prediction of shale gas favorable area and exploration work.The research and production practice in the past ten years show that,the Wufeng-Longmaxi Formation of the Late Ordovician to Early Silurian in the Yangtze area are the main target horizons in the field of shale gas exploration in China.And the fine stratigraphic division and high-precision comparison work,and subsequent isochronous stratigraphic comparison and the establishment of a three-dimensional stratigraphic framework,which provide a strong scientific basis for the future selection of favorable oil and gas exploration horizons and exploration direction deployment.Based on the Milankovitch cycle theory,combined with signal analysis,paleoecology,stratigraphy and geochemical analysis,the study of the cyclical stratigraphy of the Late Ordovician-Early Silurian Wufeng-Longmaxi Formation in the northern part of the exploration area was carried out.Based on the unique proportional relationship of the astronomical orbital period,a set of period identification method for the target area of the study area was constructed.Three representative wells in the work area were selected as the research objects,and the GR curve was used as the original data.Processing,spectrum analysis and wavelet analysis are used to study the rotation of the target interval.Combined with the paleo-biofossil identification results of Y2 well and the results of the analysis of the earth's division analysis,the following conclusions and understandings are obtained.(1)The wells selected in the study area identified the well-preserved Milankovitch orbital records,and extracted the periodic curves to obtain the target layer deposition duration of 9.61 Ma and the average sedimentation rate of the sediments of 32.25 m/Ma.Y2 well is 10.12 Ma,the average sedimentation rate of sediment is 25.28 m/Ma,the sedimentation duration of Y3 well is 9.92 Ma,and the average sedimentation rate of sediment is 26.6 m/Ma.The deposition duration of the target layer is similar to that of the latest international geological year,which verifies the correctness and adaptability of the method.(2)In the three wells analyzed and treated,the upper part of the Longmaxi Formation in each well is divided into lithological cycles by lithology or sediment particle size,and the number of cycles obtained is equal to the eccentricity orbit identified by the interval.A long-period number,a set of coarse-fine-grained sedimentary rocks during each period,indicates that the core rhythm layer of the upper layer of Longmaxi is affected and controlled by the long eccentricity orbital period.This conclusion also confirms the research method have applicability and accuracy.(3)Through the identification results of paleontological fossils combined with the distribution of organic carbon content in the target layer and the orbital period curve,when the eccentricity period changes from small to large,the climate enters the glacial period from the warm period,and the TOC shows a low value.The classification of the species identified in the species is low;when the period of eccentricity changes from large to small,the glacial period ends,the Earth's climate system enter a warm period,the TOC presents a high value,and the species of the species identified in this interval are different.The degree is higher;in the late Ordovician-Early Silurian world line,the target layer shows the trend of the orbital eccentricity from small to large and then small,and the climate as a whole shows the law from the warm period to the ice period to the warm period.The frequent fluctuations also indicate the strong and frequent changes in the climate during this period.For this reason,during this period,a large number of organisms have cluster extinction events,and the species diversity is low.(4)Combining the orbital period with the organic carbon isotope index and trace element analysis test results,it can be seen from the change of the organic carbon isotope curve that the sea level during the deposition of the target layer is from high to low and then high.The eccentricity draws the same conclusion.The eccentricity varies from small to large and then small,resulting in the climate from warm to cold to warm,making the original ice cover from ablation to expansion and then melting.The content change curve can clearly see the accumulation law of organic matter,warm period,high TOC value,sea level rise,organic carbon isotope negative drift,corresponding to the change of orbital eccentricity period from large to small,which is conducive to organic matter accumulation;ice age,the TOC is low,the sea level is falling,and the organic carbon isotope is drifting,which corresponds to the change of the orbital eccentricity period from small to large.This period is not conducive to the accumulation of organic matter.Through the spectrum analysis of the trace element ratio,the Milankovitch cycle is also founded.Existence,the change of orbital period as the original driving force affects the enrichment of organic matter,embodied in sediments,paleox redox Environmental and geochemical indicators.