Geochemical Characteristics Of Storm Coal

The study of geochemical characteristics of the storm allochthonous coal is helpful to study the paleoclimate,paleowater depth and coal seam correlation,and also has important practical significance for oil and gas exploration.In the Late Carboniferous coal-bearing strata of Yuzhou coalfield,Henan Province,there is storm coal formed in peat swamp due to strong erosion and transformation of peat swamp by storm.In this thesis,the No.1₁ coal and No.2₁ coal in Yuzhou coalfield are taken as the research objects.Optical microscope,scanning electron microscope,X-ray fluorescence spectrum,X-ray diffraction,gas chromatography-mass spectrometry and other research methods are used to explore the coal quality characteristics,provenance input and sedimentary environment of coal samples.The following understandings are obtained:The petrographic characteristics show that vitrinite content is dominant in No.1₁ coal and No.2₁ coal（73.96%and 84.61%）,followed by inertinite（26.04%and 15.39%）,and exinite is not observed.The random vitrinite reflectance R_o is 1.89%and 1.48%,respectively.In the coal seam disturbed by the storm,the common bedding and lineation in the primary structure of the coal are not observed,but its detrital inertinite and detrital vitrinite are more abundant and symbiotic with clay minerals.The analysis of coal quality characteristics shows that the total sulfur content of No.1₁ coal and No.2₁ coal in Yuzhou coalfield is 6.58%and 8.31%respectively,and the organic sulfur accounts for 81.6%and 60.6%of the total sulfur content.The total sulfur and pyrite sulfur content of No.2₁coal are low in the middle and high in the upper and lower parts in the vertical direction,and the organic sulfur content also has this distribution characteristics.It is speculated that the upper and lower parts of the coal seam are more affected by seawater than the middle.Through X-ray diffraction and scanning electron microscopy analysis,the minerals in coal are mainly calcite,pyrite,kaolinite,quartz,illite,etc.The storm coals in the upper part of the No.1₁ coal seam were affected by sea water carried by storm surge,showing that the content of callite was generally greater than that in the lower part of the coal seam.There were also pyrite-filled fusulinida structures in the storm coal,indicating that the characteristics of minerals in the coal also changed under the influence of storm.The change of pyrite content in No.2₁ coal from high to low and then to high is related to the sedimentary environment of the transition phase from the epicontinental sea carbonate platform to the brackish bay environment.By analyzing the sedimentary environment with the ash index,the SI value and RI value of No.1₁ coal reflect the high salinity and stable water,and the environment is relatively limited.The content of SiO₂+Al₂O₃ in the storm allochthonous coals shows a vertical distribution feature of increasing gradually from top to bottom after the coal seam is affected by the sedimentary differentiation caused by storm surge.Horizontally,the content of SiO₂+Al₂O₃ in the middle of coal seam is much higher than that of Fe₂O₃+SO₃and CaO+MgO.Vertically,the content of Fe₂O₃+SO₃ in the middle of No.2₁ coal is less than that in the upper and lower parts,while the content of SiO₂+Al₂O₃ and CaO+MgO is just the opposite.The content change of Fe₂O₃+SO₃ is consistent with the content change characteristics of pyrite sulfur and total sulfur.The upper and lower parts of the coal seam not only have a large amount of SO₄^2-from seawater in the brackish peat bog environment,but also have abundant Fe from the mainland,and the middle part of the coal seam is the product of fresh water or brackish cement carbon bog.The middle part of the No.2₁ coal seam is the turning point of regression and new transgression,which is consistent with the X-ray diffraction analysis.In the analysis of the combination characteristics of saturated hydrocarbon compounds,the peak distribution characteristics of n-alkane,the change of the ratio ofΣC₂₂^-/ΣC₂₃⁺,and the relationship diagrams of Pr/Ph,Pr/nC₁₇ and Ph/nC₁₈ all indicate that the organic matter in coal is derived from both lower aquatic species and higher plants.Among the aromatic hydrocarbon compounds,the sulfur-containing polycyclic aromatic hydrocarbons in coal No.1₁ are absolutely dominant,accounting for only 8.2%of the total aromatic hydrocarbons in No.2₁coal.This difference is related to the content of different forms of sulfur and the input of iron in the two layers of coal.As described in the analysis of major elements,the sedimentary environment of No.1₁ coal is more limited,the supply of Fe is limited and the excess S will be combined with organic molecules,so the organic sulfur content is higher;the S ions in No.2₁ coals first reacts with abundant Fe to form pyrite,resulting in a relative decrease of organic sulfur content.The main peak of dimethylnaphthalene in the naphthalene series is theββ-position2,6-+2,7-DMN isomer,indicating that the sample maturity is high;trimethylnaphthalene is more obviously affected by the degree of thermal evolution,and its maturity index can reflect the degree of thermal evolution to a certain extent,but it is also affected by its own properties.The methylphenanthrene index F₁ and F₂ of the two layers of coal are in the high maturity area（F₁>0.56,F₂>0.35）,which also reflects the high maturity characteristics of the two layers of coal.The characteristics of methylphenanthrene content（9-MP>1-MP,2-MP>3-MP）and the discovery of pimentone（1,7-DMP）indicate that the source of coal seam organic matter has dual inputs of marine and terrestrial sources.The analysis of trifluorene series compounds shows that No.1₁ coal belongs to carbonate platform facies sedimentary environment with high reducibility,while No.2₁coal is in transitional facies sedimentary environment,showing no obvious advantages of oxidation or reduction conditions.These data coincidence with the geological setting.