Petrological And Element Geochemical Characteristics Of The Yan'an Coals In Ordos Basin

Petrologial study is the basic work in traditional coal geology,which is also one of the foundations in modern coal chemical industry and coal-based source rock.The Jurassic coal in Northwestern China is charactered by the enrichment of the inertinite group,which is represented by the Yan'an coal in Ordos Basin.Besides,the distribution and genesis study of coal petrologial are not comprehensive.In addition,statistical data indicates that the type and content of macreals are closely related to the types of coal-bearing basins.For the further study of characteristics in the Jurassic inertinite-rich coals,together with the influences of the basin types and the factors of the coal-bearing basins.The Mesozoic Ordos Basin is taken as a typical example,for its high working degree and widespread coal seams.Based on the coal mining data in the whole basin before,combing the maceral data of coal samples taken from the underground mine in different coal seams of 6 mining areas distributed in different part of the basin,comprehensive petrologial studies were carried out.It is found that the coal seams of the Jurassic Yan'an Formation in the Ordos Basin are characterized by high content of inertinite,and the inertinite group is dominated by semifusinite.The content of inertinite in vertical are regular,which is closely related to the lake level change of Yan'an Formation in sedimentary period.The lateral distribution of the inertinite group in Yan'an coal showing a decreasing trend from the edge to the center in Ordos basin.The inertinite content in the Yonglong and Binchang coal mining area in the southern part of the basin are the highest ones in Ordos basin,which are slightly higher than the Yuanyanghu and Majiatan coal minin area in the western part of the basin.Inertinite in the northern part of the basin are lower,and the content of the inertinite group in the Yuheng mining area is one of the lowest part in the basin,which corresponds to Yuheng's highest sulfur content in the basin.The petrologial characteristics of the Jurassic coal in the northwestern part of China are similar to the Yan'an coal,which charactered by high content of inertinite.Jurassic inertinite-rich coals are distributed in the basins of Junggar,Tuha,Santanghu,Yili and Chaibei margin.Statistical analysis of the maceral data of coal and rock in dozens of coal-bearing basins at home and abroad shows that the maceral composition strong ly correlated with the original basin type,and the depression basin tends to form inertinite-rich coals.The content of inertinite group is close to or higher than the vitrinite group.The vitrinite group are extremely high in the coal seam of rifted basins,the content of inertinite group are low correspondingly.The macerals of coal in foreland basin are differ,and the inertinite content is between the depression basins and rift basins.Comparative analysis show that basin size,basin morphology,sedimentation rate and coal accumulation mode are important factors affecting the petrologial characters in the basin,and the deposition rate is the most critical one.Generally large scale,flat morphology,slow deposition rate,and coal-splitting mode dominated by the edge of the lake basin are the main reasons leading to the rich ofinertinite in the depression basin.Basic analysis,ash composition analysis,trace element and rare earth element analysis of coal samples and X-ray diffraction analysis of clay samples from the roof and floor from 7different coal seams in 5 mining areas in the Ordos Basin were carried out.Scanning electron microscopy analysis and prtrological identification were used to analyze and the trace elements abnormal samples.Distribution characteristics,trace element enrichment characteristics and mineral characteristics of the main elements were studed,and the occurrence and causes of the enriched elements were discussed based on the data.Comprehensive research show that the characteristics of major element in coal ash of the Jurassic Yan'an coal in the Ordos Basin are obvious,which can be divided into two groups,SiO₂,Al₂O₃,K₂O and TiO₂ is one group,and CaO,SO₃,Na₂O,MgO,Fe₂O₃ and MnO₂ is another group.It shows a negative correlation between the two groups,and they positively correlated with each other in both groups.The former group accounted for a higher proportion in the mudstone of the roof and floor than coal ash,and the latter is significantly higher in the coal ash,which indicating that the former group mainly originated from terrigenous detrital,and the latter are mianly from the organic matters,This feature is most prominent in the Ningdong coalfield in the western part of the basin.The major element characteristics of high Na content coal in Zhundong coalfield are consistent with the Yan'an coal,and similar characteristics exists in the Shengli coalfield in the Early Cretaceous Erlian Basin in Northeast China,except Na₂O belongs to the former group.It is concluded that the source of Na element in coal can be both organically related and land-source supply,which is closely related to the rock type in the source area.The content of trace elements in Yan'an coal are low,the enrichment states are mainly loss and normal.Ba,Sr and Mn,are the major elements that enriched in the Yan'an coal of Ordos Basin.In this study,the Sr enrichment was found in the 4-2 coal in the Majiatan mining area of Ningdong coalfield;the Ba enrichment was mainly in the 6 coal of Zaoquan mining area.Correlation analysis and SEM analysis showed that the main elements of Sr element in Ningdong coalfield is enriched in the celestite?SrSO₄?,the Ba anomaly is mainly caused by barite?BaSO₄?minerals,which are mainly formed during the later diagenesis of the coal seam.The Mn enrichment was mainly found in 5^-2 coal in the Shenfu mining area.Correlation analysis and scanning electron microscopy analysis show that Mn mainly distributed in the siderite?FeCO₃?.The enriched of Mn is associated with the relative oxidation peat environment.Comparing the geochemical characteristics of elements in different coals,the coal ash composition index of the inertinite-rich coals in the depression basin is significantly lower,that is to say,the proportion of SiO₂ and Al₂O₃ in the inertinite-rich coals are lower than that of the high vitrinite coals in rift basins.The ratios of environmentally sensitive elements shows that the formation condition of the inertinite-rich coals in the depression basin are much more oxidizing than that of the high vitrinite coals in rift basins.This geochemical feature echoes the petrological characteristics in the depression basin.