The Coal Quality And Its Controlling Factors Of Shanxi Formation In Deep Huainan Coalfield

Coal quality characteristics and coal geological control factors are not only the key to understand the formation and evolution of coal,but also the focus and hotspot of clean and comprehensive utilization of coal.Huainan coalfield is one of the important coal production bases in China.With the depletion of coal resources in the shallow Lower Shihezi Formation,it has entered the coal seam mining stage of the deep Shanxi formation,Taking the Shanxi Formation coal seam of Zhang Ji mine,Gu Bei mine and pan two mine in Huainan coalfield as the research object,the samples of coal and rock in Shanxi formation and Lower Shihezi Formation in the study area were collected.The coal quality parameters,maceral composition,vitrinite reflectance,constant and trace elements contents were measured and analyzed by using the theories of coal geology,coal chemistry,coal petrology and geochemistry.The indexes of gelation index and structural preservation index were used to integrate Sr/Ba,Cu/Zn and other geochemical indicators,summarize the coal quality characteristics,coal rock and coal facies characteristics and geochemical characteristics of Shanxi Formation coal seams,and reveal the evolution model of peat swamp and the geological control factors of coal quality.The research results show that:（1）The coal samples of Shanxi formation are ultra-low total moisture,low ash,medium high volatile matter,medium high calorific value and ultra-low sulfur coal,belonging to 1/3coking coal.The macro coal rock type is semi bright coal semi dark coal,most of which are semi bright coal,and the coal rock composition is mainly bright coal;The Lower Shihezi coal sample is ultra-low total moisture,medium ash,medium high volatile matter,medium high calorific value and ultra-low sulfur coal,which belongs to gas coal.The macro coal rock type is dim coal semi bright coal,and the coal rock component is mainly dark coal.The changes of C,H,N element contents and vitrinite reflectance from Shanxi formation to Lower Shihezi formation reflect the difference of plutonic metamorphism degree of coal under different burial depths.（2）The variation range of vitrinite content in Shanxi Formation coal samples is 37.97%-81.17%,with an average of 65.71%,the variation range of inertinite content is 11.50%-53.87%,with an average of 21.95%,and the variation range of exinite content is 2.47%-15.86%,with an average of 7.17%.The main coal facies are low stand swamp facies and wetland herbaceous swamp facies;The variation range of vitrinite content in coal samples of Lower Shihezi Formation is 53.01%-81.25%,with an average of 64.15%,the variation range of inertinite content is 8.32%-33.92%,with an average of 21.17%,and the variation range of exinite content is 0.77%-18.24%,with an average of 7.73%.The main coal facies are water covered forest swamp facies and wet forest swamp facies.Two types of micro coal rocks are identified in the coal samples of Shanxi Formation and Lower Shihezi formation,namely micro Triassic coal and micro complex mineral coal,which are mainly micro Triassic coal.The increase or decrease of micro components is corresponding to the transformation of coal facies in the section,and the hydrodynamic conditions of ancient peat swamp also change at different development stages of coal facies.（3）The minerals in Shanxi Formation coal mainly include kaolinite（Kln）,illite（Ill）,quartz（Q）,calcite（Cal）,dolomite（Dol）,pyrite（Py）,and a small amount of siderite（Sd）,gypsum（Gp）and jarosite（Jar）.The minerals in the roof,floor and gangue are mainly kaolinite,quartz and calcite.（4）Mg,B,Ga,Hg and Se are slightly enriched and As is highly enriched in Shanxi Formation coal;B,Cr,Ni and As in Lower shihezi coal are slightly enriched.Sr/Ba,Cu/Zn and ash composition index（C value）indicate that the sedimentary facies of Shanxi formation are marine continental transitional facies as a whole and in oxidation-reduction transition state as a whole;The sedimentary facies of the Lower Shihezi are generally continental and in a weak oxidation state.The environment indicated by Sr/Ba and Cu/Zn ratio is consistent with that indicated by ash composition index（C value）.（5）The coal quality is affected by many factors,such as the parent rock in the provenance area,the rate of tectonic subsidence,the distribution of provenance sedimentary system and so on.The parent rock in the provenance area has an important impact on the background values of trace elements and minerals in the coal.The Al₂O₃/TiO₂ ratio indicates that the parent rock of Shanxi Formation coal is mainly composed of medium acid rocks,and the Lower Shihezi Formation is similar to it.Geotectonic setting and ancient river flow direction indicate the terrigenous source areas of Funiu and Dabie ancient lands,which provide rich sediments to Huainan coalfield area.The difference of tectonic subsidence rate breaks the balance between peat accumulation and the growth rate of accommodation space,which has an impact on the parameters such as macerals and ash in coal.The content of vitrinite in coal of Shanxi Formation（average65.71%）is higher than that of Lower Shihezi Formation（average 64.15%）,which reflects that the basement subsidence of Shanxi formation is faster than that of Lower Shihezi Formation during coal seam deposition.In different parts of the Delta,the oxidation-reduction effects on coal forming plants are different.The closer to the distributary channel,the stronger the input of land debris,and the more partial oxidation of the coal forming environment.From the distribution of ash in different coal seams,the average ash content of deep Shanxi formation samples is lower than that of shallow Lower Shihezi Formation coal samples,which is due to the relatively high accommodation space/peat accumulation ratio of peat swamp in Shanxi formation period,Or lower terrigenous clastic input.The development of ancient peat swamp in Shanxi formation is obviously affected by seawater intrusion,and some coal seams in Lower Shihezi Formation are affected by seawater.From Shanxi formation to Lower Shihezi Formation,ancient peat swamp has experienced multiple periods of seawater advance and retreat events on the background of overall regression.