Geochemical Characteristics And Forming Environment Of Late Permian Bark Coal In Southern China

The Late Permian Longtan Formation is one of the most economically valuable coal-bearing strata in southern China,which contains abundant coal resources and paleoclimate and environmental information.This thesis collected three groups of typical bark coal from the B3 coal seam in Leping coalfield,Jiangxi Province and the C coal seam in Changguang coalfield,Zhejiang Province of Longtan coal-bearing series.Totally 30 coal samples include the roof and floor of the coal seam and parting samples.There is a combination technology of coal petrology,coal quality,sedimentology,mineralogy,stratigraphy,geochemistry for this study.The hydrocarbon generation performance,coal-forming materials,thermal evolution degree of organic matter,forming environment and genetic model of bark coal,occurrence of minerals in coal,occurrence of elements in coal,source and distribution characteristics of sulfur and nitrogen,and paleoclimatic signal of carbon isotope in coal are studied,and the following understandings are obtained:The bark coal in Mingshan and Changguang coalfields are mainly semi-dull coal and dull coal,with a small amount of semi-bright coal.Barkinite was the main maceral component,with an average content of more than 50%.The content of inertodetrinite in the three coal seams is also high,indicating that the coal seams were heavily affected by water flow during the formation process.According to the coal quality characteristics,bark coal samples can be classified as low-medium ash to medium ash coal and medium-high volatile to high volatile coal,with high hydrogen and sulfur contents.The analysis of sulfur forms and coal facies indicate that the coal seams were affected by seawater during the peat-accumulating stage,and Mingshan seam was subjected from stronger seawater influence than Changguang seams.Bark coal samples are typeⅡ-Ⅲ kerogen,which means that samples have not been subjected to high temperature during the deposition process.The thermal maturity of the samples are from early to middle maturity,consistent with good hydrocarbon-generation potential.The source materials of bark coal are mainly from terrestrial higher plants,although there is no obvious that algae were found under the microscope,the contribution of lower aquatic organisms brought by seawater invasion to source materials of bark coal could not be ruled out.Lithium,Zr,Nb,Ta,and Th are slightly enriched in bark coal,and the rest of the elements are normal enriched or depleted.Furthermore,Mingshan seam is more enriched in trace elements than Changguang seams.The minerals detected under scanning electron microscope(SEM)are carbonate minerals,silicate minerals,pyrite and many sulfate minerals including coquimbite,copiapite,halotrichite.In addition,a radial or flak-like iron and aluminum sulfate mineral was detected,which is different from most Fe-and Al-sulfate minerals in elemental composition and morphological characteristics.It is speculated that the mineral was formed in the initial stage of Al-sulfate formation but iron has not been completely depleted in the water environment.The occurrence of hydrated metal sulfate minerals indicates that the bark coal was deposited in a dry climate during the peat accumulation stage,at least a non-rainy season for a period of time,and the well-developed natural sulfur crystals on the sulfate may be related to the long dry period during the accumulation of sulfate mineral assemblages.The saturated and aromatic hydrocarbons biomarkers,as well as the trace elements indicators suggest the depositional environment during peat-accumulating stage are different.Both Mingshan and Changguang seams belong to marine-continental transitional facies.According to the vertical distribution of biomarkers as well as coal facies indicators in the profile of coal seams,it can be inferred that the seawater began to invade from the floor of Mingshan and Changguang 1 seam,and another seawater fluctuation occurred in the middle of the coal seams,accompanied by an obvious turning point of paleo-salinity and REDOX indexes.After that,a large-scale of regression began,and the regression period of Changguang 1 seam continued to Changguang 2.Meanwhile,the paleoclimate changed from warm and wet to dry and hot during the whole formation stage of Changguang seams,which means that the climatic and environmental conditions had changed greatly in the later stage of coal seam formation.According to the REDOX conditions of the medium of peat swamp,Changguang 1 seam may be deposited in coastal anoxic/hypoxic environment,while the sedimentary environment of Mingshan and Changguang 2 seams are more oxidizing.The δ13Corg of bark coal ranges from-24.5‰ to-23.4‰.From the bottom of Changguang 1 seam to the top of Changguang 2 seam,the δ13Corg value has a slow negative excursion,which is also obvious in Mingshan seam.The excursion of δ13C is similar to that of other marine and continental coal seams or carbonate profiles near the Wuchiapingian-Changhsingian boundary.Combined with the sedimentary environment of bark coal,it is speculated that the negative δ13C excursion at the boundary of Wuchiapingian-Changhsingian stage is not only a reflection of regional climatic conditions,but also influenced by the abrupt change of global climatic conditions before the mass extinction at the end of Permian.The δ15N values of bark coal range from +1.6‰ to+5.6‰.The δ15N values within the same coal profiles(similar coal rank)display heterogeneous characteristics,with the maximum offset reaching as high as 2.5‰,suggesting the control of coal-forming microenvironment on the δ15N values in coal.The sources of sulfur in bark coal and the state of the coal forming system from different coal seams are various.Under the influence of seawater,the coal forming system of Mingshan seam gradually changed from an open state to a closed state,the source of sulfur in coal is mainly from biogenic secondary sulfur,which is produced by absorbing sulfides reduced by sulfate-reducing bacteria.The bottom of Changguang 1 seam also showed an open coal-forming system caused by seawater intrusion,but then it quickly changed to a closed state and maintained this state until the whole Changguang 2 seam.For Changguang seams,the sulfur is derived primarily from the parent plant material,as well as non-biogenic secondary sulfur produced by epigenetically introduced sulfates.According to the distribution feature of nitrogen and sulfur isotope in the bark coal,the bark coal depositional model was established.Based on the nitrogen and sulfur isotope composition which are affected by sedimentary environment,and the correlation between their distribution,we proposed the influence of nitrogen cycle pathway and the open/closed state of the coal-forming system on the nitrogen and sulfur isotope geochemistry.