Deltaic Depositional System and Coal-accumulation in Jnye Coalfield

Key words: 
• Juye coalfield /
• Permo-Carboniferous /
• deltaic depositional system /
• coal-rich unit /
• stratigraphy sequence /

Abstract: Juye coalfield is a Permo carboniferous coalfield,located at the southwest part of the Shandong Province.With the guidance of the sequence stratigraphic theory and the methodology,based on the analysis of cores and well logs,the authors carry out the detailed study on the feature of the depositional system and its constitute of the Permian in the study area our.study shows that the Lower Permain is a set of sediments dominanted by deltaic depositional system.Seven sedimentary facies have been recognized in the fluvial dominated shallow water deltaic depositional system.They are distributary channel,natural levee,crevasse splay,interdistributary bay,interdistributary depression,distributary mouth bar,and distal bar.The framework of this deltaic system is the sedimentary facies combination of deltaic plain,in which the main sedimentary facies is the distributary channel deposits.Because of the rapid progradation of the distributary channel,the distributary bay formed in them.The deposition of crevasse splay and overbank action filled the bay gradually,enlarging the upper deltaic plain.At the abandon stage of the deltaic system,and the active water channel died out.Superseding was the extensive swamp and peat swamp deposits,forming thick coalbed,which provides a sound basis for the division of the sequences.The reworked distributary channel washed and destroyed the underlying coalbed,reducing the thickness of the coalbed,even completely washed away.For the same sake,the pre delta mud can rarely be found in the study area. Sequence stratigraphy and its internal composition have been recognized through drawing up the section and the planimetric maps.The framework of the stratigraphy sequence has been established.In the framework,seven third order sequences and forty two parasequences have been identified.Among these,sequence Ⅲ is the main topic in this paper,for the deltaic system develops in the upper part of this sequence.Not only the frequent changes of the sea level but also the tectonic movement and the abundance of the deposit affect the development of the deltaic system.Our study also shows that there are two or three sets of fluvial dominated shallow water deltaic depositional sequences in the Shanxi Formation.In general,Juye coalfield located in the delta plain of the large scale shallow water deltaic depositional system.The delta plain is a better place for coal accumulation,high quality workable coal seams were formed in this system.But,because of the unequilibrium of the distribution of the interdistributary swamps in the developing process of the delta,the tectonic movement and the scouring of river channel,the distribution of the coal seams are different.The distributary channel later developed often wash the earlier coalbeds.So the interdistributary swamp on shallow water delta is the best place for peat accumulation,where coal rich units generally develop well.