Distribution of Sedimentary Facies Belts and Sandstone Bodies of Permo-Carboniferous in the Central Part of Ordos Basin

Key words: 
• the central part of Ordos basin /
• Permo-Carboniferous /
• sedimentary facies /
• sandbody distribution /

Abstract: Tidal flat,delta and lake deposition of Permo-Carboniferous are well-developed in the Ordos basin with an evolution sequence changed gradually from marine to continental facies upwards.In Carboniferous,the transgression was caused by North China Sea and Qilian sea in which the former was of the largest range. The Lower part of Benxi Fm.is tidal flat facies with very large distribution range but the Sandbody in it is relatively small.The Upper part of Benxi Fm.is the mixed deposition of turbidity and fresh water in epicontinental sea,in which formed some small bars and the delta front only distributes in the northeast of the basin.Most of elongated sandbodies from well S201 of the north to well Q1 of the south extend along NNE.Transgression range of the early period of Taiyuan Fm.in late Carboniferous is larger than that of Benxi Fm. At that time,under the influence of weak tectonic movement,subdued sea-floor,weak hydrodynamism and less sediment supply,the study area mainly developed tidal flat and delta deposits.During late period of Taiyuan Fm.,the basin mainly developed tidal flat except the northeastern part of the basin where the submerged distrbutary channel of delta front prevailed .In Permian,Qinling trough began to contract again,and all north China Platform uplifted resylting in the rapid regression of sea water from east and west part of the Drdos basin.The great change of the palaeotopography and palaeoclimate led to the much wider deposition range than that of the Carboniferous. Delta plain was widespreed in the basin in the early period of Shanxi Fm.of Lower Permian.In depositional area,water became shallow and wide and the thickness of the deposits became thin.As a result of the palaeotopography,the shape of delta composed of several stacked sandbodies showed digitated,implying the obvious differential subsidence and sedimentary types between the north and south of the study area.In the north,Delta plain was well-developed,and the delta frout prevailed in the south where the submerged distributary channel from Jingbian to Ansai is the largest sandbody which is also the major reservior in the study area.In the period of Shihezi Fm.,the climate changed from warm humid to dry and hot resulting in the rapid decrease of the plant,and a set of continental clastic deposits were formed.Along with the continuous rising of the prevenance area,the paleocurrent direction was always to the south.Delta plain composed of distributary channel and flood plain was formed in the north Shihezi Fm.and digitate delta front developed in the south where riversf flowed into the lake.Widespread lacustrine mudstone formed in the Upper Shihezi Fm.and Shiqianfeng Fm. of Upper Permian became the best regional caprocks in the study area. In Permo-Carboniferou,the distributary channel sanbodies of deltaplain and submerged distributary channel sandbodies of delta front are the best reservoirs,and the tidal flat sandbody and fingered sandbody of delta front are also favorable are for exploration.