Study On The Sedimentary Environment Of Upper Carboniferous Benxi Formation Of Eastern Ordos Basin,China

The Carboniferous Benxi Formation is a new exploratory formation in the Ordos Basin.The Pangou sandstone and Jinci sandstone are the two potential sets of economic sand bodies in the Benxi Formation.The hydrocarbon exploration shown that nature gas is very active in the two sets of sand bodies.The distribution and sedimentary environment of the sandstone formation cannot be simply explained by the model of barrier island-lagoon sedimentary system proposed earlier,and it is also different from the shallow water delta model of the overlying mainly gas-bearing units in Upper Paleozoic.Until now,the understanding of the genesis and distribution of sand bodies is unclear,which lead to insufficient understanding of the regularity of natural gas accumulation in this new unit and obstruct the exploration of the Benxi Formation.The overlying mainly gas-bearing reservoirs of Shanxi and Shihezi Formation in the Upper Paleozoic are large scale sheet sandstone-facies formed by river-delta system,while the Benxi Formation sand bodies are mostly island-shaped with each other.There is a great argument about its island-shaped sand bodies' macro-distribution.Most studies suggest that the Benxi sandstone deposited in island-lagoon coast with weak seawater-energy environment,and a few studies consider it to be a tide-controlled delta system.Based on the drilling data of the Benxi Formation and observation of core intervals and outcrops,the study of stratigraphic sequence and sedimentary facies were executed.First,according the analysis of the sequence interface identification,the Benxi Formation consists of a third-order sequence,including a low stand system tract,a transgressive system tract and a high stand system tract.The Hutian member is equivalent to the low stand system tract at the bottom of the Benxi Formation.The Pangou member is roughly equivalent to the transgressive system tract.The Jinci member is roughly equivalent to the high stand system tract.This third-order sequence are divided into two fourth-order sequences,Hutian and Pangou member consist of the first sequence,and Jinci member is the second sequence.Secondly,by observation of the outcrops and cores,the sedimentary structures formed by delta and tide coexist together,and the clastic rocks,coal seams and limestone also coexist in sequence,which reflects they formed by mixed sedimentation and composite sedimentary system.The sedimentary system analysis shows that the Benxi Formation was produced by tide-delta composite system,mixed tidal flat-shelf facies system,coastal plain-peat bog system and intermittent exposed weathering system,all of which were in the epeiric sea background.Finally,the ancient water depth in Benxi period calculated by Cobalt element analysis is 0.83~77.53 m,and the average is 24.77 m.The calculation of the ancient tidal range shows that the tidal range in Benxi period is between 3.54~9.04 m,generally in 5.53 m.It can be concluded that the coast of the Benxi Formation was in the middle-large tidal range.This type of tidal could rework the sand body derived from river-delta system dramatically.In order to study the environmental change process of the Benxi Formation,according to the sea level change curve,the lithofacies palaeogeographic maps of six different sea level states are proposed,followed as the sludged trench in LST ? medium source providing and sand accumulation in early TST ? weak source providing and mixed clasolite-carbonate environment in later TST ? strong source providing and sand-accumulation in early HST?weak source and mixed clasolite-carbonate environment in middle HST?coastal marsh with coal accumulation in later HST.