Sedimentary Facies Of The Upper Fourth Member Of Shahejie Formation Of Paleogene In The Western Sag Of Liaohe Basin

The oil and gas production of the upper fourth member of Shahejie Formation in Liaohe Western Depression plays an important part of the oil field in Bohai Bay Basin.Because of the need for further exploration,the interest focus has shifted towards lithological traps.However,that the conservative understanding of sedimentary facies types affects the further exploration.In this paper,the sedimentary characteristics of the upper fourth member of Shahejie Formation in Liaohe Western Depression is studied.Based on the theory of sedimentary geology,sequence stratigraphy and multiple methods,the sequence stratigraphic framework of the upper fourth member of Shahejie Formation in Liaohe Western Depression is established.And the sedimentary characteristics in each stratigraphy are studied in detail,with the discussion on the distribution and evolution of sedimentary systems.Finally,the sedimentary facies model is established.Firstly,based on sequence boundary characteristics,sequence stratigraphy framework of the upper fourth member of Shahejie Formation in Liaohe Western Depression can be established,which is divided into two third-order sequences,SQ1 and SQ2.On this basis,systems tracts and parasequence sets are analyzed,and the fourth-order sequences stratigraphic framework is established.On the basis of sequence division,according to geological background,logging and seismic data,the types and distribution characters of sedimentary facies are discussed.Two main types of sedimentary systems in Western Depression can be recognized,including braided delta and lake facies.The braided river delta is mainly developed braided river delta front,mostly distributed in the central and southern study area.And the lake facies can be divided into shallow lake and semi-deep and deep lake,widely distributed in the whole region.The beach bar is mainly developed in shallow lake,which is distributed in Shubei area,weak provenance along the coast and far-shore area.Deep lake facies is distributed in Shubei deep depression.Beach bar is a new type of sedimentary facies in this study,which is also the next focus of the exploration area.The beach bar reservoirs in this region can be divided into five sedimentary models based on their sedimentary characteristics: the lateral reformed beach bar,the front reformed beach bar,the bedrock reformed beach bar,the flooded reformed beach bar and the tempestite reformed beach bar.The beach bars are controlled by a few factors,including: the palaeogeomorphology,the palaeodepth of the water,the palaeosource and the palaeowind,which can be profiled as “wind-source-basin” three-terminal control systems.In this paper,the recovery methods of “four-palaeo” are proposed with the example of Shubei beach bar.The palaeogeomorphology is restored by “impression method”,and the beach bar is mainly developed underwater uplift.The palaeodepth of the water is restored by ripple and thickness of bar,which is about 8 meters in study area.The palaeosource is analyzed by comparison of thin slices,concluding that the source of beach bar is from transformation of delta,erosion of buried hill and reformation of coastline bedrock.The palaeowind is inverted by the distribution of beach bars,concluding the beach bars formed by the impact of north-south monsoon with 8 grade wind.The control factors of beach bar are summarized as "wind-source-basin" three terminal control systems.The palaeogeomorphology is of great importance to the development of beach bars.According to the palaeogeomorphology characteristics,it is divided into five zones in Shubei area,including: coastal zone,buried hill zone,buried-hill draping zone,northern slope zone and deep depression zone.The control factors of “wind-source-basin” systems to the development of sandstone in each zone are described,as well as the interaction between each factor.Finally,the distribution of beach bars is predicted.