"Source-to-Sink" System Analysis Of The Lower Fourth Member Of Shahejie Formation In Bonan Sag,Zhanhua Depression,Bohai Bay Basin

Bonan Sag in Zhanhua Depression is rich in oil and gas resources,among which the lower fourth member of Shahejie Formation（Es₄^x）has a good exploration prospect and has become a key strata in the middle-deep exploration in recent years.However,its provenance direction and composition,sedimentary facies types and distribution laws are not consistent,which hinder the oilfield efficient exploration and development.Based on the theory of"source to sink"system,guided by the sequence stratigraphy and modern sedimentology,this paper carried out a comprehensive study on the provenance system,transport system,depositional system and their coupling relationship of Es₄^x in Bonan Sag on the basis of cores,loggings and 3-D seismic data.Based on the theory of sequence stratigraphy and the comprehensive analysis of seismic and logging sequences,Es₄^x in Bonan Sag can be divided into a third-order sequence and three fourth-order sequences,namely SQ1-1,SQ1-2 and SQ1-3.The sequence pattern is“onlap its lower strata and truncation of its upper strata”and mainly controlled by tectonic activity and sediment supply.Based on the composition and distribution of Pre-Paleogene bedrocks around Bonan Sag,seismic progradational configurations,the compositional maturity and component characteristics,the sand-ground ratio variation,the REE distribution model and the detrital zircon U-Pb dating,it is considered that the Chenjiazhuang Uplift（Archaeozoic granite gneiss,Paleozoic carbonate rocks and clastic rocks,Mesozoic clastic rocks and volcanic rocks）,Gudao Uplift（Paleozoic clastic rocks and carbonate rocks and Mesozoic volcanic rocks and clastic rocks）,Chengdong Uplift（Archaeozoic granite gneiss,Paleozoic carbonate rocks and clastic rocks）and Yihezhuang Uplift（Paleozoic carbonate rocks and clastic rocks）are the source rocks of Bonan Sag,and the strength of the source decreases in turn.It is speculated that the Ludong Uplift area（Proterozoic granitic gneiss）is a remote source through the east and southeast of the sag.The relative paleogeomorphology before Es₄^x in the study area was recovered by the layer leveling and 3-D visualization technology,and the depressional and the uplift topography were defined.Four first-order catchments and five second-order catchments were delineated in Chenjiazhuang Uplift,and two ancient valleys developed obviously were compared.The morphology of the sedimentary area changed from the asymmetrical"V"type to the wide and shallow symmetrical"U"type.The smaller width to depth ratio,the stronger the sand transport capacity.The alluvial fan,braided river,braided river delta,fan delta and（salt）lake were developed in the study area.The alluvial fan and fan delta were mainly developed in the south,east and north of the sag.The braided river and braided river delta were mainly developed in the south gentle slope belt,and the（salt）lake was limited in the north.Synthesize analysis results of the source system,the transport system and the sedimentary system,it is believed that Bonan Sag has mainly three coupling types of source-to-sink systems as follows:near provenance-ancient valleys on the steep slope-fan delta（the sand body is a small lobe in the north of the sag）、near and far provenance mixed source-fault troughs on the steep slope-alluvial fan（the sand body is a large lobe in the east of the sag,）、near and far provenance mixed source-ancient valleys on the gentle slope-alluvial fan-braided river-braided river delta（the sand body is transversely contiguous and extends far in the south and middle of the sag）.The larger the catchment area and slope ratio in the source area,the smaller the width to depth ratio of the transport pathway,and the larger the sediment volume and distribution area in the sink area...