Research On Sedimentary Systems Of The Second Member Of Yingcheng Formation In The Lower Cretaceous In The North Slope Zone Of Lishu Fault Depression

The Yingcheng Formation is the main reservoir layer in the northern slope zone of Lishu depression.The previous study has done a lot of work on the tectonic evolution,source rock distribution and sequence stratigraphic division of the Yingcheng Formation.However,there are still some problems need to be solved in the provenance and the sedimentation systemof the second number of Yingcheng Formation.That has greatly restricted the oil exploration and development in this area.This paper mainly based on the coredata,well logging and 3D seismic data,using the theory of high precision sequence stratigraphy,studying the provenance system characteristics and sedimentary microfacies distribution.According to the sequence stratigraphic characteristics of the second number of Yingcheng Formation,the Yingcheng Formation Second segment can be divided into three sandgroups,containing SG1,SG2 and SG3.The sandstone content of SG1 sand group is higher.The grain size is coarse and sand bodies are stacked with progradation.Sandstone of the SG2 sand group is finer and mainly composed of the mudstone intercalated sandstone.The color of mudstone is dark.The sand bodies stacking patterns are mainly the retrogradation-aggradation.The SG3 sand group sandstone particles are relatively coarse.The sand bodies are characterized by progradation.Each sand group shows the characteristics of ‘the east and the north are thinner,the west and the south are thicker'.I guess that the sedimentation center was located in the southwest of the study area.Based on the comprehensive analysis of the distribution of heavy minerals,sandy conglomerate thickness and the color of mudstone,it is found that the provenance direction of the second number of Yingcheng Formationhave some difference in difference periods.The provenance of SG1 sand group is mainly from the north and northwest of the basin.The SG2 sand group is deeply influenced by the northern provenance.The SG3 sand group is mainly affected by the provenance of the north and east.According to the sedimentary face-marks,we can recognize the underwater distributary channel,underwater diversion bay,estuary bar,foreshore lake beach,sand-muddy flat and coastal-shallow lake mud microfacies.The SG1 usually develops underwater distributary channel and mouth bar.During the depositional period of the SG2 sand group,the water is deep and it mainly developsshore-shallow lake mud.The SG3 sand group mainly develops braided river delta underwater distributary bayand mouth bar microfacies.Withthe combination of the provenance system characteristics and of the sedimentary facies distribution,it is clarified that the second number of Yingcheng Formation developed sedimentary facies characteristics in different periods.During the SG1 period,the depth of the lake was shallow.In northern area,the front sand body of braided river delta was widely distributed,and moved to south by progradation.During the SG2 period,the lake basin descended,the water became darker,and braided river delta front sandbody was narrowed in the northern area.The shallow lake muddy sediments were developed in the southern area.During the sedimentary period of SG3 sand,the lake surface decreased,and the front sand bodyof braided river delta showed belt distribution in north area.In the vertical direction,the development of the front sand body was characterized by inheritance and weak cycle.