Research On The Sedimentary Facies Of Jiufotang Formation In The Northern Ludong Depression,Kailu Basin

The Ludong sag belongs to the secondary negative tectonic unit of the Kairu Basin in the southwestern margin of Songliao Basin,and the main oil-gas enrichment strata are the Lower Cretaceous Jiufotang Formation.Although some achievements have been made in recent years,the understanding of the sedimentary facies in the northern part of Ludong sag is not uniform and the distribution of sedimentary facies is not clear,so this study is of great significance to guide the oil and gas exploration in the next step.Based on the regional geological characteristics of the Jiufotang Formation in the northern part of Ludong Depression,this paper,based on the theories of sedimentology,sequence stratigraphy and seismic sedimentology,analyzes and studies the sequence characteristics,sedimentary types and facies zone distribution of the Jiufotang Formation in the northern part of Ludong Depression through core observation and description,combined with seismic data and single well geological data.The results show that:(1)Based on regional stratigraphic development characteristics,the identification of second-order sequence interface and third-order sequence interface is completed by comprehensive use of logging data and seismic reflection characteristics.On this basis,through the combination of well and earthquake,the four-stage sequence interface was calibrated,and the Jiufoteng Formation in the northern Ludong Depression was divided into eight four-stage sequences sq1-1,sq1-2,sq1-3,sq2-1,sq2-2,sq3-1,sq3-2,sq3-3,and sequence stratigraphic framework was established.(2)On the basis of stratigraphic division,the identification of lithofacies,logging facies and seismic facies in the study area can be divided into four sedimentary facies types,namely fan delta,lake,braided river delta and near-shore subaqueous fan.According to the analysis of single well facies,the sedimentary environment and characteristics under the control of sequence framework are determined.(3)The relevant seismic sedimentology technology is used to conduct a comprehensive analysis of the combination of stratum slice and seismic multi-attribute of Jiufotang Formation.Combined with the vertical sedimentary evolution characteristics,the distribution of sedimentary facies zone in the study area is constrained and the sedimentary plane facies map is drawn.(4)The southeast of sq1-1 and the upper member of Jiufotang Formation in the study area mainly developed near-shore subaqueous fan deposits,which were mainly siltstone with small distribution range.The inheritance developed from sq1-2 to sq2-2.Fan delta deposits were mainly developed in the southeast with a larger distribution range.The front fan body extended northward to lacustrine facies,and the lithology gradually became thinner.The distribution range of sand bodies in different periods is related to the rise and fall of lake plane,among which,when sq2-2 is in the middle part of Jiufotang Formation,the sand body has the largest range and pushes the furthest northward,and part even connects with braided river delta.In the northern provenance area,braided river delta front deposits are mainly developed.