Research On Depositional Systems And Prediction On Favorable Sandstone Body Of The Second Menber Of Shahejie Formation In Qikou Sag, Bohai Bay Basin

Under the study of the filling framework of the basin and the palaeogeomorphology and structure evolution,we reconstruct the depositionnal system tract of Qikou sag using the method of single-factor thoroughly analysis and multiple factors to explain, which has a certain role on model guiding on the research on the depositional system.Carrying out the depositional system identification and characterization and prediction of sand body in Qikou sag which is such an important oil-rich sag,which has important theoretical value and practical significance on the subtle reservoirs exploration in Qikou sag.From the angle of sedimentary studies,we study the depositionnal system and the provenance in pan qikou sag by the scientific ideas and methods which is to get fine evidences of single factor and to explain on multi-factors comprehensive.1,Depositional system types is identified of the second member of Shahejie fomation of Paleogene using core, log, seismic, paleontology, petrology, geochemistry and other informations, and we identified four main depositional systems of the second member of Shahejie fomation in the study area: fan-delta system, braided delta system, deep lake gravity flow system and the lake system.depositionnal system is reconstructed of the second member of Shahejie fomation, and the space allocation rules of depositionnal system is summed up.2,We carried out a systematic analysis of control factors of the ancient sediment source system, comprehensive identification marks of the ancient sediment source system are established, regularity development model of the ancient sediment source system is summed up. We find out four Qikou sag main sediment source (the northern Yanshan source, the western Cangxian source, the southern chengning source and the eastern shaleitian source) and its spatial distribution, and evaluate the different sediment sources and their main contribution of Qikou sag and the sphere of influence.3,In the sequence stratigraphic framework, we characterized sand distributed system of the second member of Shahejie fomation using drilling informations in drilling-rich zone, while in less pore area and pore-free zone using seismic facies and seismic attribute analysis, which laid a solid foundation for the reservoir prediction, sediment source of research and reconstruction of depositional systems tracts.4,We use various means of geological and geophysical to depict typical deposition systems and the research on deltas in Qibei sag can be done on different scales.After the works of the application of seismic attributes,the tracing of seismic faces and the analysis of the sand distributed system ,we found that the composite delta is divided into western parts and migratory eastern parts.The seismic facies present that there is similar to the "s"-type foreset reflectance characteristics of the Delta of the second member of Shahejie fomation in Qibei Sag, which is lateral migration of seismic reflection characteristics of delta.Based on the discovery of the phenomenon of lateral migration, we track the axes in seismic profiles in plane,control the scope of the the migratory delta in plane of each period of time, finally acquire the area and evolution of the delta with all the periods of time,and find that in the lowstand systems tract of the second member of Shahejie fomation the East Delta migrate eastwardly six times while in the highstand systems tract the East Delta migrate eastwardly four times.The West Delta continually deposits vertically.Analysis of the differences development characteristics of composite delta of the second member of Shahejie fomation in Qikou sag, and we compared the geological background of typical depositional systems, found that and the channelized Beidagang burial hill, which the west fan delta is controlled by strike-slip faults channelized, and the braided delta of the eastern part is controlled by the normal faults channelized.5,We have done research on reservoir properties of different depositional systems and different genetic facies,which come to a conclusion that reservoir properties are best in fan delta front and braided delta front, and the underwater channels and mouth bar in the fan delta front and the underwater channels in the braided delta are best favorable reservoir.We found that the existence of the phenomenon of lateral migration of the eastern braided delta makes it different from other deltas of its heterogeneity characteristics, while deposition control the reservoir heterogeneity.Research on the favorable sand prediction is conducted under the guidance of the depositional systems, the potential reservoir are consisted of two types of the second member of Shahejie fomation:delta sand body and turbidite sand body, and delta system front sand body is more important. Summarize the favorable sand body and lithologic traps:1,deep water turbidite and turbidite fan lithologic traps; 2,the pinch zone of steep slope fan delta front which pass by sub-sag center; 3,incised valley lithologic traps in Beidagang burial hill and the southern margin of Qinan sag; 4,the stratigraphic traps favorable zone of the second member of Shahejie fomation; 5, the lithologic traps formed by braided delta front with lateral migration characteristics in Qibei sag;6, braided delta front lithologic traps in southern basin.