Seismic Imaging And Prediction Of Carbonate Reservoirs Under The Strong Shield Of The Laoshan Uplift In The South Yellow Sea

The South Yellow Sea is located in the east of the coast of Shandong and Jiangsu provinces,it is the only sea in China's offshore where no industrial oil and gas has been discovered.The South Yellow Sea Basin is not only an extension of the Yangtze platform in the sea,but also the main body of the lower Yangtze block,it is a multi-cycle superimposed basin composed of marine Mesozoic-Paleozoic residual basins and continental meso-Cenozoic faulted basins superimposed on pre-Sinian metamorphic rock basement of the lower Yangtze platform.The main tectonic units of the basin are the Qianliyan uplift,Yantai depression?northern depression?,Laoshan uplift?central uplift?,Qingdao depression?southern depression?,and Wunansha uplift.Many scholars believe where the Paleozoic deformation is relatively weak,relatively stable structure,and developed thick Meso-Paleozoic carbonate rock formations in the Laoshan uplift of the South Yellow Sea.In addition,continental scientific drilling of CSDP-2 well?the only well drilled to the Lower Silurian in the Laoshan Uplift?also confirmed there has more complete Meso-Paleozoic strata of the Triassic-Lower Silurian Gaojiabian Formation in the Laoshan Uplift.However,the Mesozoic and Cenozoic strata had suffered a lot of erosion in the Laoshan uplift due to the influence of the Indosinian movement.Downward energy of seismic wave is weak because of the strong shield formed between the Neogene and the Meso-Paleozoic,in addition,there is little difference in the reflection coefficients of the inner strata of the Paleozoic,and shallow water-related multiples are rich developed,and there are many reverse faults and high dip complicated stratum in the Meso-Paleozoic strata,so the quality of seismic imaging of the Meso-Palaeozoic is poor.In addition,currently only 5 wells have been drilled into the Upper Paleozoic in the entire South Yellow Sea.Therefore,no technical method has been established for carbonate reservoir prediction of the Meso-Paleozoic,and the process of oil and gas exploration for the Meso-Paleozoic in the Laoshan Uplift is still slow,due to the factor constrained of oil and gas exploration costs and risks and so on.This article aims at the problems of the Middle-Paleozoic seismic imaging and carbonate reservoir prediction in the Laoshan uplift of the South Yellow Sea,the southeast part of the Laoshan uplift with weak tectonic deformation is taken as the study area.Technical research of broadband seismic imaging under the strong shields have carried out,it mainly includes frequency extension,shallow water combined multiple wave suppression,and high-precision velocity grid tomography modeling,so Middle-Paleozoic seismic imaging under the strong shield has improved effectively.Next,the prediction of porosity and fracture-karst reservoir have implemented with the sea-land comparison idea of combining existing drilling data in the sea and stratum physical properties in the land according to the characteristics of carbonate reservoirs in the Carboniferous-Lower Permian Qixia Formation,it mainly includes post-stack sparse pulse impedance inversion,pre-stack simultaneous inversion,and spectral decomposition,the distribution range of favorable reservoirs of the Carboniferous-Lower Permian Qixia Formation has predicted in the study area,and its cause analysis is analyzed.Now,a few aspects of knowledge and results have been achieved by carrying out exploratory research on seismic imaging technology and carbonate reservoir prediction in the study area.?1?The effective frequency range of seismic reflection of the Middle-Paleozoic in the Laoshan Uplift is 7-35Hz because of affecting by strong shielding layer,and its frequency band is narrow.In order to widen the frequency bandwidth of seismic reflection signals,firstly,a linear radon transform least square solution method is used to effectively attenuate the ghost wave at the shot and receiver point,so the frequency band of the signal has been widened,especially the energy of low-frequency effective signals is increased;and Q compensation technology is used to compensate the signal energy at high frequency.Through ghost wave suppression and Q compensation methods,a wideband processing attempt of shallow water low signal-to-noise ratio conventional streamer seismic data has been realized.and the wave group characteristics of the Meso-Palaeozoic under the strong shield have been improved,at the same time,it provides reliable low-frequency data information for the next seismic inversion.?2?The water depth in the study area is relatively shallow with an average water depth of 50m,the short period multiples correlated from the subsea are rich developed in seismic data,and the long-period multiple correlated from the subsea which formed between the strong shielding layer and the seawater surface are also commonly developed,and interlayer multiples below the strong shielding interface may also exist.The technical solutions of combined shallow wave suppression of the shallow water and residual multiple wave attenuation have been implemented by analyzing the multiple wave generation mechanism and characteristics,in particular,the combined method of deterministic water-layer demultiple and generalized surface multiple prediction has achieved better than the previous conventional technology,and seismic effective reflection of the Meso-Paleozoic have presented,the deep energy masses in the velocity spectrum have converged.?3?A set of modeling methods suitable for high-accuracy velocity modeling of Laoshan uplift was summarized acoording to the low exploration degree in the study area and the relatively poor signal-to-noise ratio of the Meso-Paleozoic seismic reflection.Firstly,the multi-method quality control velocity analysis was used to extract the Meso-Paleozoic velocity information,and the velocity change information of the formation near the lower part of the strong shielding layer?average depth of about 700m?has revealed by the first arrival tomographic inversion method using the first arrival information of the long array seismic,which guide the accuracy of Middle-Paleozoic velocity extraction,and so a prestack time migrate initial velocity field has been established;then the grid tomography velocity inversion has carried out by the prestack depth migrate according to the initial velocity field of the prestack time migration velocity,during the iterative inversion process,the principle of shallow first and then deep is followed,that is,the more accurate Cenozoic stratum velocity is obtained first,and then the Triassic-Silurian stratum velocity is obtained;and so a medium-shallow stratum velocity model with high signal to noise ratio seismic has obtained;Secondly,the Triassic-Sinian(T₉-T₁₀)layer velocity grid has established based on the formation velocity of existing wells in the sea,horizon interpretation and stratigraphic understanding of the terrestrial Sinian-Silur(T_g-T₁₁)from the land,then among of the fistly established grid tomographic velocity model,the velocity of the seismic data with low signal-to-noise will be filled and smoothed using the Triassic-Sinian(T₉-T₁₀)layer velocity;Thirdly,a high-precision velocity model of the research area is obtained by iterative inversion of grid tomography based on horizon constraints,according to the flattening effect of common imaging point gathers and the imaging improved effect of prestack depth migration,a high-accuracy velocity model is provided for carbonate reservoir prediction.?4?systematic analysis of the carbonate characteristics of the Carboniferous-Lower Permian Qixia Formation carbonate reservoirs with existing drilling and land data is must,and the key sensitive elastic parameters of carbonate reservoir prediction are summarized based on the log curve correction through log seismic response analysing and petrophysical analysing,these is:the high vp value is a lithologically sensitive parameter that distinguishes carbonate and clastic rocks,low??is an important porosity-sensitive elastic parameter.low?/?and low?are sensitive elastic parameters of oil and gas fluids.?5?Porosity and fracture-karst types are the characteristics of carbonate rocks in the Carboniferous-Lower Permian Qixia Formation,based on high-accuracy velocity model,migration imaging data and existing logging data,the porosity-type reservoirs has been predicted by the method of lithologic prediction using poststack sparse pulse impedance inversion and prestack simultaneous inversion fluid detection;the fracture-karst reservoirs has been inversed through multi-scale fracture prediction in the frequency domain,RGB fusion karst weathering crust characterization,and time-frequency analysis of oil and gas detection methods.Based on this,a technical method for carbonate reservoir prediction of the Carboniferous-Lower Permian Qixia Formation in the Laoshan Uplift was established,and the favorable reservoir distribution of the Carboniferous-Lower Permian Qixia Formation was effectively predicted in the study area.?6?The main controlling factors of carbonate reservoirs of the Carboniferous-Lower Permian Qixia Formation in the Laoshan Uplift have summarized,it is thought that the reservoir is mainly fracture-karst type,supplemented by porosity type,and the reef-bank facies reservoir is locally distributed,secondary pores of the reservoir have developed although subject to deep burial compaction,and the pore space was further improved by the development of later tectonic movements,fractures and karsts.