| The application of geophysical prospecting methods in seismic exploration and development is crucial.It is also an indispensable part of the sustainable development of hydrocarbon resources and the construction of national economy.Nowadays,the exploration targets have been gradually focused on the complex geology and complex lithologic reservoirs(zhangjing,2013).In the wake of developments in oil exploration and increasingly complex in exploration targets,the demanding accuracy of underground reservoirs prediction has been increasing accordingly.The targets of petroleum exploration in our country are mainly about sedimentary facies hydrocarbon resources.Cenozoic petroliferous basin are mostly composed of thin sand layers and mudstone deposits,and also containing a small amount of thin carbonate,shale and gypsum.In addition,the lateral variation of lithology and thickness are relatively large,and the multilayered layered reservoirs of multicycle formation are formed between sand-shale thin interbedds(Cai Xiling,2007).In recent years,the development of oil exploration has got to the stage of a delicate phase of exploration in the study area of this paper,the key jobs are determining the rule of spatial distribution of the thin reservoirs as well as its nature.Thin interbed refer to a kind of geological phenomenon that thin layers alternating occurrence with different lithology,which is to say the two adjacent layers are different in lithology.For example,the sand-shale thin interbeds.In the area which sand-shale thin interbedds are relatively developed,the seismic wave propagation effects,tuning interference and lateral variability in reservoir,the seismic wave amplitude,frequency and phase are affected in varying degrees.The main reasons leading to the above conditions are among the following changes,such as the layer's velocity,density and single layer thickness of sand-shale thin interbed.So the seismic wave field characteristics become more complicated as well.At present,the research on thin layers and even thin interbeds is becoming increasingly important in seismic exploration.In order to make full sense of the seismic response of the thin interbed stucture,this paper carry out the research of seismic response which is calculated by Zoeppritz equation,the approximate equation given by Aki and Richards,Shuey's two-term approximation,Shuey's three-term approximation and the propagator matrix.First,we study the theories of the above five forward methods,analyze the difference between Zoeppritz equation and approximations and propagator matrix algorithm.Then we reach the following conclusions: accurate Zoeppritz equation based on single interface reflection only consider the reflection effect;although above approximate equations have different emphases,due to the theoretical limits,the seismic response of these approximate equations cannot restore thin interbeds structure properly.However,the generalized propagator matrix theory belongs to the reflectivity method,that based on consecutive interface conditions to calculate the reflection coefficient of each frequency in the frequency domain,can fully consider the dynamical characteristic of seismic wave in the process of reflection and transmission.Compared with other methods,the propagator matrix method is more suitable for getting seismic response of thin layered model.In this paper,two groups of thin interbedded reservoirs with different lithology and structure are used to analyze the above forward methods as well as verify the above conclusions,which provide the proper forwarding method for coming inversion method.In order to fully understand the rock physics relationships in the research region,In this paper,we forecast sand and shale shear wave velocity of four wells in the researched region;establish the rock physics template as follw the sand porosity,shale porosity,P-wave impedance,S-wave impedance and S-wave velocity.The study find that the traditional linear or nonlinear fitting method,is not suitable for logging data in the researched region.In order to solve this problem,this paper proposes a statistical template of rock physics,which can explain the physical relationship between sands and shale.And this provides a priori information for forward modeling and inversion algorithms.Before put forward the inversion theory of thin interbed,we apply the generalized S transform which is more mature in actual production to do time-frequency analysis of theoretical model and real data.The calculation results show that the time-frequency analysis can reflect the reflection characteristics of thin layers,but due to the limitation of resolution,it is difficult to predict accurate structure of thin interbeds directly.Then we study the frequency-dependent AVO algorithm,the dispersion dependent attribute of DP and DG can predict properties correlated to fluid.Also due to the limitation of resolution,this algorithm is not suitable for thin layer structure identification either.Based on the above analyses,In the next stage it will be applied to solve the thin interbeds structure of the study area,further technique will be develop in this paper,then apply to actual seismic data to verify the validity of the method.According to the above analyses,the subject in this paper is aimed to solve the question of thin interbed seismic identification and description.First,we analysis the logging data in researched the region to establish the rock physics template between thin interbedded sand porosity,shale porosity and elastic parameters below marked layer.After that,we design typical thin interbeds geological model,then calculate the model AVO seismic response through propagator matrix.And we develop log scale high-resolution synthetic seismogram algorithm simultaneously.Last but not least,we develop the inversion technique which is based on the waveform comparison to inverse the sand-shale thin interbeds structure below marked layer,and the obtained results can be used for the prediction of favorable reservoirs. |
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