Forward Modeling Of Time Frequency Characteristics Of Cyclic Thin Interbeds

With the increasing difficulty in the exploration and development of oil fields in our country,the prediction of thin interbed reservoir has gradually attracted many researchers.However,due to the characteristics of thin thickness,complex structure,fast transversal change and strong anisotropy,thin interbed reservoir is very difficult to predict.Generally,It is a good idea to separate thin interbeds into different patterns according to their structural characteristics,and then study different patterns of thin interbeds respectively.Cyclical thin interbeds are an common pattern in thin interbeds and a common formation assemblage in sedimentary strata.The cyclic thin interbeds correspond to sedimentary cycles in sequence stratigraphy,therefore,solving the problem of cyclic thin interbeds is not only an important proposition in sequence stratigraphy research,but also has important guiding significance in the research of general thin interbeds prediction.In the study of sedimentary cycles,some main problems need to be solved,including the unit division,location,pattern discrimination and lateral tracking of cyclical thin interbeds.The application of logging data in sedimentary cycle is an common method in the practical work.It is accurate for sedimentary cycle units at well points,however,which is limited to predict lateral distribution of sedimentary cycle units in the absence of wells.Instead,the seismic data is not restricted by the well point,which is beneficial to the study of lateral tracing of sedimentary cycle units.Starting from the seismic data,we set up different sedimentary cycle geological models around the main problems concerned in the identification of sedimentary cycles,and used wave equation forward modeling to simulate seismic data.Then,those research has been done including the comparison of different time frequency analysis methods to distinguish the sedimentary cycle,the comparison of different timefrequency analysis post-processing methods to distinguish the sedimentary cycle,analysis of the influence factors and the mechanism of time-frequency analysis methods distinguish the sedimentary cycle and the study of lateral tracing method for sedimentary cycle.This research obtained the following conclusions:1.In some of these time-frequency analysis methods,affine class time-frequency distribution can not only suppress the cross-term interference of Wigner-Ville distribution effectively,but also has a good effect on random noise suppression,having the strongest ability to identify sedimentary cycle.It can reduce the impact of reflection coefficient mutation,layer of discontinuity and combination mode change of sedimentary cycle unit on the dominant frequency by spectral smoothing which can smear out the details of instantaneous spectrum and underline the tendency of the dominant frequency,having good stability.2.In some of these time-frequency analysis post-processing methods,synchroextracting transform can not only effectively extract the weak high-frequency signal components corresponding to the thin layers,but also ensure a better robustness to noise.The time-frequency spectrum calculated by synchroextracting transform can clearly describe the tendency of the dominant frequency,having a good focus and the highest resolution.We can combine the twomethods to enhance the discriminant accuracy of sedimentary cycle when synchroextracting transform can't accurately extract the thin layer's frequency components alone.3.If not considering the effects of noise,absorption and attenuation and wavelet filtering,the main factors affecting the identifying ability of sedimentary cycle modes are interlayer number and thickness gradient.In addition,through the mechanism analysis,it is recognized that the time-frequency spectrum of the thin layer and the cyclical thin interbed has a uniform regularity with the change of the thickness of the layer.This law of unity weakens as the gradient of thickness changes.4.The top interface tracking method has been established based on the characteristics that the thin layer position of the top interface corresponds to the high frequency maximum.The limitations of that method are discussed by analyzing the time-frequency characteristics of models mentioned above.Based on the characteristics that the synchroextracting operator has the narrow arch shape at the top interface thin layer,a human-interactive,feedback and revision scheme is presented to improve the accuracy of tracking results.The feasibility and practicability of that method are proved by the top interface tracking experiment of the theoretical model finally.