Elastic Wave Response Analysis On Thin Interbed Of Marine Gas Hydrates

China is the biggest energy producer,consumer and carbon emitter in the world.Replacing traditional fossil energy with clean and low-carbon new energy,promoting the structural transformation of energy industry and speeding up the energy transformation are important guarantees to realize China's sustainable development and achieve the goal of carbon neutrality before 2060.Compared with coal and oil,natural gas is a cleaner and more efficient energy source.When generating the same calorific value,the CO₂ emission from burning natural gas is only 59%of that from burning coal.A considerable part of natural gas is stored in natural gas hydrate,which is widely distributed in the shallow sediments of the offshore slope and continental permafrost regions,and is considered as a potential alternative energy.Relevant research shows that China is rich in marine gas hydrate resources.Since the beginning of the 21st century,a number of marine hydrate reservoirs have been delineated through exploration and investigation by means of geology,geophysics,geochemistry,drilling and other multidisciplinary methods.However,due to China's marine geographical environment,hydrates mainly exist in clay silt or silty clay with low permeability and stiffness,with thin reservoirs,low average saturation,uneven and discontinuous spatial distribution,which brings great challenges to the safe and economic exploitation of marine hydrates.In view of this,in the field of seismic exploration,it is an important prerequisite for realizing commercial exploitation of marine hydrates in China to break through the technologies of accurate identification of hydrate reservoirs,fine characterization of reservoir parameters and evaluation of favorable development areas as soon as possible.It is found that the spatial distribution of hydrate in seabed sediments is inhomogeneous,the vertical saturation difference is obvious,and the reservoirs are mostly thin layers in seismic exploration frequency band.For reservoir prediction and saturation evaluation,the traditional AVO technique based on single interface is no longer applicable.Therefore,a elastic isotropic horizontal hydrate-bearing thin interbed(or layer)model is proposed for the diffusion hydrate reservoir in South China Sea.The purpose of this study is to explore the seismic elastic wave response anomalies of hydrate-bearing thin interbed(or layer),and provide new ideas for detecting marine hydrate by seismic multi-conponent technique.First,on the basical research of hydrate,this dissertation focuses on the reservoir-forming environment and microscopic occurrence characteristics of marine hydrate in China,as well as the rock physical properties of pure gas hydrate and gas hydrate-bearing sediments,and briefly analyzes the applicability of existing seismic rock physical models of hydrate-bearing sediments to reservoirs with different hydrate occurrence types.Combined with coring data in South China Sea,the elastic wave velocity and density of gas-bearing sediments and gas hydrate-bearing sediments are predicted by using Helgerud effective medium theory model,and the relationship between formation porosity,hydrate saturation and formation equivalent elastic parameters is established.Second,based on the propagation matrix of horizontal layered media,the differences of elastic wave AVO response between single-interface of hydrate reservoir,thin hydrate reservoir and hydrate-bearing thin interbed are compared from the angle of frequency domain reflection coefficients,and the influence of reservoir thickness and hydrate saturation on seismic response is analyzed.Then,AVO synthetic seismograms of hydrate-bearing thin layer are calculated by reflectivity method,and the reflection waveform anomalies caused by different thickness and saturation of hydrate thin layer and whether the underlying strata contain free gas are simulated.Finally,the periodic thin interbed and non-periodic thin interbed models are established.By changing the vertical structure of hydrate thin interbed and the gas-bearing state of the underlying strata,the AVO waveform characteristics of elastic waves are systematically simulated and analyzed.The analysis results show that it is feasible to invert the elastic parameters and thickness of a single hydrate thin layer based on AVO waveform or reflection coefficient spectrum in principle.The joint inversion of PP and PS wave has theoretical advantages.The response of PS wave can constrain the velocity inversion of reservoir and its underlying layer,avoiding overestimation of reservoir saturation when there is strong BSR caused by free gas.At the same time,the AVO response characteristics of P-wave and S-wave can be integrated,and the main factors resulting in the BSR can be identified.There are still many difficulties in inversion of thin interbed containing hydrate,especially in areas without well-log data constraints.