Abstract: Traditional AVO forward modeling usually uses constant parameters to construct the petrophysics model,however,these parameters have uncertainties in the exploration process,resulting in great uncertainty for the process of forward modeling. The petrophysical model was firstly established based on the wireline logs and the tools of probability density function(PDF)and information entropy(IE)were used to characterize the uncertainty of petrophysical parameters. Then based on the typical reflection interfaces,the parameters of AVO forward modeling,including the ratio of compressional wave velocity,shear wave velocity and density between the media above and below of reflection interfaces,are analyzed to the influence of AVO forward modeling. Finally,the Markov Chain Monte Carlo(MCMC)simulation method was used to model the reflection characteristics and AVO attribute response of different lithological interfaces. The IE was then used to analyze the uncertainty in AVO analysis. The results show that the PDF and IE indicate that petrophysical parameters are different in different lithologies,but there is a certain overlap for each other,which results in uncertain and multi-solution for the identification of lithology using petrophysical parameters. The AVO response indicates that the changes of the petrophysical parameters of the upper and lower media and the differences of lithology of the lower media result in different responses of AVO reflection coefficient. The attribute of AVO intercept(P)and the AVO gradient(G) shift to the type Ⅲ,as the gas saturation increases. The study effectively evaluates the uncertainty in the AVO forward modeling through statistical methods,which can provide a priori understanding for reservoir prediction and is helpful for the risk assessment and decision-making optimization of reservoir prediction.

Key words: tight sandstone reservoirs, AVO forward modeling, petrophysical model, uncertainty, probability density function, information entropy, Markov Chain Monte Carlo