AVO Intercept-Gradient Crossplot Analysis And Application Of P-P Wave

With the continuous increasing in difficulty of oil and gas exploration and development and more perfect in seismic technology, the main task has developed into structural interpretation, lithology interpretation, the oil and gas reservoir properties (porosity, permeability, hydrocarbon saturation) explanation and hydrocarbon forecast from the simple structural interpretation, so that more and more information must be extracted from seismic data. As a lot of useful information is included in the pre-stack seismic data, and AVO technique can get the information as more as possible just in time which is related to lithology and hydrocarbon properties, so it is a technique which adapts to the exploration need and has the development potential.AVO is a method of finding oil and gas using the relationship between seismic reflection amplitude and offset (incident angle). It bases on the Zoeppritz equations which describes the reflection and transmission of plane wave derived from elastic wave theory. One of the most important analytical tools is the AVO gradient and intercept(P-G) cross-plot which is a perfect method to detect the AVO response differences which is related to lithology and fluid types.In this paper with the basis of the basic theory of AVO technique, we get the AVO theoretical model using the complete Zoeppritz equations and fit the P, G according to Shuey approximate expression. Reservoir thickness, Poisson's ratio, velocity changes have an impact on the cross-plot which is analysed by getting the angle traces, fitting P, G value and drawing. By comparing analysis, P, G fit a linear for two-layers media, and an ellipse for three- layers media. Both P and G are effected by reservoir thickness, which has no impact on the slope of the trend line. P and G is the biggest and the trend line is the longest atλ/4; Poisson's ratio has a main impact on the G value and less on the P value; and the speed change is opposite to the Poisson's ratio.It shows that, from the actual data analysis, the cross-plot analysis can protrude the abnormal occurrence caused by hydrocarbons, and determine whether they are beneficial to the oil and gas reservoir by the P-G cross-plot characters.