Modification of Zoeppritz Equations and Its Implications for Hydrocarbon Exploration: A Case Study of an Onshore Niger Delta Sedimentary Basin

Abstract

Zoeppritz equations are used to determine the reflection coefficient against the angle curves, which are often valid only for small seismic parameter changes across reflectors, but generally inaccurate close to the critical angle. These inaccuracies affect the quality of amplitude variations with offset (AVO) analysis, which might results in systematic errors when estimating relative seismic parameter variations at the reflectors. Thus modifying the Zoeppritz equations at the given angles allows for more accurate estimation of the usual AVO attributes, such as intercept, the gradient, and a possible third coefficient, which often leads to a better estimation of seismic-parameter contrasts at reflecting interfaces. The modification of Zoeppritz equations was analyzed using well data from oil fields in a sedimentary basin, onshore of Niger Delta area. This paper analyzed the modification of Zoeppritz equations and using them in AVO analysis to collect information on how seismic amplitudes vary with incident angles, which when combined with the P-P reflectivity (RPP) or the P-S Reflectivity (RPS) expressions, is used to obtain information on the properties of the earth layers, with the emphasis only on interface reflectivity, while thin-bed effects, attenuation and other propagation factors well known to influence AVO measurements are not considered. The modified equations are subsequently used to generate the AVO reflectivity curves, the results obtained shows that the modified Zoeppritz predicted the AVO effects correctly for the different zones of interest in the basin. The results show that the Shuey’s approximation gives better accurate results up to angle of 300 compared with others approximations, while the 3-term approximation shows that the modified Zoeppritz equations predicted AVO response accurately to about 500 of angle of incident. The result obtained can also be used to classify the different sand base types and their fluid contents, either oil, gas or brine sand base.