Abstract
AVO based Zoeppritz’s plane wave and elastic spherical wave anisotropic synthetic modelling have been investigated in Derby field, Southeastern Niger delta. The objective of the study is to investigate the influence of anisotropy on plane and elastic wave AVO synthetics modeling over shale-gas sand horizons in the field. Well log data was check shot corrected and correlated with pre-stack data and a zero phase wavelet was extracted after well conditioning and petro physical analysis. Elastic and plane wave algorithms were then used to generate offset-dependent anisotropic synthetic seismograms respectively, using p-wave sonic, s-wave and density logs and Thompsen’s epsilon (e) and delta (σ) anisotropic logs for a transversely isotropic media. Results revealed that seismic anisotropy for AVO based analysis is better modelled with elastic rather than the conventional Zoeppritz’s plane wave model and their approximations for transversely isotropic media at larger offsets. This is attributed to the breakdown of the Zoeppritz’s plane wave model and their linearized approximations at large offsets or near the critical critical angle. Using elastic synthetic model for anisotropic AVO analysis will not only ensure that the reservoir will be adequately imaged, but misinterpretation of data and misplacement of the well will be circumvented.
Highlights
Seismic synthetic modelling is the key to obtaining a better fit with the real seismic data
Results show that Thomson’s epsilon (ε) and delta (σ) anisotropic logs are low in gas reservoir sands and high in non-reservoir shale formations in the well. This is an evidence of fact that anisotropy exist and is stronger in shale than gas sands
Careful inspection of the Zoeppritz’s plane wave and elastic spherical wave anisotropic synthetic models over the gas sand tops HDI, HDII and HDIII, respectively, show that reflections are stronger at near offsets and weak at far offsets
Summary
Seismic synthetic modelling is the key to obtaining a better fit with the real seismic data It forms the basis for understanding the seismic signature especially, in anisotropic AVO analysis of pre-stack seismic data for lithology prediction and direct hydrocarbon indication (Castagna, 2001). These reservoir variables are mainly extracted at mid-to-far offsets were anisotropy of rock formations becomes significant. Anisotropy decreases the amplitude of reflection coefficients of pwaves with increasing offset (Williams and Jenner, 2002) This is largely due to the effect on the shape of the incident wave front which determines the magnitude of the incident angle. The shape of the p-wave incident wave front is determined by two principal anisotropic parameters: Epsilon (ε) and delta (σ) in the overburden. ε is the p-wave anisotropy and σ is a nonintutive amalgamation of constants of elasticity that controls the form of the attenuation at intermediate angles that affects logging responses and AVO directly (Thomsen, 1996)
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