Estimation of anisotropy parameters using intrinsic rock properties in a selected swamp fields in Niger Delta

Abstract

Total disregard of anisotropy in seismic velocity analysis often accounts for suboptimal imaging especially when prestack depth migration algorithm is used in depth positioning and focusing. The type of anisotropy commonly observed in most sedimentary basins, like the Niger Delta, which comprises of about 70% shale, is the Vertical Transverse Isotropy (VTI), which often affects imaging processes. This type of anisotropy can be accurately quantified by estimating the three Thomsen parameters namely: epsilon (ε), delta (δ) and gamma (γ), determination of these parameters will greatly enhances the accurate imaging of events subsurface in prestack depth migration which is used for the proper placement of the events. For this study, the Thomsen parameters, which were derived from well log suites of selected swamp fields of the study area, were used to completely characterize the vertical transverse isotropy (VTI) of the system in our study area, a total of five elastic stiffness moduli were estimated and the values obtained were subsequently used in the estimation of the anisotropy parameters needed. The following values were estimated for the anisotropy parameters from the selected wells; parameter delta (δ) values estimated lies within the ranges -0.16 ≤ δ ≤ 0.13, while for parameter epsilon (ε), the values lies within the ranges -0.07 ≤ ε ≤ 0.11. The values for parameter gamma (γ) estimated lies within ranges -0.42 ≤ γ ≤ 0.4 while the values for eta (η) lies within the ranges -0.13 ≤ η ≤ 0.27, respectively. The anisotropy parameters values estimated for the selected swamp fields correlate well (0.95%) with the values of the same parameters obtained analytically with a combination of seismic moveout velocity (V_NMO) and vertical velocity from the check-shot data. The anisotropy estimated was found to be higher in shale than in sands, while P-wave anisotropy parameters are observed to be generally smaller than S-wave anisotropy parameters. In the study area, the plots of the anisotropy parameters within the depobelt show a weak anisotropy for the study area.