Seismic anisotropy in Trinidad: A new tool for lithology prediction

Abstract

A great challenge facing explorationists is identification of potential intervals of reservoir quality sand in areas with little well control. This is especially acute in deep water where reservoir presence can be elusive and well costs are considerable. Substantial evidence suggests the occurrence of transversely isotropic (TI) anisotropy in the subsurface associated primarily with shales, while intervals of clastic material show greatly reduced and/or near-zero TI anisotropy. Although this has been demonstrated conclusively on core samples, the need exists to translate these concepts into a geologic framework on a basinwide and prospect scale. This article presents a method for estimating interval anisotropy from nonhyperbolic moveout on common midpoint gathers and relating these values to subsurface geology. Alkhalifah and Tsvankin (1995) demonstrated that for TI media with a vertical symmetry axis (VTI media), only two parameters are sufficient for performing all time-related processing, such as NMO correction (including nonhyperbolic moveout correction, if necessary), DMO correction, and prestack and poststack time migration. One of these parameters, η, is dimensionless; its departure from zero implies anisotropy. The other parameter, Vnmo, is the short-spread NMO velocity for horizontal reflectors. These two parameters can be obtained solely from surface P -wave data, using estimates of stacking velocity for reflections from interfaces having two distinct dips (Alkhalifah and Tsvankin, 1995). VTI media induce nonhyperbolic moveout of reflections that depends on Vnmo and η at large offsets. Alkhalifah (1997) showed that these parameters can be obtained by analyzing the moveout at far offsets. The direct output from such an inversion (or velocity analysis) is the effective values of Vnmo and η that include the combined (average) influence of the overburden. These effective values are converted to interval values using layer-stripping techniques. Using data from offshore Africa, Alkhalifah et al. (1996) showed that anisotropic …