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Bright spot (including stacked seismic brigh-spot and traditional AVO anomaly) does not mean high gas saturation hydrocarbon (maybe non reservoir, or brine, or low gas saturation hydrocarbon), and high gas saturation hydrocarbon maybe not exhibit bright-spot (maybe nonbright-spot) behavior especially in deep water area or deep gas situation. But high gas saturation hydrocarbon must exhibit low elastic properties (Poisson’s ratio, or density, etc.) behavior in the northern South China Sea and Gulf of Mexico. Quantitative elastic prediction is the effective solution of non-gas bright-spot and non-bright-spot gas identification problem in those areas.

Summary Rock physics study shows that Jurassic gas-sand with low water saturation (Sw) is highly correlated to density properties. Moreover, density property of Jurassic gas-sand is much less than that of shale. Seismic-derived density property is used for the prediction of Jurassic gas-sand with low water saturation. Density contrast (reflectivity) is derived from 3D pre-stack time migrated CRP gather data and velocity data, and then density reflectivity are inverted to density volume. Two low density anomalies (including one new anomaly discovered) was drilled and proved to be gas-sand with low water saturation.

Summary It is commonly accepted that pre-stack seismic inversion (quantitative AVO) is a powerful tool to predict fluids. This study investigates the use of pre-stack density inversion to predict gas-saturated carbonate reservoir using well log data and pre-stack time migrated data (CRP gather). In M block, the upper Jurassic limestone reservoir is dominant. The comprehensive rock physics analysis indicates that density is the most sensitive elastic parameter to fluids. Low density is the main behavior of gas-saturated carbonate reservoir. Density data within upper Jurassic zone is used to discriminate gas-saturated limestone from water-saturated limestone.

Determination of the conversion point is an important step in P-SV converted-wave survey design and in data processing. Unlike the midpoint determination in P-P wave exploration that is determined geometrically, the conversion point in P-S exploration is determined by physical properties of the formations. The depth-dependent conversion-point position is often approximated by asymptotic conversion point, which is at a constant offset to the source. The influence of polar anisotropy on the P-S conversion point has to be considered together with the effect of reflector depth, offset, as well as layering. We developed a general raytracing algorithm for multi-layered modeling using exact velocity equations for weak anisotropy to map the raypath and the conversion point location. The conversion point can also be determined by using the eff γ method, where eff γ refers to effective velocity

The important aspect of converted-wave (P-S) seismology is that the exact location of P-S conversion point at the reflector is not well known. For a single horizontal layer, the position of the conversion point can be calculated exactly from an analytic expression. In multi-layered strata, the conversion point is not at a constant offset from the source, but traces a trajectory that moves towards the receiver as the depth decreases. The earth is known to be anisotropic, although basic seismic survey planning and data-processing are based on the isotropic assumption. The most common anisotropic case is Vertical Transversely Isotropic (VTI) media. In VTI media, there can be a large difference between the true coordinate of the conversion point and the one obtained from the isotropic single-layered model. This horizontal displacement of the conversion point in VTI media from that in the isotropic case is dependent on the offset-to-depth ratio, velocity ratio, and anisotropic parameters e and δ defined by Thomsen. The relationship linking the displacement and the anisotropic parameters, and offset-to-depth ratio, can be complicated. An algorithm to calculate this relationship is developed using Thomsen’s anisotropy equations, both the linear approximation and the exact forms. A VTI model is designed using NORSAR2D software and the common-shot raytracing is performed to obtain the conversion-point coordinate. The displacement of the conversion point increases with the increasing offset to depth ratio and the anisotropy parameter e. The value of δ can also have large influence on the displacement of the conversion point. We conclude that when the anisotropic parameter e is smaller than δ, the conversion point is displaced towards the receiver relative to its location in an isotropic medium. When e is larger than δ, the conversion point moves towards the source compared to that in the isotropic medium. There is no large difference between the results from Thomsen’s linear equations and the results from exact equations at small offset-todepth ratios. INTRODUCTION Converted waves have been more and more widely used in seismic exploration, for they can provide high-quality images where conventional images are poor (Stewart et al., 1999). Figure 1 shows the geometry of the converted wave in a thick, uniform isotropic layer. A ray emitted as a P-wave at angle θP from the source reflects from the bottom of the layer as an S-wave at an angle θS and is received at the receiver at a certain offset. These two angles are related by Snell’s law