Distinguishing fizz gas from commercial gas reservoirs using multicomponent seismic data

Abstract

Partial gas discrimination is a challenging problem because low and high gas saturation can result in very similar seismic AVO, bright spot, and velocity sag anomalies. This is typically explained using Gassmann's theory: i.e., (1) small amounts of gas in the pore space cause large decreases in rock incompressibility while further increasing gas content does not reduce rock incompressibility significantly, and (2) the shear modulus is not affected by nonviscous fluids in the rock pore space. In addition, rock bulk density varies gradually with water saturation, as predicted using the volume-average equation. Consequently, low-gas saturation reservoirs and high-gas saturation reservoirs can have similar VP and VP/VS values (or, equivalently, Poisson's ratios). Therefore, in many cases, high and low gas saturations cannot be distinguished using existing hydrocarbon indicators and techniques. These indicators include those based on VP variations, such as velocity sags, and those based on Poisson's ratio vari...