Velocity versus offset (VVO): a new tool for direct detection of gas in low-impedance sand reservoirs

A new direct hydrocarbon indicator is proposed using velocity information, called velocity variation with offset (VVO) or velocity variation with angle. Velocity is extracted from seismic data to predict hydrocarbon distribution in the reservoir. The theory is derived based on an equation for the dependence of velocity with direction in rocks with weak polar anisotropy. Fluid content in the reservoir affects the anisotropic properties of the rock and thereby alters the variation in interval velocity with offset. We have tested VVO over a low-impedance gas sand reservoir that gives rise to a class 3 AVO anomaly. The seismic data from a known gas field were processed with amplitude preservation and offset-based velocity analysis, and AVO and VVO attributes were generated. The VVO attributes are consistently in agreement with AVO responses around the gas discovery well and other bright amplitudes away from the well location. Thomsen’s anisotropy parameters ε and δ can also be derived from VVO analysis. They have anomalously large positive values at the locations with bright amplitudes.