Amplitude variation with offset (AVO) analysis via fluid replacement modeling (FRM) for characterizing the reservoir response of Cretaceous sand interval

摘要

The reservoir pore fluid at various saturation levels can be modeled and analyzed through fluid replacement modeling (FRM) by applying Gassmann's theory. The FRM technique was applied to a gas-prone reservoir (Cretaceous C-sand interval) of the Lower Goru Formation in the Sawan gas field, Middle Indus Basin, Pakistan. 3D post-stack seismic data and wireline log data of two exploration wells (Sawan-01 and Sawan-08) were utilized in the study. The petrophysical analysis of wells was carried out to initially predict the gas-bearing zones of the reservoir interval at well locations, and later on to predict areas away from the well through seismic to well tie. The amplitude variation with offset (AVO) behavior was analyzed by replacing the in situ fluid saturation level with a modeled 80% gas and 20% water saturation in the reservoir. The elastic properties of the reservoir sand were estimated through rock physics based on Gassmann equations. The AVO response under both in situ and FRM conditions indicated that the reservoir gas sand exhibits a typical class Ⅳ signature. The increase of reflection amplitude with angle in FRM synthetic gathers showed AVO sensitivity to changes in fluid type and saturation. The change in reservoir parameters with a change in fluid saturation levels and their AVO response was well-captured in the analysis for effective identification of lithology and fluid concentration in the reservoir. Moreover, the methodology followed in this study will be helpful for better characterizing hydrocarbon-bearing reservoirs within the study region and in various other parts of the world.