Seismic low-frequency amplitude analysis for identifying gas reservoirs within thinly layered media

Seismic low-frequency amplitude anomalies can be correlated with the existence of hydrocarbon reservoirs, but the generation mechanism of these low-frequency amplitude anomalies is very complicated. We investigated the effect of the Biot slow P-waves on the seismic reflection amplitude at low frequencies from fluid-saturated porous media using techniques of both theoretical calculation and physical modelling. First, we used theoretical calculation to study the effects of the frequency, the fluid content and the heterogeneity of layered media on the conversion of the Biot slow waves, and the reflection coefficients. Then, we used physical modelling data to show a low-frequency shadow of the air-saturated layered model. The phenomenon was similar to that in the theoretical calculation. A combination of the theoretical calculation and physical modelling results provided a reasonable explanation of the low-frequency shadow. The conclusion is that whether the actual hydrocarbon reservoirs exhibit low-frequency shadow phenomena depends on the frequency, the consolidation, the thickness of the hydrocarbon reservoirs, the homogeneity of the porosity and the fluid distributions of the reservoirs. The proposed method can be used to analyse the low-frequency anomalies and to identify the fluid properties using seismic data.