Estimation of porosity and fluid saturation in carbonates from Rock-physics templates based on seismic Q

Rock-physics templates establish a link between the seismic properties (e.g., velocity, density, impedance, attenuation) and the reservoir properties such as porosity, fuid saturation, permeability and clay content. We focus on templates based on attenuation (seismic Q or quality factor), which is highly affected by those properties, and consider carbonate reservoirs, which constitute sixty percent of the world oil reserves and a potential for additional gas reserves. The seismic properties are described with mesoscopic-loss models, such as the White model of patchy saturation and the double double-porosity model, which includes frame and fuid heterogeneities. We perform the ultrasonic experiments, and estimate the attenuation of the samples and the reservoir by using the spectral ratio method and the improved frequency-shift method. Then, multi-scale calibrations of the templates are performed by using laboratory, well-log and seismic data. On this basis, reservoir porosity and fuid saturation are quantitatively evaluated. We frst apply the templates to ultrasonic data of limestone using the White model. Then, we consider seismic data of a carbonate gas reservoir of MX work area in the Sichuan Basin, southwest China. A survey line in the area is selected to detect the reservoir by using the templates. The results indicate that the estimated porosity and saturation are consistent with well-log data and actual gas production results. The methodology shows that the micro-structural characteristics of high-quality reservoir can effectively be predicted using seismic Q.