Attenuation study of a clay-rich dense zone in fractured carbonate reservoirs

Seismic attenuation in clay-rich dense zones remains unknown, despite the importance of such zones in hydrocarbon reservoirs, where they delimit the reservoir zones and isolate them from nearby aquifers. We have determined that a dense zone separating two carbonate reservoirs of an onshore oilfield in Abu Dhabi, United Arab Emirates, exhibits the highest intrinsic attenuation even though the zone contains no hydrocarbons. The frictional movement due to the elastic contrast between the hard carbonates and soft clay is most likely the dominant mechanism in the dense zone. The compressional sonic and vertical seismic profile (VSP) attenuation are on the same order of magnitude and are both maximum in the dense zone. Therefore, it is possible that the same attenuation mechanism in this zone exists at low and high frequencies; whereas the intrinsic attenuation mechanism in the reservoir zones, which are more permeable and porous than the dense zone, can be explained by the coexistence of global and squirt-flow mechanisms. Moreover, sonic attenuation exhibits higher magnitudes than VSP attenuation in these zones. This is due to the fact that the squirt-flow mechanism, which can take place between pores and fractures, is more important at sonic frequencies. The scattering mechanism is also important in the reservoir zones; this is due to the high heterogeneity and the presence of fractures in these zones.