Variation of the Shale Properties in the Offshore Equatorial Guinea and Its Impact on the Seismic AVO of the Reservoir Sands

Wan, JC¹, Michelle Thomas¹, John Graves¹, Jacques Leveille¹ (1) Amerada Hess Corporation, Houston, TX

Seismic AVO was successfully used in finding the Ceiba field in Rio Muni basin in the offshore Equatorial Guinea. In general, AVO responses depend on both the bounding shale and the reservoir sand properties. Shale acoustic properties in this basin vary significantly and seemingly cast no clear trends with depth below mudline (DBM), which makes the prediction of seismic attributes of reservoir sands difficult. Closer examination of seismic data, well logs and core data revealed that the shale elastic properties in this basin vary between two end-members: the soft shale trend and the hard shale trend, i.e. the clay-rich/hemipelagic shale versus the canyon/channel filling silt-rich mud. The difference in compressional velocities between the soft and hard shales often exceeds 20%. Depending on the type of shales, very different seismic AVO may be produced for sands. However, from seismic interpretation and well control, we observed that most of sands reside in the canyon/channel systems, i.e. bounded with hard shales. Consequently, oil sands tend to show Class III AVO while wet sands show Class II AVO.