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.