Reservoir characterisation using 4C seismic and calibrated 3D AVO

In this case study, modern seismic technology is used to characterise the Lower Tertiary Siri reservoir (Norwegian-Danish North Sea border). Based on 4-component (4C) seismic and calibrated 3D AVO (Amplitude Versus Offset) data, it is possible to define fluid types, assess reservoir quality and map the geometry of the trap. It is well-known that the ratio of P wave velocity and S wave velocity (V-P/V-S) in porous media is sensitive to both lithology and type of pore fluids. As a consequence, both these effects influence the AVO response. Whereas extensive well-log information is usually required to calibrate the lithology effect if AVO analysis is to be used for mapping of pore fluids, this paper presents a procedure for using seabed 4C seismic profiles to calibrate the lithology effect and thereby enables pore-fluid mapping from 3D AVO. Such a methodology is attractive at the exploration or appraisal stage when the well control is limited. The potential of the procedure is demonstrated in a case study of a Late Paleocene fan system from the North Sea. Firstly, 3D spectral attributes are used to divide the seismic data into depositional packages. Then the 4C seabed data are processed using a novel model-based technique. The model consistency obtained between all data sets allows a calibration of the 3D AVO cubes to be computed. Finally, the calibrated 3D AVO cubes can be inverted for fluids in each lithological unit recognised.