Amplitude variation with offset modeling of a North Sea well

Abstract

Amplitude variation with offset (AVO) modeling has been performed on well logs from exploration well 25/5-6. The well was drilled on the Mon prospect in 2009 and encountered water-bearing sandstones in Paleocene Hermod Mbr. The objective of the present work is to obtain an understanding of what the seismic data represents and possibly be able to use this knowledge in future projects. Long offsets is becoming more common as the information obtained at far offset is valuable in the process of quantitative interpretation. As the offset in the data increases, the seismic signal changes as offset-dependent tuning and refractions may occur. In processing, normal moveout (NMO) stretch and the breaking down of the travel time equation at far offsets need to be accounted for.

The AVO modeling has been performed by use of the Hampson-Russell AVO software and the well data has been received from Lundin Norway AS, operator for the Mon well. Experimental studies on cutting data have been combined with the modeling in order to tie the seismic response to the true geology. The synthetic gathers have also been compared to real seismic in the area. Gathers from NSR09-41155 2D line and a sub-cube of the TO06R06M1 3D cube, which the original interpretation was done on, have been available.

A separate study on the response of cemented stringers of various thicknesses and numbers was performed. It was desired to assess the capability of the modeling algorithms and to examine how the response changed by the application of different wavelets and target zones for modeling.

The response from the separate events observed in the well logs has been studied by modification of the logs. The events have been flattened out one by one and in combination.

Modeling results show that refracted energy occurs at offsets of about 2500 m – 3000 m. The refracted energy is probably set up by siderite cemented zones at top and base of the reservoir. The base has the strongest influence, and the observed spikes do not seem to contribute.

In the 2D seismic data, the whole reservoir is represented by one strong reflector probably caused by tuning, that changes appearance at about 2400 m offset. A class III AVO anomaly observed in the soft event above the reservoir may have been the same event analyzed pre-drill if the original 3D data comprised the same effect as observed in the available 2D data.