Rock physics diagnostic of North Sea sands: Link between microstructure and seismic properties

Velocity in high porosity sands strongly depends on the position and amount of intergranular material. Velocity is high if the original grains are cem ented at their contacts. It is low if the pore-filling material is placed a way from the contacts. The amount and type of intergranular material can be theoretically determined from velocity and porosity data by adjusting an e ffective-medium theoretical model curve to a trend in the data and then ass uming that the microstructure of the sediment is such as used in the model. We apply this diagnostic method to clean sands in the Heimdal formation (t he North Sea). The results show that the reservoir-zone sand in one well un der examination has slight contact cementation, while it is completely unce mented and friable in another well, This conclusion is supported by microsc opic grain images. The rock physics diagnostic is important for establishin g a velocity-porosity trend consistent with local geology. Such trends can improve the accuracy of porosity prediction from seismic data.