Cokriging geostatistical mapping and importance of quality of seismic attribute(s)

Multi-component seismic provides a new way of looking at reservoirs properties. Application for seismic based both on P and S waves (PS signal) is constructed on the difference in reflectivity of these waves’ types. One of the most important property of S-waves is their relative insensitivity to saturation (P-waves are very sensitive). The 3D seismic was acquired and interpreted in the Molve field. The reliability of each seismic attribute was not enough to interpret gas zones, porosity and water saturation. The reason was complex reservoir geology, represented by even 4 lithofacies: lithofacies IV (mostly retrograde metamorphosed gneisses ; Palaeozoic and older), lithofacies III (metaquartzites of Permian and Triassic ages), lithofacies II (mostly dolomites of Jurassic and Triassic age) and lithofacies I (Miocene limestones). It is why seismic attribute and reservoir parameter couldn’ t be correlated. It is why new type of multi-attribute was created for each of four reservoir lithofacies. Such multi-attribute was much more easily correlated by petrophysical variables. Based on Kalkomey’ s approach, the probability that calculated correlation was false had been estimated at 0.15. Moreover, there was established correlation between attribute and porosity, especially in reservoir lithofacies III, consisting of quartzite, schist and greywacke. It is why the porosity was interpolated using geostatistical method of cokriging, i.e. using secondary seismic variable. Also, multi-attribute analysis makes the best reservoir visualization in limestone lithology of lithofacies II. It could be expected that in several years new multi-component seismic data could be recorded for the most important Croatian hydrocarbon fields. The Molve is one of the most important. New seismic data will improve the quality of interpreted attributes, and make possible to establish clear correlation between seismic and petrophysics in all lithostratigraphic units of such heterogeneous reservoir.