Mapping and characterization of thin chalk reservoirs using data integration: the Kraka Field, Danish North Sea

Abstract

The integration of 3D seismic data, well logs and synthetic seismic data has been used to identify an additional Intra Danian seismic horizon in the chalk reservoir of the Kraka Field, Danish North Sea. Mapping of this seismic horizon has allowed production of a separate thickness map for the main reservoir unit, the Danian Porous, in the greater Kraka area. The unit is less than 25 m thick in most areas and, to produce reliable reservoir maps, it has been necessary to use well data to guide the seismic interpretation. It is impossible, however, to resolve the reservoir stratigraphy properly in areas where the Danian Porous is thinner than c. 15 m due to tuning effects. The lateral porosity distribution has been mapped using a combination of well log data and seismic data inverted for acoustic impedance. The Danian Porous unit is characterized by average porosities over 28% and shows no evidence of depth-related porosity reduction. Rather, the impedance data indicate the presence of positive porosity anomalies both over the crest and downflank towards the southeast. Comparison of impedance-derived porosities with those derived from well data indicates that the seismic-based data reflect the variations in porosity but underestimate the highest porosity by 3–4%. Faults and fractures are important for production of the Kraka Field. Detailed mapping of seismic horizons, supplemented with seismic attribute mapping, has proved useful for outlining areas with high fault intensity in the northwestern part of the field but has been unsuccessful in identifying individual faults as recognized from log data.