The Roar Field consists of a gentle inversion anticline in which porosity, permeability and hydrocarbon saturation distributions in the Danian–Upper Cretaceous Chalk reservoir have been mapped using seismic data. The seismic data are inverted to support porosity prediction in a stochastic modelling approach. The uniform chalk matrix causes seismic impedance to correlate mainly with porosity and fluid content. Replacement of oil by water has negligible effect on seismic impedance but correction for fluid content in the gas cap is needed to increase correlation between seismic impedance and porosity. Porosity in excess of 40% is characteristic, probably reflecting early invasion of hydrocarbons. A slight tilting since Miocene times is suggested by higher porosity on the southwest flank. Permeability is generally below 10 mD in the reservoir and around 1 mD below. A slight south-dipping free water level is consistent with a regional water zone pressure gradient, if local high permeability is considered. A gas cloud that produces a velocity anomaly not apparent in well velocity data hampers depth conversion. The velocity anomaly is revealed from horizontal wells and has led to the identification of a new culmination at the north end 15 m shallower than the culmination at the south end; this has increased the estimated initial hydrocarbons in place from around 510 to 720×109 SCF gas. Analysis of uncertainty on hydrocarbon in-place calculations resulting from the characterization is limited to studying the effects of the gas cloud anomaly and the value added by the two horizontal wells on porosity field determination.