The Zechsteinkalk of the Hewett Field is characterized by extremely heterogeneous porosity distribution. The two most closely spaced wells (only 200 m apart) show one of the largest differences in average porosity (1.8 to 7.6%). In order to locate future wells and achieve 'reasonable' OGIP estimates, it is therefore necessary to model porosity distribution. Facies type has some influence on porosity development. Tidal flat muds developed at the shoreward margins of shoals have only very low porosity, whereas inter-shoal and main shoal facies have moderate porosity, on average. Extreme porosity variation within facies is caused by differential diagenesis. Porosity has been created by leaching and dolomitization and destroyed by early carbonate cementation and by anhydrite cementation. Anhydrite is volumetrically the most significant, occluding 20% (bulk volume) porosity in many intervals and completely cementing many fractures. The scale of the variability that can be derived from modelling of porosity-affecting diagenetic processes is mostly much greater than that found between closely spaced wells, particularly in the case of anhydrite cementation. In spite of this uncertainty the geological model represents an improvement on simple independent porosity contouring. The gross trends of porosity distribution, defined by the sedimentological model, have helped enhance confidence in a seismic porosity model and have helped with estimates of gas resources.