A unique true distribution of rock properties exists in each petroleum reservoir as a result of a complex sequence of physical, chemical, and biological processes. Although some of these depositional and diagenetic processes are well understood, it is impossible to define the initial and boundary conditions in sufficient detail to provide a deterministic picture of the reservoir. There is unavoidable uncertainty in the distribution of rock properties. Geostatistical techniques are increasingly used to quantify this uncertainty and to create numerical models that mimic the physically significant features of rock property variation.