Abstract Conventional geological modelling of oil and gas fields has in practice been restricted to illustrations on paper. Complex spatial configurations are usually represented as a limited number of 2D maps and cross-sections, perhaps complemented by a conceptual reservoir model in a block diagram. This approach precludes a quantitative assessment of essentially 3D characteristics such as reservoir connectivity and effective permeability or transmissibility of reservoir intervals. The computer modelling package Monarch, developed in Shell Research, enables the user to overcome this problem by subdividing the reservoir volume into a 3D grid of discrete volume cells (voxels) which can be used as a data base to integrate all available information. A set of modelling tools is provided to assign properties to groups of voxels, according to predefined geological rules. For this purpose, well data are related to geological information via a genetic classification scheme. Monarch first models the distribution of geologically meaningful objects, such as genetic sand-body types or lithofacies units within an undeformed stratigraphic framework. If sufficient information is available, these objects are mapped directly in three dimensions, using any information available on their expected geometry. If insufficient information is available, their distribution is modelled by means of stochastic techniques, honouring established stratification patterns and architectural trends. Subsequently the structural configuration is restored, and the effects of through-fault communication are established. In a last step, the distribution of reservoir properties within the genetic elements is included. By evaluating the resulting models statistically, probable hydrocarbon volumes connected to existing or planned wells can be calculated and optimal well locations selected. The method has been used over the last five years to assist in the development planning of oil and gas fields in the appraisal stage, to delineate optimal infill locations in producing fields and to locate remaining oil in a mature field.