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Geological structures of both tectonic and sedimentary origin have been classified based on outcrop exposures. These structures may assume a very different appearance on core surfaces or borehole images because of the cylindrical shape of boreholes. A modular object-oriented solid-modeling system is used to simulate common geological structures on such curved surfaces. Starting from simple geometric primitives, geological layers are stacked and arbitrarily deformed in a number of steps specified by the user. The final object is calculated directly from the surface normals of the original object and a transformation matrix. Subsequent intersection of the body with a cylinder results in a core-like object, which may be viewed from any angle. Such models are presented for a number of sedimentary and tectonic structures with considerable changes in layer geometry over vertical distances ranging from centimeters to meters. Diagnostic features for some structures such as folds and cross-beds are found to differ from the outcrop. Because of the curved boorehole surface, dips and dip azimuths of key geological surfaces can be accurately measured. The models presented here help to properly recognize geological structures in boreholes and thus provide an important basis for improved reservoir models. [ABSTRACT FROM AUTHOR]