A profitable development of an oil or gas field starts with a good understanding of the subsurface. As part of a subsurface evaluation team, geophysicists and geologists interpret subsurface data and provide geological models that form the basis for development planning. Construction of maps and sections of the subsurface, including integration of seismic and well data, forms an important part of the evaluation. The result should also provide illustration material on which management should be able to make decisions. Computers are currently used to construct subsurface models and calculate reserves. A check for consistency with structural models against a quick manual evaluation should always be done. This course provides the tools for this. Assessment of the connectivity between hydrocarbon-bearing areas is critical in understanding in the fluid flow within a reservoir, knowledge on faults dimensions and properties with respect to transmissibility is discussed such that will learn how to perform fault seal analysis.