Manual interpretation of well log measurements is a lengthy process that requires an extensive knowledge of the area's geology and experience in interpreting well log measurements. The aim of the master's thesis is to develop an expert system that will help me with data processing and well log interpretation. Expert system is developed in VBA language and is used together with Excel, where data is stored. Code of the expert system is composed in such way, that well log measurements, such as neutron porosity, density, short and long normal electrical resistivity, caliper and laterolog, determine which fraction is located at each depth. By knowing the area's geology and lithological properties of the layers I set the criteria which are specific to each fraction. Developed program checks whether the data of well log measurements meet the criteria of the fraction and writes the result to the specific location. In addition to the basic results, notes which help us to interpret well logs are also produced. This system determines the position of coal, sand and clay fractions in the area of Velenje Coal Mine. Well log interpretations, results and images of the results are presented for each well, together with basic data. That way correlation between input data and results is visible. Expert system has proven to bea n effective aid to the maual well log interpretation.