This article presents research on the structure of energy mixes and the dependence on imports of the EU-27 member states, with a particular emphasis on Poland. During the conducted research, a spatial information system was used. GIS tools made it possible to build layers presenting information based on the countries’ energy mix, the level of dependence on the import of this fuel, and the share of the Russian Federation in fuel imports. It was also examined whether the level of dependence on imports from Russia was dependent on the geographical location. Since it has been shown that the share of Russian fuel is significant in the energy mixes of many member states, and that security does not depend solely on import dependence, an energy security assessment measure has been created (SES). As the level of security consists of many factors, assessing each of them separately is very difficult and unclear. Therefore, in order to simplify this analysis, it was necessary to determine one indicator that would take into account all the factors influencing the level of energy security. Poland is privileged in terms of access to fossil fuels due to its rich coal deposits; however, the potential of this fuel is not used, which is also indicated by the level of the SES measure. In the case of Poland, SES amounts to less than 16% and is almost three times lower than the EU-27 average. The indicator made it possible to indicate not only those factors that positively affect the level of energy security but also those that adversely affect it. It also enabled the identification of possible remedial measures.