Being a powerful instrument for modelling, analysing, and simulating dynamic systems, Petri Nets have been used many times in the past to solve software engineering problems and to model different aspects of information system. However, none of these research attempts have succeeded in modelling complete information system's behaviour in formal and executable way, at the same time. In the field of requirement specification and modelling of the information systems, it is usual to find object-oriented (OO) methodologies (UML) instead of some formal methods such as Petri Nets. In this paper, we propose a model based on extension of Petri Nets for creating formal static and dynamic models of information systems. The proposed model was designed as an executable replacement of the UML dynamic diagrams. For modelling static structure of the system we still suggest using OO methods, but proposed Petri Nets extension provides complete integration of the OO static elements into Petri Nets used as a dynamic model. The main contribution of the proposed Petri Nets extension is that we get the formal model that is, unlike other known formal models, directly and completely executable. More precisely, we created a model of a Petri Net that represents real information system with all its functionalities. Moreover, we created a CASE tool that allows modelling and execution of a system using the proposed model.