Defining the transition zone in clastic oil formations with mixed or water-preferential wettability is useful in estimating the initial oil resource, locating production and injection wells, choosing drilling intervals, predicting water intrusion into production wells, and modeling the reservoir for simulation purposes. The purpose of this article is to define the initial distribution of water in oil reservoirs and especially the knowledge of the transition zone, presenting theoretical concepts and practical methods for planning the necessary experiments and interpreting the data collected to define this part of the oil structure. The article presents the theoretical elements of the methods of obtaining and using capillary pressure curves, data on interfacial tension and contact angle. The dynamics of the saturation state during oil reservoir formation and the most optimal method for determining fluid contacts and free water levels are also described.