Yibal field is one of the most important fields to Petroleum Development Oman (PDO). At a certain time, the field used to produce one quarter of PDO's total production. The principal hydrocarbon reservoir is Shuaiba (ca. 1300 meter below sea level). Drilling into the reservoir is preceded by drilling the Tertiary Formations that consist of the Umm er Radhuma (UeR) Formation, Rus Formation, Dammam Formation and Fars group. The Fars and the upper UeR are aquifers whereas the rest may be considered as aquitards. Usually those Formations are associated with lots of problems that include losses, inefficient cement jobs, casing corrosion, water flowouts and blowouts that in some incidents had caused the loss of the whole drilling rig. The field contains 474 wells where some of them are 30 years old. Older wells are more apt to the well integrity problems mentioned before. The most recent water flowouts are the Y-86 and Y-177, which happened while writing this study. Th concern to thoroughly understand the geometry of those formations. Until now limited information is known about the hydrogeological characterisation of the Yibal overburden Formations. This study aims at characterising the Yibal Tertiary Formations both from geological and hydrological perspectives. This forms the base to building a conceptual model where this study deals with the static part of it. The model could be used to test for the conditions that would permit communication between the Fars and UeR It could also be used for monitoring any contaminant in the future. The dynamic part of the model is going to be built in a separate project. Data for the study mainly comes from seismic data, data of 417 wells and aquifer test data. From the seismic data that was acquired in 1993, various illumination plots have been constructed to make an interpretation of the shallow subsurface faults. Following an extensive quality checking of the Formation tops data using gamma ray logs, top structure maps of the various Formations have been constructed. Aquifer tests data of the Umm er Radhuma and Fars aquifers have been used to establish the transmissivity and hydraulic conductivity. Bringing all the results together ended up with building a 14-layers model with average matrix properties. Inadequate quality of aquifer test data and lack of basic geological data (e.g. Core data) are the sources of deviation from proper characterisation of the Tertiary system and hence building a more realistic 3-D flow model.