This paper presents the results of pressure transient analysis carried out in several wells in a carbonate reservoir sequence, which has facilitated the assessment of the degree of vertical communication across various flow units on a larger reservoir scale. This enables the reduction of uncertainties in the full field reservoir simulation model, which is a key for the successful reservoir development plans. One of the major uncertainties that impact any reservoir development of relatively thin flow units, in a stratified carbonate reservoir, is the incomplete knowledge of the vertical inter-reservoir communication. This includes characterisation of vertical and lateral flow barriers such as, horizontal baffles, compartmentalisation and sealing faults. A well testing campaign was implemented in the high uncertainty areas of the reservoir to assess the degree of communication. The paper discusses a few examples of the well tests to demonstrate the approach utilized for the assessment. The testing procedure for the dynamic data acquisition in this study included extended flowing periods followed by extended pressure build-up periods that were analyzed by analytical and numerical modelling methods. The results indicated that there is vertical communication in the vicinity of some wells due to the discontinuity of the natural barriers, whereas for some other wells in the field a complete isolation has been exhibited within their drainage area. Introduction This study was conducted on the most mature field on production in Saudi Arabia. The main producing interval is a thick carbonate reservoir sequence with varying properties of excellent to fair limestone and dolomite. The best rock properties are located towards the upper zones of the main carbonate sequence with flow capacity values (kh) ranging from 100,000 md.ft to more than 1 million md.ft. The vertical wells in swept areas have been sidetracked and placed at the very top of the reservoir to produce the remaining attic oil column. The top 30 ft attic oil has relatively lower reservoir quality1, compared to the main producing interval, with porosities ranging from 15% to 20% and flow capacity values ranging from 1000 md.ft to 5000 md.ft. Figure-1 shows the location of the attic oil interval compared to the main reservoir carbonate sequence. Some recent saturation logs run in wells close to flank injectors have suggested original oil saturation values in this upper most zone leaving a very promising target for attic oil production. This study is focused on this attic oil interval, and it aimed to assess the degree of communication with the main carbonate sequence below in the areas of high uncertainty. A successful outcome in minimizing the geological uncertainties of the vertical flow barriers across the field will contribute for a better well placement strategy for the attic oil zone and guide the pressure support strategy of this interval. Geology of Attic Oil Zone The attic oil interval is divided into two main sections (Figure-2), the upper and the lower lobes1. The upper lobe is a 5 ft to 10 ft thick limestone separated from the lower lobe by a thin impervious anhydrite layer of 1 ft to 5 ft thick. The lower lobe is a 15 ft limestone with low dolomite content. The basal layer of the lower lobe is often a very tight dolomite which might act as vertical flow barrier in some areas. As the anhydrite and dolomite layers could act as vertical flow barriers, its distribution across the field is crucial for developing the untapped reserves in the attic oil zone.