Assessing The Uncertainty In Hydrocarbon Distributions In The Transition Zone Of A Giant Carbonate Reservoir

Considerable oil reserves are still held in the transition zone in some of the giant, tight carbonate reservoirs of the Middle East. These reservoirs are characterized by poor rock quality, heterogeneity and mixed wettability. Estimating and modelling the hydrocarbon distribution in the transition zones of these reservoirs is difficult. A multi-deterministic scenario workflow was applied to one of the giant and complex carbonate reservoirs (“Field X”) in the Middle East that comprises a thick transition zone. This approach had the objective to explore a broader range of geological uncertainties and analyse the impact of different modelling techniques, with the aim to provide to more robust STOIIP estimates compared to models that are anchored to a single base case. The uncertainty in modelling the initial hydrocarbon distribution in Field X changes STOIIP estimates by up to 20% and is mainly related to the chosen reservoir rock typing approach and the way saturation distributions are modelled. These uncertainties do not only affect STOIIP estimates, they also are expected to impact predictions from reservoir simulations and the quality of history matching exercises, and hence influence future development strategies and reserve estimates.