Advanced geological modeling and uncertainty analysis in a complex clastic gas reservoir from Saudi Arabia

Abstract

This study deals with quantitative detection of factors affecting volumetric uncertainty in Jauf reservoir in Saudi Arabia. The Devonian clastic Jauf reservoir is complex, pinched-out, highly faulted, and stratigraphic trap, limited by an erosional truncation due to a regional Pre Unayzah Unconformity (PUU). The Jauf Formation was developed in a tidally-influenced sequence of shallow marine to fluvial sandstones. Latest technology for geological modeling allowed the construction of a sophisticated model, integrating various types of data. This complex geological model incorporates a set of faults and an erosional unconformity surface, explicitly determined from seismic, and facies interpreted from core and log evaluation. A special study was conducted to explain the factors of volumetric uncertainty. Structural uncertainties, affecting both the unconformity and faulted blocks, were studied to explain the volume of the reservoir. Additional sensitivity analysis was performed for facies, porosity and gas water contact to evaluate the normalized volumetric uncertainty. Uncertainty analysis was made to identify the number of uncertain attributes. The simulation models were built through the derivative tree of ten different scenarios with hundreds of realizations using critical geological factors such as facies, porosity, structural and fluid contacts. The study showed that, using currently available data, the major factors affecting the uncertainty are the erosional unconformity surface (PUU), and the facies distribution.