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Seismic Characterization of the Upper Unayzah Fairway in Eastern Saudi Arabia

Abstract

The Late Carboniferous-Early Permian Unayzah Group encompasses some of the key Paleozoic clastic reservoirs in Eastern Saudi Arabia, constituting one of the main targets of Saudi Aramco's exploration efforts to develop significant, nonassociated gas reserves. The Unayzah Group is subdivided into three main reservoir intervals, grouped for the purposes of this paper in two main zones: Unayzah A (Upper zone) and B/C (Lower zone). These were deposited on the paleotopography created by the uplift and erosional event known locally as the “Hercynian Unconformity.” The Unayzah A interval is comprised of interbedded ephemeral fluvial, playa, and eolian sandstones. The area to the south and southeast of Ghawar field is characterized by a preponderance of eolian sediments that are relatively continuous within a west-east trending belt, with an average total thickness of approximately 100 meters, forming the main Unayzah A eolian fairway. This reservoir facies contains large natural gas reserves, comprising of well-sorted sandstones with excellent porosities and permeabilities, in spite of being located at 4500+ meters of depth. Lateral seals are provided by facies changes from reservoir sands to tight interdune/playa deposits, while vertical seals are provided by paleosols on the upper part of the interval and the overlaying formations. Understanding reservoir and non-reservoir facies distribution within the Unayzah A interval, as well as being able to determine presence of hydrocarbons pre-drill, is of critical importance for exploration and delineation planning. Seismic characterization of the reservoir facies and its hydrocarbon content based on 3D data facilitates these efforts, but in the exploration context may be challenging due to the large areas covered and limited well control. Newly acquired broadband seismic data has helped address this by providing a more reliable indication of the subsurface lateral variability. A log-based seismic lithofacies classification was undertaken first to understand the facies elastic properties. Analysis of seismic attributes shows that in the post-stack domain, Acoustic Impedance (AI) is usually a reliable indicator of reservoir quality facies, but it cannot detect the presence of hydrocarbons within them. In the pre-stack domain, the combination of Velocity Ratio (VP/VS) and AI can be used to isolate hydrocarbon-charged, good reservoir intervals. This approach has proven successful in a number of recent wells.