Looking into the Deep: Seismic Reservoir Characterization for Understanding the Offshore Early Permian Clastic Reservoirs in Abu Dhabi

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Abstract With the current increase of domestic and global demand for gas, exploring any gas accumulations has never been more attractive in Abu Dhabi. The deepest targets, such as the clastic Permian formations, are especially attractive due to their H2S free (sweet gas) characteristics. Nevertheless, understanding the deep reservoirs is still one of the main exploration challenges in Abu Dhabi. The primary Pre-Khuff reservoir potential within offshore Abu Dhabi is considered to lie within the siliciclastic deposits of the Unayzah Fm. The Unayzah Fm. comprises predominantly continental deposits ranging from alluvial fan, braided river, lacustrine facies in western Saudi Arabia to fluvial, eolian, overbank and interdune facies further east. Deltaic to shallow marine deposits are also present in eastern Abu Dhabi and Saudi Arabia. Seismic reservoir characterization poses as one of the most significant options for understanding the reservoir architecture and properties. Pre-stack seismic inversion was conducted targeting the Pre-Khuff zone of two offshore fields in Abu Dhabi. The available seismic data is a full azimuth high-quality 3D ocean bottom cable (OBC), covering 1270km2, with wide radial offset range of approximately 9400 meters. Due to the long offsets, four partial angle stacks (up to 40° of mid angle) processed to relatively true amplitudes, after pre-stack time migration, allowed the extraction of seismic reflectivity information at the deep sections with confidence. Elastic properties were derived from seismic reflectivity data and reservoir properties were sub consequently obtained. Although the inversion resolution was led by the seismic bandwidth, lateral continuity of inverted properties (P-impedance and Vp/Vs ratio) benefitted from the good seismic coherency of reflectors and signal to noise ratio. The comparison between the recorded seismic and the inverted synthetics exhibited a high cross-correlation throughout the whole area, for all partial stacks. The novelty of this study is the applicability of pre-stack seismic inversion techniques to the characterization of deep clastic reservoirs in offshore Abu Dhabi. The final products represent valuable information not only for further qualitative analysis in terms of reservoir geometry interpretation but also to derive some reservoir properties that could potentially optimize future exploration and appraisal activities.

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