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.
In 2017 the Abu Dhabi National Oil Company (ADNOC) made the decision to image the entire Emirate’s subsurface (onshore and offshore) with 3D seismic. In addition to covering areas little explored before, and refreshing old images with modern technologies, the goal is to explore specifically for unconventional resources and shallow gas. Imaging both shallow and deep targets, in areas known for low signal-to-noise ratio, requires intense field efforts and elaborate processing sequences. Acquisition started in late 2018 and is scheduled to finish early 2022 with a final image delivery two years later. This is an extremely tight timeline considering the almost 90,000 km2 to be covered.
Seismic data offer valuable spatial information that contributes to the development strategy, such as drilling decisions and well planning. Inverting 3D seismic reflectivity data to impedance unlocks the potential of using seismic data to estimate the lateral variability and to predict reservoir geomechanical behaviour. The geomechanical model contributes for the evaluation of wellbore stability, frackability and drilling direction. The objective of this work is to illustrate the use of seismic derived models for reservoir and geomechanical properties, such as total porosity, total organic carbon (TOC) and rock rigidity. These properties are instrumental to characterize tight reservoirs and useful tools for defining a horizontal well trajectory. The study area has an exploration potential of unconventional reservoirs, targeting tight carbonates from Upper Jurassic. To date, only one well has been drilled in the area of the interest and high quality 3D seismic data is available. The process consists of conducting seismic inversion, using the well and the 3D PSTM seismic data, and resulting in the generation of 3D subsurface impedance model. The well-log relationships between impedance and reservoir quality (effective porosity, source richness) and completion quality (brittleness) were determined and showed high correlation. Hence, 3D rock quality and geomechanical volumes were derived to characterize the behaviour of the reservoir target. Seismic derived properties models yield a better description of the reservoir heterogeneity than using seismic reflectivity data only. The horizontal well trajectory was planned taking into account the observed and significant lateral variability revealed by the 3D models estimated based on seismic inverted data. The results obtained using this methodology demonstrate the importance of integrating geophysical characterisation as a relevant supporting factor in the drilling decision making process. In particular, it validates the use of seismic derived reservoir and completion quality models as an additional tool in the definition of a horizontal well trajectory in a tight/unconventional deep gas target.
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