A Better Understanding of the Depositional Setting and Hydrocarbon Trapping Mechanism of a Challenging Arab D Formation (Oolite) for a Reliable In-Place Volume Estimations and Enhanced Field Development Planning – A Gas Bearing Field in Onshore, Abu Dhabi
Authors: Khan R. , Aamir M. , Hassan A. Y. , Ahmed J. Z. , Aziz L. A. , Mannaee A. M. , Hosani K. M. 
Published: November 2024

The depositional setting and hydrocarbon trapping mechanism of the Arab-D Formation are challenging to understand due to the complex Oolitic shoals’ distribution, diagenetic alterations, structural deformation, and lateral variations. The objective was to provide a better understanding of the depositional setting and hydrocarbon trapping mechanism of the Arab-D Formation in the gas-bearing field. The study integrates a multidisciplinary approach that combines geological, geophysical, petrophysical, and reservoir engineering data and methods.

Sequence stratigraphic well correlations, core data (7 wells), 3D Seismic volume, MDT pressure, and well test data were integrated. The seismic amplitudes were sliced at different reservoir levels which provided great support in understanding the Oolite depositional system.

The vertical and horizontal well placements across the Oolitic shoals along with the seismic attributes provided controls to better understand the shoal's geometry and its orientations. In addition, seismic inversion work was performed to generate the acoustic impedance 3D volumes and corresponding seismic-based porosity volumes. This helped in estimating reliable pore and hydrocarbon gas in-place volumes.

The formation is interpreted as a pro-grading shoal system moving from NW to SE within the field. The Hith Anhydrite is interpreted as deposited in the supratidal environments, which makes the onlap to the main Oolite in the western part of the field, and the tight carbonate, which is the time equivalent of Hith Anhydrite, is onlapping to the Oolite from the east. The pro-graded shoals represent NNW-SSE-oriented elongated ridges that are isolated, partially connected, and fully connected at different levels from the top to the base of the reservoir. The MDT pressure data supports the isolated carbonate ridges with supercharged pressures and variable fluid gradients at wells. Multiple traps were discovered within the field at the Oolite level, where two compartments were confirmed. The hydrocarbon trapping mechanism is most likely the combination of both Hith Anhydrite juxtaposition to Oolite and Faults. The gas in-place estimations performed earlier were revised based on this new understanding of narrow elongated ridge patterns. The new understanding has helped in constructing the new generation static model for improved field development planning. This includes but is not limited to well placement optimization, reduction in in-place volume estimations uncertainty, and revised field development strategy.

The study has helped in optimizing the well-placements of horizontal wells to avoid the tight carbonate zone. The orientation of the future development wells will be aligned with the elongated pro-grading Oolitic shoal ridges to target the best reservoir quality. This will reduce the drilling challenges and costs with efficient reservoir contact. Consequently, maximizes the recovery and production rates. This work can be utilized across the UAE for the development of Oolite / Arab-D reservoirs.