The Arab-D member – Arab formation (Upper Jurassic) – is one of the most prolific oil reservoirs in Saudi Arabia. Located onshore in the Eastern Province and offshore in the Arabian Gulf waters, many huge oil fields have been discovered since the forties. In addition to their size, several Arab-D oil fields present another similarity: the original oil water contact (OOWC) is apparently tilted.

Several theories have been advanced in the past, but none were able to account successfully for the tilt.

The lack of flank wells intersecting the OOWC in a clean reservoir section makes it even more difficult to ascertain the origin of the tilt. In addition, special care must be devoted to avoid using information coming from edging wells in areas where crestal production has already been high, resulting in a dynamic OWC, not the OOWC.

A new model has now emerged to account for the observed tilt of the OOWC. This model is based on the existence of a higher geothermal gradient close to the Dammam dome area (Saudi Arabia Eastern Province). Studies are underway, using SAUDI ARAMCO multi-disciplinary teams to integrate these facts in a tectonic model of the area.

For the Ghawar field, it has been shown that the observed tilt of the OOWC can be explained by the evolution of the regional geothermal gradient, and by an inflow of fresher water on the western flank of the Haradh area. What was thought before to be a hydrodynamic-related tilt is now interpreted as a pure static equilibrium with varied fluid densities.

The tilt of the OOWC has been back calculated and found to be in good agreement with the values measured in key wells. Applications of this particular type of static equilibrium in reservoir simulation will be presented to conclude this integrated approach, which was successful to establish a consistent picture of the Ghawar OOWC tilt.

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