The Artistic Science of Reservoir Quality Prediction of the Khuff Formation Gas Reservoirs in the Subsurface of Eastern Saudi Arabia

By

 Rami A. Kamal¹

(1) Saudi Arabian Oil Company, Dhahran, Saudi Arabia

 Within the confines of eastern Saudi Arabia, Late Permian Khuff sediment was cyclically, and overall aggradationally, deposited on the very broad, shallow marine, low gradient, Arabian Shelf. It will be sedimentologically demonstrated that today’s structural highs correspond closely to Late Permian topographic highs. Optimal reservoir development was contingent on the palaeotopographic highs that enabled early porosity-enhancing diagenetic mechanisms to kick in. Different types of reservoir development occurred on and around these topographic highs. Strings of diversely-shaped topographic highs are irregularly strewn, archipelago-style, along today’s Ghawar substructures. Regional and even subregional reservoir predictability is contingent on finding the original topographic highs (including partially subaerial “islands”).

Throughout the history of Khuff deposition, water depth has also continued to change. The nature of the sedimentary depositional components throughout the history of Khuff deposition was a function of relative sea depth. The consequence of this additional dimension meant that the products of diagenesis, i.e. the reservoir facies, vary in the vertically-stacked Khuff A, B and C reservoirs.

Traditional sequence stratigraphic methodology will not have much bearing on the quest to predict reservoir quality development in any of the three Khuff reservoirs. This is chiefly due to the fact that the reservoirs are shelf, not shelf-edge, deposits, and seismically-identifiable clinoforms are indiscernible. The trick is to map the ancient topographic highs that are today relief features on irregular structures. The surviving ancient topographic highs are identifiable through depositional sedimentology and the reservoir facies that they carry. These topographic highs and their surrounding topographic lows can be initially mapped through well control. Acoustic impedance maps are the best hope for defining the interwell extensions of previously identified reservoir facies. Improved seismic imaging and improved seismic resolution will have a direct impact on improving the ability to more precisely predictively map our reservoir facies in the Khuff Formation.