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.