Saudi Sandstone Correlations

Riyadh
Ulayyah
Hilwah
Hawtah
0 50 100
Abu Jifan
Dilam
Raghib
ST
Hazmiyah
Ghinah
Km
Nuayyim
Khurais
Qirdi
Ain Dar
Seismic line
The recent discovery of large volumes
of super light oil in the
Unayzah sandstones in central
Saudi Arabia (figure 2.1) has pleased
explorationists from the Saudi Arabian
Oil Company (see Middle East Well
Evaluation Review, Number 11, 1991).
Since these discoveries were made,
Saudi Aramco's geologists and engineers
have been trying to decipher the depositional
history of the deeper pre-Silurian
sandstones. Initially, they were thought
to be equivalent to the Saq Formation
and the Hanadir member of the Qasim
Group (figure 2.2), which lies beneath
the major oil-bearing zones to the east.
There were good reasons to assume this.
The new sandstone units were remarkably
similar to the more familiar rocks
found in the east, they occurred at similar
depths, had approximately the same
thickness (about 3000 ft) and lay below a
major Silurian shale source rock.
By the late 1980s, wireline logs
recorded in the new fields of Dilam,
Hawtah, Hilwah, Nuayyim and Raghib
were being compared with logs in the
more easterly Ghawar Field. The logs
were significantly different in the deeper
part of the wells, below the main oil-bearing
intervals. Characteristic marker
shales from the deep Saq and Qasim formations,
which were obvious in the
HRDH
Ghawar
Bahrain
28 Middle East Well Evaluation Review
Qatar
Ghawar logs, could not be found in logs
from the new wells. The absence of distinctive
‘marker beds’ made it difficult to
divide the new sandstone sequence into
separate stratigraphic units. Lacking
clear lithostratigraphic information, and
without fossil evidence to help identify
and date the rock layers, geologists faced
a tricky correlation.
Surface seismic surveys taken across
the central region did not make the picture
any clearer. The resolution of the
deep seismic data was poor in the
deeper parts of the basin, partly due to
the overlying Khuff carbonate layer
which absorbed much of the seismic
energy. A conventional wireline log correlation
between wells was attempted
but this did not help to identify the units.
Though the wells had been extensively
cored it was impossible to produce a reliable
correlation of the sandstones
between wells. Age dating of fossils
which were found in minor shales within
the sandstones indicated that the
sequence may have been younger than
the familiar Saq Formation with which a
correlation was being sought, but in the
light of the overwhelming lithostratigraphic
similarity of the units, this scenario
was considered very unlikely.
Fig. 2.1: Location map
of the area of interest in
central Saudi Arabia.