Saudi Sandstone Correlations

Basin basics
Detailed analysis of sedimentology and
palaeocurrent directions in the five
Upper Ordovician formations (Units 1 to
5) has helped in the understanding of
basin development during the Middle
and Upper Ordovician. It has also
explained a great deal about the difficulties
geologists have experienced in the
past.
Unit 1 is a well-sorted fine-grained
quartzarenite which reaches thicknesses
of 400 ft (122 m) and has a conglomerate
layer at its base. On logs, the formation
appears as a ‘clean’ blocky low gammaray
interval with high porosities. The
sandstone contains steeply dipping
cross-beds with a consistent trend. It is
not bioturbated and there is no evidence
of marine influence. Unit 1 was deposited
on land and is the result of wind action in
a semi-desert or desert environment.
Unit 2 was deposited in an alluvial fan
environment (figure 2.3a) with sediments
sourced from the west and north. The
unit, which can be up to 600 ft (183 m)
thick, is composed of various sediments
ranging from siltstone to coarse conglomerates
and pebbly sandstones (figure
2.3b).
Several features identify this unit.
• There are low-angle tabular and trough
cross-beds.
• The unit displays a moderately high
gamma-ray response with a serrated
appearance.
• There are variable dips which trend
east to south.
• Bioturbation (sediment disturbance
caused by burrowing organisms) is rare.
• There is moderate diagenetic alteration
with patchy cementation.
• Average grain size increases toward
the western margin of the depositional
basin. In general, grain size increases as
we approach the sediment source.
Mountains
Fault
line
Fanhead trenches
Streamflow
Proximal fan
Mid-fan
Distal fan
Fig. 2.3a: A continental
alluvial fan - the probable
depositional environment for
the sediments which comprise
Unit 2.
30 Middle East Well Evaluation Review
1ft
Fig. 2.3b: Excellent
pebble conglomerate in
Ordovician Unit 2. This
example is taken from
a typical Ulayyah well.