Page 358 - Carbonate Facies in Geologic History

Facies Patterns 345

edge in the Shuaiba and Karaib limestones. However, it is not yet possible to
evaluate the role of Cretaceous structural movement in localizing shoal facies on
individual domes, major anticlinal trends, or shelf margins. It is known that local
movements cause thinner deposition over structures but exact facies control is not
easily demonstrated.
The usual, open marine facies of the Upper Thamama consists of chalky,
peloidal, molluscan, and Orbitolina-rich wackestone and packstone with 20-40%
grains. In places argillaceous streaks with attendant ball and flow compaction
structures (causing nodular limestone) are present with hard-ground bored sur-
faces and represent still-stands of marine deposition. Burrowing is common in
these micritic sediments. Porosity is uniformly above 20% in the micrite matrix
but permeability is low. In the shoal limestone area a number of grainstone-
packstone units occur in the upper 300 m of the Thamama. These average 30-
40 m thick each and are separated by dense, stylolitic zones. The topmost of these
units (A zone), the Shuaiba equivalent and the underlying Murban B zone pro-
duce considerable quantities of oil in the eastern Persian Gulf. Detailed petro-
graphic study of the chalky but grainy reservoirs, has been made at Id EI Shargi,
Urn Shaif, Zakum, and Murban Bab Dome. The main facies recognized are:
a) Rudist buildups or biostromes, probably mostly ofmonopleurids.
b) Molluscan-burrowed normal marine wackestone.
c) Orbitolina wackestones (Plate XXX B).
d) Rounded, coated, altered, chalky shell debris and algal lumps forming grain-
stones and packstones with high porosity and permeability. Dasycladacean
remains occur but the most common alga is the irregular and encrusting
codiacean, Lithocodium.
A particularly detailed petrographic study of the Shuaiba facies (Harris et ai.,
1968) has delineated a possible shelf margin trending across the northern edge of
the major Bu Hasa structure south of Murban. Figure XI-13 is a sketch map of
the area. The Bu Hasa dome is 35 km long and 18 km wide. It is gently arched
with dips of about 2 degrees. The facies and related thickness outline an upper
Shuaiba buildup which faces northward. The significance of such an obvious
topographic relief at the very top of the Thamama is great. Porosity-permeability
development is known regionally along the Albian-Aptian unconformity because
of subaerial exposure and leaching of the chalky rock. The presence of depositional
relief underlying the unconformity means an even better opportunity for reservoir
development. Similar patchy rudist reef occurrences are known atop the Id EI
Shargi structure west ofthe major shoal limestone development.
In the Murban area, the Shuaiba reefy facies reaches 170 m and thins
northeast off the flank of Bu Hasa dome to 40 m of dense basinal facies. The
inclined slope of the original bank as interpreted from facies, is only 1/2 degree.
Equivalent strata in the large Bab dome, 20-30 km to the northeast, are
wackestones indicative of offshore, open circulation. The Bu Hasa buildup may
be an isolated low carbonate bank atop the structural culmination. Figure XI-14
is a cross section of the reefy margin taken from the above cited work by
Abu Dhabi Petroleum Company geologists (Harris et ai., 1968). The inter-
preted geologic history is as follows: during early Shuaiba time the whole
area of Bu Hasa and Bab domes was the site of an algal biostrome of