RETRACTED ARTICLE: Depositional environments and porosity distribution in regressive limestone reservoirs of the Mishrif Formation,Southern Iraq |
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| Authors: | Moutaz Al-Dabbas Jassim Al-Jassim Saad Al-Jumaily |
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| Institution: | (1) College of Science, University of Baghdad, P.O. Box 47138, Baghdad, Iraq |
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| Abstract: | Eight subsurface sections and a large number of thin sections of the Mishrif Limestone were studied to unravel the depositional
facies and environments. The allochems in the Mishrif Formation are dominated by bioclasts, whereas peloids, ooids, and intraclasts
are less abundant. The sedimentary microfacies of the Mishrif Formation includes mudstone, wackestone, packstone, grainstone,
floatstone, and rudstone, which have been deposited in basinal, outer shelf, slop followed by shoal reef and lagoonal environments.
The formation displays various extents of dolomitization and is cemented by calcite and dolomite. The formation has gradational
contact with the underlying Rumaila Formation but is unconformably overlain by the Khasib Formation. The unconformity is recognized
because the skeletal grains are dominated by Chaophyta (algae), which denotes the change of environment from fully marine to lacustrine environment. Thus, the vertical bioclast
analysis indicates that the Mishrif Formation is characterized by two regressive cycles, which control the distribution of
reservoir quality as well as the patterns of calcite and dolomite cement distribution. Mishrif Formation gradationally overlies
Rumaila Formation. This was indicated by the presence of the green parts of Chaophyta (algae) as main skeletal grains at the uppermost part of well Zb-47, which refer to lacustrine or fresh water environment.
Petrographical study shows that the fossils, peloids, oolitis, and intraclasts represent the main allochem. Calcite and dolomite
(as diagenetic products) are the predominant mineral components of Mishrif Formation. Fossils were studied as an environmental
age and facial boundaries indicators, which are located in a chart using personal computer programs depending on their distributions
on the first appearance of species. Fifteen principal sedimentary microfacies have been identified in the Mishrif Formation,
which includes lime mudstone, mudstone–wackestone, wackestone, wackestone–packstone, packstone, packstone–grainstone, grainstone–floatstone,
packstone–floatstone, packstone–rudstone, and wackestone–floatstone. Markov chain analysis has been used to study the transitional
pattern of different microfacies types vertically in each well and laterally in all wells as a composite section. The vertical
analysis indicates that the Mishrif Formation characterized by two regressive cycles, the main one started with basinal or
outer shelf environment, slop environment followed by shoal or reefal environment, and ended with a lagoonal environment.
The lateral analysis shows the same regressive cycle, and by using the lithofacies association concepts, we built the depositional
model of the Mishrif Formation environment. |
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