RETRACTED ARTICLE: Depositional environments and porosity distribution in regressive limestone reservoirs of the Mishrif Formation, Southern Iraq

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, mudstonewackestone, 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.

Sequence Stratigraphic Analysis of the Mid‐cretaceous Mishrif Formation, Southern Mesopotamian Basin, Iraq

“…The type section of the Mishrif Formation was described by Rabanit (1952) at well Zb-3 (Zubair oilfield). Sedimentological, stratigraphic and palaeontological studies of the formation have recently been published including Gaddo, 1971;Reulet, 1982;Aqrawi et al, 1998;Bernaus and Masse, 2006;and Al-Dabbas et al, 2010. However, the sequence stratigraphy of the formation has not been considered in detail (except in some M.Sc.…”

Identification of facies‐controlled eogenetic karstification in the Upper Cretaceous of the Halfaya oilfield and its impact on reservoir capacity

“…Subsequently, influenced by the Alpine diastrophism, the strong compression of the Zagros Orogeny (60–10 Ma) resulted in a series of gentle northwest–southeast anticlines in the study area (Agard et al, ; Almutury & Al‐Asadi, ; Molnar, ). Then, the tectonic movement during the Cretaceous was relatively weak, leading to a generally flat topography, which provided accommodation for the later massive deposition of bioclastic limestones (Al‐Dabbas, Al‐Jassim, & Al‐Jumaily, ; Al‐Dabbas, Al‐Jassim, & Al‐Jwaini, ; Flügel, ). These limestones are mainly shallow marine shelf carbonate deposits (Aqrawi et al, ).…”

Net pay determination by artificial neural network: Case study on Iranian offshore oil fields

“…It is a very important oil reservoir in the Middle East, deposited in a regressive cycle, i.e. starts with basinal (outer self) environment, followed by slope environment, then shoal or reefal environment, finally ends with lagoon environment (Al-Dabbas et al, 2010). Oil field of which Mishrif is investigated through (we call it First Field here) is a relatively small field in the Persian Gulf, with the trend of North-South.…”