Abstract
This paper includes studying the distribution of microfacies, diagenetic processes and depositional basin of the Mishrif Formation in selected wells of Ratawi oilfield (Ratawi-2 and Ratawi-5), southern Iraq. More than 200 thin sections of samples were examined of the above-mentioned wells. The Mishrif Formation is divided into three major microfacies: lime mudstone, wackestone, and packstone, which are deposited in sub-basinal, lagoonal open shelf margin, and, shoal environments. Benthonic foraminifera, planktonic, Echinodermata and algae represent the fossils in the Mishrif Formation. Dolomite and calcite are the main minerals components of formation. Seven diagenetic processes were recognized in the Mishrif Formation, showed positive and destructive effects on the reservoir quality; dissolution and neomorphism (recrystallization) had highly positive effects through creating and improving porosity and permeability, which led to improving reservoir quality. Cementation, micritization, and compaction have destructive effects, through reducing porosity and permeability and led to reducing reservoir quality. Other processes such as, dolomitization, authigenic minerals (pyrite) did not have strong effects on reservoir quality. Based on the genetic classification of the porosity, most of the porosity within the Mishrif Formation in this field was formed by diagenesis processes, in which the predominant pore types are vuggy, intraparticle, fractured, and moldic. Analysis of microfacies showed that there are three main facies and eight secondary facies in the Mishrif Formation. The longitudinal section of the depositional environments showed, that Mishrif Formation deposited in the sub-basinal, lagoonal, open shelf margin, and shoal environments depending on fossils observed in a number of microfacies.
Highlights
The Cretaceous rocks occupy a distinct position within the stratigraphic column in southern Iraq, where the content of this era represents good oil potential rocks
There are four microfacies, which recognized in the present study, as followed: 1. Lime mudstone microfacies Lime mudstone was defined by Dunham (1962) as a kind of limestone that its infrastructure composed of microcrystalline calcite, which corresponds to the term micrite that was launched by Folk (1965)
The most important digenesis processes that affected this microfacies are neomorphism. These facies could be compared with standard microfacies SMF19, 23 that was deposited in facies zone Facies Zone (FZ)-8, 9 according to Wilson (1975) and Flügel (1982 and 2004), which represents the restricted lagoonal environment
Summary
The Cretaceous rocks occupy a distinct position within the stratigraphic column in southern Iraq, where the content of this era represents good oil potential rocks. Some of these formations were considered as source rocks, while the others were reservoirs with high oil potentiality (Idan et al, 2020). While Kendall (2007) defines facies as representing histological, structural, and synthetically properties that result from deposition processes and caused modification and accumulation within the depositional environment. The stratigraphic, depositional environment and facies studies were given by recognized four general facies within the Mishrif: restricted shelf, rudist build-up, open shelf and sub-basinal.
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