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Environmental scenarios around the Cenomanian-Turonian ocean anoxic event (OAE2) interval are scarcely studied in the southeastern Neo-Tethys and not discussed in terms of variation of primary mineralogies of marine carbonates. Based on petrological and geochemical analyses, this study focuses on the increased aragonite abundance in the oxygen-decreasing interval before the OAE2 and its environmental implications in the southeastern Neo-Tethys. In central Iraq, the upper Cenomanian-lower Turonian Mishrif Formation represents marine carbonate records in the southeastern, tropical Neo-Tethys and includes five members from the lowermost Mi-5 to the uppermost Mi-1. From Mi-5 to Mi-1, dominant carbonate components and primary carbonate mineralogies vary. Mi-5 is dominated by primarily calcitic grains (mainly calcite shells of rudists and echinoids); Mi-4 and Mi-3 dominated by primarily aragonitic grains (mainly green algae and aragonitic bivalves) and planktonic calcitic foraminifera; and Mi-1 dominated by primarily calcitic ooids. Paleoenvironmental interpretations suggest: i) open marine, offshore settings for Mi-5 to Mi-3, and a shallow marine shoal setting for Mi-1; ii) a sea level rise from Mi-5 to Mi-3, followed by a sea level fall to Mi-1. The increasing aragonite abundance in Mi-4 and Mi-3 is exceptional and suggested to most likely reflect enhanced preservation of shallow-marine aragonite transported into open marine offshore settings. This enhanced aragonite preservation is likely related to an oxygen-decreasing level before the OAE2 in the offshore settings. Remarkably, these primarily aragonitic components are commonly preserved now as moldic pores in Mi-4 and Mi-3. The moldic pores are interpreted to be produced via deep burial dissolution of recrystallized aragonitic grains, based on petrological and geochemical analyses. Moreover, the primary carbonate compositional and mineralogical changes from Mi-5 to Mi-1 provide some potential clues for the environmental scenarios around the OAE2 interval in the southeastern, tropical Neo-Tethys: i) from Mi-5 to Mi-4 and Mi-3, the increasing abundance of primarily aragonitic components is accompanied by a sea level rise, increasing seawater temperature and decreasing oxygen level of bottom water; ii) from Mi-3 to Mi-1, the decreasing abundance of primarily aragonitic components is accompanied by a sea level fall, decreasing seawater temperature and enhanced microbial carbonate production. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ten carbonate microfacies deposited in five sedimentary facies belts are identified in the Rumaila Formation. • Rapid sedimentary facies changes were organized into three RST and two TST caused by frequent relative sea-level changes. • Reservoir quality is related with carbonate microfacies types. • Differential early diagenesis causes distinct contrasts of pore evolution for the same primary rock texture. • Reservoir quality of the Rumaila is jointly controlled by sedimentary facies variation and differential early diagenesis. The Middle Cretaceous Rumaila Formation comprises a thick carbonate succession and forms an important reservoir unit in the Central Mesopotamian Basin. Microfacies analysis was performed based on the cores and thin sections observations to investigate the sedimentary and diagenesis evolution, and physical properties and mercury injection capillary pressure measurements were conducted to evaluate the reservoir quality. Ten microfacies types are identified showing that Rumaila carbonates deposited in five sedimentary facies belts: (i) platform-margin rudist shoals, (ii) platform interior grain shoals, (iii) platform interior inter-shoal, (iv) slope & toe-of-slope, and (v) deep shelf. These facies are organized into three regressive systems tracts and two transgressive systems tracts. Microfacies types present prominent vertical variations but are relatively stable in lateral extension, revealing a wide carbonate shelf sedimentary environment with frequent fluctuations of relative sea-levels. There is a good correlation between microfacies types and reservoir quality, ten microfacies types are classified as three classes with distinct reservoir quality. Thereinto, the microfacies rudist grainstone, bioclastic peloidal grainstone and worn rudist grainstone bear the best reservoir quality. The Rumaila Formation provides a typical case that reservoir quality and reservoir heterogeneity are jointly controlled by sedimentary facies variation caused by frequent relative sea-level changes, and differential early diagenesis. The frequent changes of relative sea-levels lead to the rapid changes of sedimentary facies (rock texture) and the frequently stacking of limestones with different primary porosity. Differential early diagenesis resulted in distinct contrasts of pore evolution for the same rock textures, which further enhance reservoir heterogeneity and also illustrate the destruction of cements on reservoir quality. This study provides an insight into reservoir quality prediction in the study area and other carbonate reservoirs undergone similar sedimentation and diagenesis process. [ABSTRACT FROM AUTHOR]