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The results of a multiproxy study on Mishrif carbonates from the margins of the Central Mesopotamian Basin of Iraq display a wide range of rock fabrics and diagenetic features, all affecting reservoir quality and flow properties in a complex manner. Based on petrographic and facies analyses of the Mishrif Formation in the AD oilfield, the model of a homoclinal ramp geometry with twelve characteristic microfacies types is proposed. A relevant observation is that the ramp, judging from the core data of the AD oilfield studied here, reflects low-energy conditions as documented by an abundancy of fine-grained carbonates in nearly all microfacies types. This notion is perhaps best understood in the context of an intrashelf basin with significant wave-seafloor interaction leading to wave degradation and a shallow effective wave base. In contrast to the rudist bioherms that typify the more basin-marginal settings, we find mainly evidence for small, low-relief buildups characterized by green algae. The Mishrif reservoir properties are shaped by two main diagenetic features: (i) early diagenetic, aquifer-related marine-meteoric mixing zone dissolution inducing abundant fabric selective porosity and (ii) the preferential cementation of Thalassinoides burrows leading to low-porosity concretionary carbonate nodules with reservoir properties that differ significantly from those of the host carbonate. Given the somewhat more distal position of the AD oilfield, the effects of prolonged stages of subaerial exposure related to sea-level fall, as reported from more basin marginal settings, are not recognized. The bending and splaying of seams around nodules and the well-preserved skeletal material within these concretionary carbonates demonstrate that the onset of nodule cementation was early diagenetic/pre-compactional in nature. Based on integrated microfacies and reservoir properties, seven rock types (i.e., Rock Type I-VII) with distinctive reservoir qualities are recognized. Nodular concretions are mainly developed in the Rock Type I lithology and result in distinctive cm-to dm-scale heterogeneity. The contrast between small, poorly-connected pore systems in nodules and medium-sized, well-connected pore systems in the host rock are best described as an inconspicuous, dual permeability system. Data shown here and evidence from other studies dealing with the Mishrif in the Central Mesopotamian Basin are important to understand and predict reservoir properties in other carbonate fields with similar properties. • The Lower Mishrif in Iraq was deposited on a low-energy homoclinal ramp. • A genetic model to explain the relationship between bioturbation and concretion, and their impact on reservoir properties. • Seven reservoir types with distinctive porosity system are recognized based on microfacies and reservoir property. [ABSTRACT FROM AUTHOR]

High-permeability zones (HPZ) are common occurrences in carbonate reservoirs of the Middle East and act to improve productivity in primary depletion but lead to water breakthrough in secondary depletion. Bioturbation aggravated the reservoir heterogeneity of the Khasib Formation carbonates in AD oilfield, Iraq, resulting in the development of HPZ. The observations of cores and thin sections, measurements of porosity, permeability, capillary pressure and probe permeability were conducted to investigate the characteristics and genesis of the HPZ. Well log responses of the HPZ in cored wells were analysed to identify the HPZ in uncored wells and predict its distribution. Results demonstrate that the HPZ is demarcated by Glossifungites ichnofacies and Thalassinoides ichnofabric. The burrow systems filled passively with overlying sediments that were apparently different from the unbioturbated matrix, in terms of the lithologic associations, pores types, and pore connectivity. The burrow systems, which contribute to the high permeability of the HPZ, were merely composed of grainstone with well-connected interparticle pores and vugs. The matrix comprises packstone and hardground-associated strongly cemented grainstone, which developed some fractures and exhibited poorly connected pores, including intraparticle, moldic, and residual interparticle pores. The arithmetic mean permeability of the HPZ is 241.0 mD, which is much higher than that of the non-HPZ (17.3 mD). The entry pressure and pore throat radius of the burrows in HPZ is 0.02–0.05 MPa and 5–20 μm, respectively, which reveal the high-quality pore structure of the burrow systems. There is strong heterogeneity within the HPZ. The permeability of burrow systems could reach up to 10133.5 mD with an arithmetic mean permeability of 712.2 mD, while the permeability of the matrix is only 0.05 to 69.29 mD, with an arithmetic mean permeability of 8.7 mD. Interconnected burrow networks in the HPZ were formed by bioturbation. The efficiency of connectivity of the HPZ was further enhanced by the early dissolution occurred within burrow architecture and matrix in the meteoric freshwater environment. These resulted in the formation of the high-permeable network composing of interparticle pores, vugs, and the dissolution-enlarged fractures. The high-permeable burrow systems and low-permeable matrix jointly contributed to the distinct well log responses, which are markedly different from the conventional shoal HPZ. The bioturbated HPZ can be identified by medium natural gamma (GR), high bulk density value (RHOB), low acoustic transit-time value (DT), low compensated neutron (NPHI), high deep induction value (RILD), and high microspherically focused resistivity value (MSFL). A well log-model based three cutoffs, namely RHOB nor /DT nor , RILD*MSFL and GR, was built and shows good prediction of the HPZ. • Bioturbation-associated high-permeability zones are systematically characterised in all aspects. • The development of the HPZ is jointly controlled by bioturbation and early dissolution. • Well log responses of the bioturbated HPZ is synthesised responses of burrow and matrix. [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]