The Barremian-Aptian, Kharaib Formation is among the main reservoir units in the Middle-East. The Kharaib Formation is broadly divisible into three main zones: the Upper, Middle and Lower Kharaib, which are defined based on their distinct sedimentological characteristics. The Kharaib interval has been extensively studied across the region; however, the inherent heterogeneity associated with sediments always makes it challenging for interpretation of the depositional facies, stratal geometries, and the prediction of their reservoir properties. This study focuses on the Middle and Upper Kharaib by integrating sedimentological, diagenetic, and pore-scale datasets to provide a better understanding of the depositional framework, diagenesis and its effects on the reservoir properties. Dunham/Embry & Klovan classification (1971) scheme is used to describe the sediments, which determines lithofacies. Lithofacies that are genetically relatable are grouped into larger sets of associations. The lithofacies association represent their sedimentary environments of deposition. The grouping/stacking pattern of the sediments reflects a broad third-order sequence. Higher-order variations/sequences are either from localized topographical changes or energy variations or from sea-level changes, which thereby imparts lateral reservoir heterogeneity. X-ray diffraction (XRD) analysis defines the mineralogical composition, whereas pore-scale fabric/textural characteristics have been defined via conventional light microscopy and scanning electron microscopy (SEM). Sedimentological description of the Kharaib Formation showed varying textures ranging from mudstone to grain-rich floatstone, which are interpreted to be deposited in a broad homoclinal ramp setting. Petrographic evaluation of the sediments from the studied section highlighted the effects of diagenesis on reservoir properties with calcite cementation significantly reducing the reservoir quality (i.e., lower porosity and permeability). In contrast considerable micro and macropore enhancement due to secondary dissolution appears to negate the effects of cementation. In the studied sediments, the porosity comprises considerable abundance of primary and secondary macropores. The measured porosity varies from moderate to very good, while permeability is low to moderately high. Interrogation of conventional core analysis data establishes that the primary control on reservoir quality distribution is sediment texture and composition. The genetically distinct characteristics of lithofacies related to allochem assemblage, their abundance and size aid in defining the associations, which thereby provides the former fabric for the subsequent diagenetic alterations. The abundance of detrital clays in the middle Kharaib appears to be the primary cause of relatively poor reservoir quality. This study shows that both the original depositional texture/composition and diagenesis have had an important impact on shaping reservoir properties. This integrated approach sheds light on the sedimentological make-up, depositional setting, and diagenetic overprint of the Kharaib Formation and their effect on the reservoir quality of various lithofacies. This understanding will further develop work related to pattern prediction of the Kharaib Formation, which may be extrapolated to uncored intervals for reservoir quality correlation and assessment.

The Cenomanian-early Turonian Mishrif Formation is an important reservoir rock throughout the Middle East. Although extensive studies have been carried out on the Mishrif Formation across the Middle East, very limited information is available about the Mishrif Formation from the western region of Abu Dhabi. The primary objective of this study is to review the sedimentology of the Mishrif Formation in the oilfields of the western region of Abu Dhabi and to define the petrology, the depositional and diagenetic history, and the reservoir quality. The Mishrif Formation is sedimentologically characterized using an industry-standard lithofacies classification scheme. Subsequent sedimentological interpretations define the depositional evolution of the sediments. Petrologic assessment highlights the depositional texture and successive diagenetic alterations that have affected the sediments, thereby modifying the reservoir properties. The Mishrif sediments represent deposition on a carbonate ramp system modified by a potential barrier shoal, along with localized relief developing shoal-like geobodies. A broad range of lithofacies represent the range from wackestone to boundstones, with the sediments interpreted to represent deposition in a shoal complex through backshoal and rudist buildups to a distal mid-ramp to an outer ramp setting. A petrographic study reveals multiple diagenetic phases have affected the sediments. Porosity is enhanced considerably due to multiple leaching phases, which are followed by subsequent cementing phases comprised of calcite and dolomite that degrades the porosity in places. Fracture events do not seem to have any significant impact towards enhancing the reservoir properties as they remain dominantly filled by cements but locally, a few fractures, which are open, contribute to enhancing the connectivity. A moderate to very good reservoir quality is inferred for the sediments of the Mishrif Formation. The porosity values range between 7% to 34% and permeability values from 0.06mD to >10 D. Interrogation of the core analysis data indicates a depositional control along with a diagenetic influence on the reservoir quality. This study helps to resolve the complexities associated with the sedimentological makeup and diagenesis of the Mishrif Formation in the western region of Abu Dhabi. The understanding from this study will help in further works related to reservoir geomodelling and pattern prediction, which may also be extrapolated to uncored intervals for representativeness and regional correlation of the Mishrif Formation.

Objectives/Scope This study focuses on assessing the uncertainties related to sedimentological heterogeneity and the diagenetic variability within the gas-condensate reservoirs of the Shuaiba Formation, Sharjah, United Arab Emirates. Methods, Procedures, Process For characterizing the sedimentology of the Shuaiba Formation, a lithofacies scheme has been developed on the basis of Dunham's (1962) and Embry & Klovan classification (1971). The lithofacies are grouped on the basis of their genetic relationships which also correspond to their depositional environment, and are designated as lithofacies associations. A pore-scale fabric/textural investigation was completed using conventional thin-section microscopy and Scanning Electron Microscopy (SEM). Results, Observations, Conclusions The Shuaiba sediments are characterized by skeletal-rich wackestone/packstones to floatstones deposited in an inner ramp setting. The stacking pattern of the inner ramp deposits define broad third order trends observed across the studied field.These trends are relatable to the regional sequence stratigraphic framework of Sharland et al. (2001). In higher order sequences, lateral variations in lithology occur, defining the reservoir heterogeneity, which are most likely forced by topographic/hydrodynamic variation as well as sea level changes. Reservoir quality distribution is controlled by various factors, including the depositional texture and allochem assemblage (abundance, type, and size). Diagenetic alteration of the textures played an important role in determining overall reservoir quality. The pore enhancing phases are defined by dissolution events, where later stage dissolution was the dominant phase to enhance micropores and also to create meso- to macropores which partially to completely negated the effect of previous cementing phases. In these Shuaiba deposits, the porosity comprises common matrix-hosted as well as grain-hosted micropores along with variably distributed intraparticle and rare mouldic meso- to macropores. The measured porosity ranges from very poor to moderate (0.5-17%) while permeability is very low to low (<0.001-1.49 mD). The detailed petrographic analysis highlighted that changes in micritic fabric shows a variation in the reservoir properties. From SEM observations, it was noted that microcrystalline calcite crystals of polyhedral to sub-rounded morphologies with intercrystalline contacts ranging from facial to sub-punctic, which display relatively a good microporosity developement, whereas crystals that show anhedral compact character with coalescent/fused intercrystalline contacts are rarely associated with any microporosity. Novel/Additive Information In addition to SEM characterization, porosity data and elastic properties (e.g., Young's moduli) generated from the interpretation of the well-log data, were used to investigate the prospective relationship between the microporous carbonates and elastic properties. The comparisons highlight that an increase in porosity values results in a decrease of Young's moduli values, thereby reflecting a decrease in the stiffness of the rock. On the other hand, the increase in porosity maybe linked to the evolution of anhedral, compact, micritic fabric to polyhedral/sub-rounded micritic fabric. The understanding of this relationship provides a powerful tool to be utilized in reservoir architecture prediction based on integrating the sedimentological framework and diagenetic overprint.

The Early Cretaceous Kharaib Formation is one of the most prolific hydrocarbon bearing reservoirs in the Middle East. Based on sedimentary characteristics, the Kharaib Formation is divided into three main intervals/zones: the lower, middle and upper Kharaib. Interpretation of the depositional fabrics and prediction of reservoir parameters is often difficult due to the Kharaib Formation's diverse array of sedimentary characteristics. To address the considerable heterogeneity of the Kharaib Formation, this study investigates depositional characteristics and the diagenetic impact on reservoir quality within the Sajaa Field. The Kharaib Formation sediments were characterized using a Dunham/Embry & Klovan (1971) defined classification scheme. Analytical techniques via thin section analysis, scanning electron microscopy (SEM) and capillary pressure data were used to qualitatively and semi-quantitatively assess porosity and the diagenetic processes involved in its evolution. Key reservoir quality parameters for Kharaib intervals were defined by combining petrographic analysis and conventional core methods. The sedimentological characterization of the Kharaib sediments in the Sajaa Field showed a range of Orbitolina-bearing carbonate textures, which were interpreted as inner-ramp deposits. The stacking pattern of the depositional facies showed intermediate-order trends. The conventional core analysis measurements showed poor to good reservoir properties with porosity values of 0.7 – 23.2% and permeability values from negligible to 2 – 3 md. The pore system is dominated by grains and matrix-hosted micropores, which are primarily generated via diagenesis. Qualitative thin section analysis identified grain-hosted micropores as the most significant microporosity type, highlighting that grain-rich textures are relatively more porous compared to mud-rich textures. Diagenesis transformed both the matrix and various grain types (e.g., micritized grains, foraminifera etc.), generating considerable micropore volume. High resolution SEM imaging, along with semi-quantitative assessment, highlighted that diagenetic modification of the micritic sediments led to an abundance of sub-rounded to polyhedral crystals exhibiting primarily sub-punctic to punctic intercrystalline contacts that generated a porous fabric. Mercury injected capillary pressure (MICP) data, confirmed the abundance of micropores within the pore size distributions of studied samples. This study demonstrates the importance of considering textural variation within the Kharaib Formation when assessing the reservoir coupled with the fractured nature of the unit. The established microporosity-depositional relationships when combined with the updated core descriptions now allow for the integration of textural trend information in the field that can be applied to refine reservoir property and abundance distributions.

Rock typing represents an important tool for classifying clastic reservoir rocks, especially in reservoirs showing low permeability and low porosity values. For this study we first approach these challenges from a geological perspective and subsequently translate the results of traditional facies and RQ analysis into rock types by integrating previously obtained results with mercury injection capillary pressure (MICP) data. The rock typing workflow proposed by Rushing et al. (2008) is applied to the low porosity-low permeability Unayzah Formation penetrated in three offshore wells, UAE. The characterisation of this type of low reservoir quality (RQ) rock confronts the geologist with some challenges. The Unayzah Formation is an important clastic reservoir unit in the Middle East, especially in the UAE. It is characterized by low porosity and low permeability values and therefore, according to Holditch (2006) can be classified as a tight gas reservoir where a tight gas reservoir is "a reservoir that cannot be produced at economic flow rates nor recover economic volumes of natural gas unless the well is stimulated by a large hydraulic fracture treatment or produced by use of a horizontal wellbore or multilateral wellbores". In order to characterize the Unayzah Formation we used newly acquired geological and laboratory data obtained from detailed core description, facies, petrographic- and reservoir quality analysis. In a second step we translated these geological laboratory related data into rock types by integrating these data with MICP data considering pore throat geometries for a more accurate reservoir quality characterisation of the Unayzah Formation.