Challenges in using calcium isotope proxies to interpret Cenomanian–Turonian carbon isotope excursions: Insights from the Mishrif Formation, Southern Iraq

The Mishrif Formation was deemed the main oil reservoir in the Fauqi oilfield, southern Iraq. This paper aims to predict characteristic properties of the Mishrif Formation depending on the interpretation of well logs data. The data were collected from five wells of log images which had been digitized with Didger software version 5. 12. 1762 to be analyzed and interpreted using interactive petrophysics 3.5.1.2 software. The main lithology of the Mishrif Formation in this region was determined using standard (M-N xplot) based on the integration of density-neutron logs. The results confirmed that the Mishrif Formation is mainly composed of limestone units. A Gamma-ray log was used to estimate the shale volume in the Mishrif Formation depending on old rock function. As a result, shale volume relative to the bulk volume was about 19%, and an increasing volume was noticed in the upper reservoir, reaching the highest in the MA and MB11 units. The Mishrif Formation porosity was calculated using the interpretation of neutron, density, and sonic logs. The results showed that the Mishrif Formation is distinctive with low to medium porosity values (4.8 to 17%). The calculation of cores manifested the dominance of secondary porosity in the MB21 unit of the formation in comparison to the other stratigraphic units in the studied area. These results led to the fact that the MB21 unit was more exposed to dissolution diagenetic processes in the south dome, which provided a good permeability integrated with the medium effective porosity, puttinh it as the main reservoir unit.

The Mishrif Formation is considered the main oil reservoir in the Buzurgan oilfield, southern Iraq. This study aims to characterize and evaluate the reservoir properties of the Mishrif Formation based on the interpretation of well logs data. The logs data for six wells have been analyzed and interpreted by using Techlog 2015.3 software. The lithology of the Mishrif Formation was determined by using the M-N cross plot method based on the interpretation of density, neutron, and sonic logs. The results showed that the Mishrif Formation is mainly composed of limestone. The shale volume in the Mishrif Formation has been estimated from the gamma-ray log. The results illustrated that the shale volume is about 20% of the bulk volume, and may increase to reach the highest value at the upper part of the MA unit of the formation. The porosity of the Mishrif Formation was calculated based on the interpretation of neutron, density, and sonic logs. To achieve accurate values of porosity, the log-derived porosity has been correlated with the core-derived porosity, and the comparison showed a good correlation between the two types of porosity. The results showed that the Mishrif Formation is characterized by low to medium porosity (about 5% to 18%). The secondary porosity of the formation is most dominant in the MB21 unit compared with the other stratigraphic units of the Mishrif Formations. This indicates that the MB21 unit was affected by the diagenesis processes. The MB21 unit of the Mishrif Formation represents the most dominant reservoir because it was delineated by high effective porosity and high oil saturation.

Facies analysis of the Middle Cretaceous Mishrif Formation in southern Iraq borehole image logs and core thin-sections as a tool

The Mishrif Formation constitutes a significant oil reservoir within the Ratawi oilfield located in Southern Iraq. Three wells, R T-2, R T-3, and R T-5, situated 70 km northwest of Basra, were chosen to examine the petrophysical attributes with potential for oil production. Gamma-ray (GR), resistivity, and porosity logs were used to define the upper and lower parts of the Mishrif Formation. Water saturation of oil and non-oil areas, shale volume and porosity were calculated. The results show that the amount of oil shale is less than 15% in most wells, and the formation contains varying percentages of produced hydrocarbons. The Cretaceous Mishrif Formation is widespread in Iraq. Due to erosion, truncation, and lateral face changes, the thickness of this formation varies. The Mishrif Formation consists of four units from top to bottom: MA, CR-1, MB-1, and MB-2. The bottom of the Mishrif Formation (MB) extends from the Middle Mishrif, where the Middle Mishrif is at the top and the Rumaila Formation is at the bottom. This unit can be considered a cover rock for the Middle Mishrif Reservoir due to the low saturation rate in well 3, lower than the other wells in the two units, and the low saturation rate in the unit MB-2 in well 2. The results of analyzing and interpreting the MA unit records showed that they contain a very small percentage of the volume of oil shale in wells 5, 3, and 2, respectively, and the percentage of water saturation in wells was higher than in wells 3 and 5. CR unit is composed of a high percentage of shale or the volume of oil shale in well 2, followed by wells 3 and 5, considered the lowest percentage of oil shale. Analyzing and interpreting the logs showed they contain more water saturation in well 2 than in wells 3 and 5. MB unit can be found in all of the studied wells. The studied 3 wells have porosity ranges between 0.08 and 0.16 and water saturation ranges between 0.6 and 0.9.

The majority of petroleum studies have traditionally focused on rocks that store hydrocarbons due to their economic significance, particularly in a period where oil is the primary global energy source. In the Cretaceous period, Iraq, and the Middle East, in general, are of particular interest due to the presence of formations that are significant for hydrocarbon storage and generation. The Mishrif Formation, dating to the Cenomanian-Early Turonian period, is a notable oil reservoir in southern Iraq. This formation’s distinctive features and extensive surface area make it a significant hydrocarbon repository, ranking second economically after the Zubair Formation in the region. This study analyzed six oil wells from the Mishrif Formation using various well logs (gamma ray, density, neutron, spontaneous potential, acoustic and resistivity) to extract petrophysical properties. The study identified rock types, layer boundaries, thicknesses, formation depths, and hydrocarbon-bearing zones. Through qualitative interpretation of these logs, the reservoir's characteristics were evaluated, including porosity, shale distribution, water and oil saturation. The results were visualized in three-dimensional models using the Petrel software, showing the distribution of these characteristics across the study wells. The Mishrif Formation is composed of four units (MA, CR-1, MB-1, and MB-2), with varying properties that were detailed in this study. Shale volume, averages between 0.04 % and 0.16%. Effective porosity ranges from 0.09% to 0.19%, indicating good reservoir quality; permeability ranges from 0.001 to 10 mD. Hydrocarbon saturation varies from 0.1to 0.7, while water saturation ranges from 0.3 to 0.9. The distribution of effective porosity in the Middle Mishrif reservoir unit is good throughout the field. In the Lower Mishrif reservoir unit, the effective porosity distribution is good in the central part of the field but poor in the northern and western parts. The formation thickness is homogeneous, indicating stable deposition. Hydrocarbon accumulations are present in both the Lower and Middle Mishrif reservoir units.

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.

The Middle Cretaceous Mishrif Formation is considered as the main productive reservoir in the south of Iraq. The rudist-bearing productive zones are characterized by high heterogeneity due to the effects of diagenesis and depositional environment. The Mishrif Formation represents a carbonate succession deposited through the Middle Cretaceous (Cenomanian - Early Turonian) within the main retrogressive depositional period succession. This research is identifying the effect of each diagenetic process on the reservoir properties using the utility of core data and thin section collaborated with the well logs along the wells ZU-46, ZU-41 and ZU-42 in the Zubair oilfield. The Mishrif Formation have been subjected into various diagenetic processes such as micritization, neomorphism, dissolution, cementation, dolomitization and pressure solution or stylolitization. Effect of eight diagenetic processes have been recognized in the Mishrif Formation, these processes have affected constructively and destructively on the reservoir quality of the Mishrif Formation; dissolution and fractures have been affected positively, while micritization, cementation, dolomitization, compaction, bioturbation and neomorphism have negatively affected. The porosity logs (Neutron, density and sonic) and gamma ray log are interpreted to find the effective porosity and clay content to determine the reservoir quality. The porosity results show a wide variation, the primary porosity is decreased in the well ZU-46 at the depth of 2300-2450 due to the cementation while the secondary porosity is enhanced due to the dissolution and fractures. In well ZU-42, at the depth of 2225-2400m there is an increasing in the porosity due to the increasing of grains to clay while in the ZU-42 at the depth of 2215-2300m, the porosity is very low due to cementation, compaction and micritization affects as well as the high content of clay. In general, the porosity is enhanced in the reefal zones due to the geometry nature of the pore throat.

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, mudstone–wackestone, 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.

This paper reports on the sedimentology and stratigraphy of the mid‐Cretaceous Mishrif Formation, one of the principal carbonate reservoirs in Central and Southern Iraq. The Cenomanian Mahilban, Maotsi and Fahad Carbonate Formations of Central Iraq are the lateral chronostratigraphical equivalents of the Mishrif and underlying Rumaila Formations of Southern Iraq. Together, these units represent a single mid‐Cretaceous carbonate succession in the Mesopotamian Basin.The Mishrif Formation in Central Iraq reflects the continuous deposition of shallow‐shelf carbonates; periodic rises in sea level led to episodes of deeper‐water sedimentation, during which the outer‐shelf and basinal deposits of the Rumaila Formation were laid down. A ramped platform was the principal depositional setting for the entire Cenomanianearly Turonian carbonate succession.The best reservoir conditions in the Mishrif Formation occur in rudist‐bearing facies, such as rudstones and rudistid packstone/grainstones. Reservoir units are characterised by porosities of >20% and by permeabilities of 100 mD to 1 Darcy. Other carbonate facies, such as pelagic mudstone/ wackestones, bioclastic wackestones and peloidal packstones, are less significant as reservoir rocks. All the carbonates were affected by a range of diagenetic processes, among which dissolution and dolomitization led to the formation of secondary porosity; porosity was reduced by compaction, stylolitization, micritisation, neomorphism and cementation.The Mishrif Formation is divisible by a prominent unconformity into two large‐scale regressive sequences, which are particularly distinguishable in the east of the Mesopotamian Basin. Multiple reservoir units are present in both sequences. The west of the basin is dominated by the lower sequence, which has relatively few reservoir intervals. The shallow‐water reservoir units in the east of the basin are thick, reflecting relatively high subsidence rates throughout the Cenomanian (e.g in the Amara oilfield and nearby areas). Subsidence rates in the western side of the basin were lower, and reservoir units are thinner and more limited. The Mishrif Formation carbonates wedge‐out in the western and SW deserts of Iraq.

The history basin of the Mishrif formation in Southern Iraq was analyzed by building a model of sedimentary basin, which enabled us to know the geological events that occurred during and after the deposition it. Mishrif formation of age (Late Cenomanian - Early Turonian) is considered one of the most important geological formations containing oil in Southern Iraq, so nine wells were chosen from several oil fields to cover the study area: (Zb-114, R-270, WQ-17, Rt-5, Lu-2, Ns-5, Ri-1, No-2, and Hf-5), located between the Eastern lines (582400-749080) and the Northern lines (3534600-3357197). Mathematical models and equations were used to calculate the original thicknesses and sedimentation rates of the Mishrif formation and the formations above it through the Backstripping method. Estimated of the sedimentation rates and erosional thicknesses of the regional unconformities surfaces that located in the stratigraphic column in the study area. While Mishrif formation was characterized a moderate sedimentation rate that ranged between (2-6cm./1000y.), where increase in the Northeast of the study area in wells (Ri-1, No-2, and Hf-5), these rates an indicator to the center of sedimentary basin. The geological burial history curves show three levels of subsidence rates (Fast, moderate, and low), the burial history of the Mishrif formation and the other formations indicated that the sedimentation rates are in a direct relationship with the total subsidence of the basin. Three regional unconformities surfaces were found which the effectiveness of the tectonic movements, it’s had strong effective to the sedimentary basin of the Mishrif formation and other formations, where exposed it to during the Cretaceous and Tertiary periods. These movements had a major impact on shaping the sedimentary character through its influence on the process of advancement and retreat of the sea level, which formed successive depositional cycles, as the top of the Mishrif formation was exposure to uplifting and erosion processes, especially in the Northeast parts, where formed the first regional unconformity surface about (89-90 m.y.) ago.

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 environ- ment from fully marine to lacustrine environment. Thus, the vertical bioclast analysis indicates that the Mishrif Forma- tion 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 distribu- tions on the first appearance of species. Fifteen principal sedimentary microfacies have been identified in the Mishrif Formation, which includes lime mudstone, mudstone- wackestone, wackestone, wackestone-packstone, pack- stone, packstone-grainstone, grainstone-floatstone, packstone-floatstone, packstone-rudstone, and wacke- stone-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 environ- ment, 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.

Mishrif Formation is one of the main oil reservoirs in southern Iraq. This study analyzes the microfacies and depositional environment for Mishrif Formation in North Rumaila oilfield. The study was based on the analysis of 17 wells core. The Mishrif Formation represents deposition in a carbonate platform ramp system, with scattered patch reefs and shoals developed across the ramp margin and the platform top. It is characterized by skeletal grains (bioclasts) which are dominated such as foraminifera, rudist, calcareous Algae and other skeletal grains included mollusks shell fragments with Chondrodonta sometimes, and Echinoderms while non-skeletal grains are less abundant which are represented by Peloids and Ooids. According to the petrography analysis of Mishrif Formation, the fossils are dominated in the formation, four groups of these fossils are diagnosed, and these are Oligosteginid, Alveolinids, Dicyclina and Miliolids. Can be identified and build the sedimentary model with microfacies which apply to the Mishrif Formation. The sedimentological and stratigraphic analysis of Mishrif Formation core led to identification of 16 facies association, seven of which are found in the mB unit (lower part of the Formation) whereas they are deposited in outer ramp, mid ramp, ramp margin and lagoon, while nine in the mA unit (upper part of the formation) which are deposited in intraplate basin, mid ramp, inner ramp, ramp margin and lagoonal supra tidal.

The Upper Cretaceous Mishrif Formation of Southern Iraq deposited as part of a large carbonate platform system spanning across the Arabian Plate from Iraq to Oman. The carbonates are the host of some of the most prolific hydrocarbon accumulations and form one of the major reservoir units of the Rumaila Field. The Mishrif Formation is part of the second-order systems tract spanning from the upper-most Albian to the Turonian. The Ahmadi Formation is part of the Transgressive Systems Tract (TST), and the Rumaila and the Mishrif Formations are part of the Highstand Systems Tract (HST). The Mishrif is bound at the top by an Arabian plate-wide unconformity. The Mishrif Formation was further divided into five 3rd-order sequences. The lower three of these sequences comprise the Mishrif B (mB) while the upper two sequences make up the Mishrif A (mA). Extensive core studies helped to understand the depositional environment and the impact of the sequences on reservoir quality of the Mishrif reservoir.

The Microfacies analysis of the Cretaceous Mishrif Formation was studied in Nasiriyah oil field in Nasiriyah Governorate, southern of Iraq using integrated borehole data set that included, core samples and well logs in newly drilled well to analyze the microfacies and the Diagenesis Process of the formation, 100 thin sections for selected well. The results show that the formation composed of: (top to base) a fine-grained, limonitic fresh water limestone containing Charophytae. Four facies’ associations were distinguished in the Mishrif Formation. These include: - These include: - Restricted shallow Open Marin environment is represented by Wackstone and Mudstone to Wackstone. The shallow open marine environment is represented by bioclastic wackestone and packestone, as well as rudistid floastone in rare situations. Shoal Environmentis represented by bioclastic packstone to grainstone, as well as rudstone. Deep Marine environment is represented by mudstone to packstone. six diagenetic processes affected on Mishrif Formation: Cementation, Micritization, Recrystallization, Dissolution, Compaction and Pressure Solution (Stylolization), and Dolomitization, found in thin sections.

This paper aims to identify the impact of the diagenesis processes on the carbonate reservoir’s characteristics (porosity and permeability) of the Mishrif Formation. A detailed investigation of available core samples and microscopic study of more than 200 thin sections of the core limestone samples were examined from two wells (Ratawi-2 and Ratawi-5) in Ratawi oilfield, which are located in the Basrah Governorate, southern Iraq which revealed that different diagenetic parameters influence the reservoir quality in Mishrif Formation. Diagenetic features and four identified diagenetic environments that have affected the carbonate rocks in the Ratawi oilfield were the meteoric vadose, meteoric phreatic, marine phreatic and mixing zone. Petrographic examinations revealed that diagenesis varies in intensity from microfacies to another, micritization, dissolution, cementation, compaction, neomorphism, dolomitization and authigenic minerals (pyrite) are the dominant diagenetic parameters which are identified in the current field of study. Among all the observed diagenetic features, dissolution and neomorphism (recrystallization) were formed as a result of subaerial exposure during the meteoric diagenesis and contribute to porosity enhancement and reservoir quality. The cementation (calcite and dolomite), compaction, dolomitization, and micritization have a negative impact on the reservoir behavior, through reducing porosity and permeability that led to reducing reservoir quality. Other processes such as authigenic minerals (pyrite) did not have strong effects on reservoir quality. Based on the genetic classification of porosity, most of the porosity within Mishrif Formation is a combination of depositional, diagenetic and fracturing. The diagenetic porosity is the far more dominant types of porosity such as vuggy, intraparticles, fractured and moldic, implying that carbonate Mishrif reservoir is the type of diagenetic reservoirs.