Zubair Oilfield Research Articles

Tectonic evolution and structural geology of the Jurassic succession in the Zubair oil field, southern Iraq

This study investigates the structural style and tectonic evolution of Jurassic succession in southern Iraq, within the Zubair oil field, which are crucial for charging oil to the Cretaceous reservoirs, utilized 3-D seismic reflection data to explore its complex geological history. The field is located in the Mesopotamian plain on the northeast Arabian Plate in southern Iraq. Four folds recognized: Hammar, Shuaiba, Rafdyia, and Safwan, each shaped by specific geodynamic processes. Hammar, Shuaiba, and Safwan are characterized as salt-related folds, influenced by salt tectonics. Rafdyia is identified as a compression-related fold formed due to the convergence of the Arabian and Eurasian plates. Forty-seven faults were identified, with diversity patterns and spatial distributions suggesting a tectonic history marked by three distinct phases: extensional tectonics, extensional with concurrent salt activity, and plate convergence. These phases have created a series of graben structures that, along with the identified folds, highlight the role of three primary tectonic processes: basement tectonics, salt dynamics, and plate interactions in shaping the structural style of the Zubair oil field during the Jurassic time.

Gas Lift Optimization for Zubair Oil Field Using Genetic Algorithm-Based Numerical Simulation: Feasibility Study

The gas-lift method is crucial for maintaining oil production, particularly from an established field when the natural energy of the reservoirs is depleted. To maximize oil production, a major field's gas injection rate must be distributed as efficiently as possible across its gas-lift network system. Common gas-lift optimization techniques may lose their effectiveness and become unable to replicate the gas-lift optimum in a large network system due to problems with multi-objective, multi-constrained & restricted gas injection rate distribution. The main objective of the research is to determine the possibility of using the genetic algorithm (GA) technique to achieve the optimum distribution for the continuous gas-lift injection flows in the structure of the network of Zubair, oil field with 10 gas-lift injected wells. This will be done through numerical simulation and modeling studies. The overall enhancement of the filed production rate is found to have increased from 15767 STB/day to 19847 STB/day. The well's reservoir pressure and water cut sensitivity studies are carried out to study the possible impacts of these elements upon the well and its efficiency through the course of the field. Our understanding of the potential benefits of utilizing gas lift techniques in a field from a technical and economical point of view is deepened by the use of examples from economic analysis. Furthermore, even though the idea of employing GA in this manner is not new, this work discusses GA-based optimization methodologies for increasing the oil production rate by using gas lifting in a Zubair oilfield. In order to assign gas injection rates to specific wells in a network throughout the field using limited gas injection rates, the model for optimization will be laid out step-by-step making it simple to understand and employ as a guide, especially for the front-line production technicians involved in the development and design of gas-lift systems.

An Experimental Study to Assessing the Efficacy of Oil to Prevent Differential Stuck Pipe Incidents in Zubair Oil Field, Southern Iraq

This study focuses on the important role of drilling mud in successful drilling operations, particularly in preventing and treating differential stuck pipes. Differential pipe sticking (DSP) is a common challenge when drilling through a pay zone with water-based drilling mud containing bentonite clay. However, using oil as an additive in a water-based mud (WBM) system offers distinct advantages and can significantly improve drilling performance. The research utilizes the Fann VG meter, a self-fabricated tester, and a tester of stickance to investigate the impact of oil on water-based drilling mud (WBM) and controlling DSP to address this challenge. The empirical study demonstrates that adding oil to the drilling fluid enables it to maintain its rheological properties (PV, YP, and API fluid loss) while reducing filtration loss from 7 to 4.8 cc and friction coefficient from 0.30 to 0.05. This indicates that emulsion mud can mitigate the risks of differential sticking by maintaining the oil concentration within the recommended limit (not exceeding 3%). Balancing the maintenance of rheological properties with the reduction of friction coefficient is essential to reaching optimal drilling performance and preventing differential pipe sticking.

A Review on Pressure Transient Analysis in Multilayer Reservoir: South Iraq Case Study

Multilayer reservoirs are currently modeled as a single zone system by averaging the reservoir parameters associated with each reservoir zone. However, this type of modeling is rarely accurate because a single zone system does not account for the fact that each zone's pressure decreases independently. Pressure drop for each zone has an effect on the total output and would result in inter-flow and the premature depletion of one of the zones. Understanding reservoir performance requires a precise estimation of each layer's permeability and skin factor. The Multilayer Transient Analysis is a well-testing technique designed to determine formation properties in more than one layer, and its effectiveness over the past two decades has been demonstrated. In order to conduct MTA, a combination of rate profiles derived from production data and transient rate and pressure measurements at multiple surface rates is necessary. Numerous experimental and analytic approaches to calculating multilayer characteristics, performance, and flow behavior in multilayer systems have emerged. This technology was implemented at the Zubair oil field in southern Iraq. In the last four years, the number of wells producing under saturation pressure has been increased in the Zubair oil field, particularly for the Mishrif and Zubair reservoirs. In the design of secondary and tertiary recovery, the study of the reservoir in the form of an individual layer to determine the pressure, permeability, and damage of each layer with commingled formation is important. This research describes previously available methods, factors that affect Multilayer Transient Analysis an economic indicator of Multilayer Transient Analysis and a case study

Large Benthonic Foraminifera Biozonation of Mishrif Formation at Tuba and Zubair Oilfield, Southern Iraq

Mishrif Formation was deposited during The Cenomanian-Early Turonian, which has been studied in selected Tuba and Zubair OilFields, these wells (TU-5, TU-24, TU-40, ZB-41, ZB-42, and ZB-46) are located within Mesopotamian basin at southern Iraq and considered as a major carbonate reservoir in Iraq and the Arabian Gulf. The palaeontological investigations mainly depending on benthonic foraminifera of the studied wells of Tuba and Zubair Oilfields in Mishrif Formation, twenty-four species belonging to fourteen genera are recognized of benthonic foraminifera, which has been recognized through this study, especially benthonic foraminiferal, indicating four zones as follows:

Identifying Average Reservoir Pressure in Multilayered Oil Wells Using Selective Inflow Performance (SIP) Method

The downhole flow profiles of the wells with single production tubes and mixed flow from more than one layer can be complicated, making it challenging to obtain the average pressure of each layer independently. Production log data can be used to monitor the impacts of pressure depletion over time and to determine average pressure with the use of Selective Inflow Performance (SIP). The SIP technique provides a method of determining the steady state of inflow relationship for each individual layer. The well flows at different stabilized surface rates, and for each rate, a production log is run throughout the producing interval to record both downhole flow rates and flowing pressure. PVT data can be used to convert measured in-situ rates to surface conditions. Different types of Inflow Performance Relationship (IPR) equations can be used for SIP interpretation, including the Straight-line method, Fetkovitch method, and Laminar Internal Turbulent (LIT) relations. Although the SIP method can be used for single-phase flow, the interpreter can restrict the IPR’s calculations to a particular phase. This research discusses the difficulties in estimating the average reservoir pressure in multilayered reservoir completed wells over their production life. The SIP technique has been applied to some producing wells in the south of Iraq, which are completed in multiple producing reservoirs previously tested with a formation tester to estimate reservoir pressure and other parameters. Two wells are taken in the south of Iraq region, Zubair Oil Field, one with cross flow between perforations and the other well with no cross flow. An average pressure is not calculated for layer A in Well-1, because there is no contribution rate. While the average pressure for Well-1, layer B is 3414.49 psia. Also, the average pressure for Well-2, layer H is not calculated because there is no rate contribution from this layer, and the maximum average pressure was calculated in layer G, which is about 2606.26 psia. It is also found that the presence of cross flow has no effect on SIP calculations.

High resolution sequence stratigraphy of the Mishrif Formation (Cenomanian-Early Turonian) at zubair oilfield (al-rafdhiah dome), southern Iraq

The Mishrif Formation (Cenomanian -E Turonian) is one of the most important geological formations in the Middle East and Iraq because it contains enormous petroleum accumulations. It is considered to be the first reservoir in the region, and is still being studied because of its economic significance. The carbonate of the Mishrif Formation derives from a variety of depositional settings, including mid-ramp, shoal, lagoon, and intertidal. The five main microfacies discussed in this paper are wackestone, packstone, grainstone, floatstone, and bindstone. The most frequent fossilised components found in the Mishrif Formation are rudists, benthic foraminifera, echinoderms, burrows molluscs, and algae. According to the microfacies and analysed wireline log data, the sequence stratigraphy of the studied formation is composed of two regression cycles. Five parasequences of transgressive–regressive cycles make up the depositional sequence of the Mishrif Formation. The standard depositional environments seem to demonstrate a gradual regression, beginning with a short period of the outer ramp, then a steady period of the mid-ramp, and ending in the intertidal environment. Additionally, the study recorded two regional maximum flooding surfaces: K-135 and K-140. The former is present in the lowermost part of the formation, while the other lies in the middle. This study shows a close relationship between facies (environments) and hydrocarbon accumulation. The increased accumulation focuses on the lower part of the studied formation, and seems to be lower in the upper part of the formation as a result of changes in the environment of deposition.

Stratigraphy Analysis of the Nahr Umr Formation in Zubair oil field, Southern Iraq

The Nahr Umr Formation is considered one of the main reservoirs produced in southern Iraq. It is one of the important siliciclastic deposits of the Cretaceous sequence of Iraq oilfields. Zubair oil fields ZB-190 and ZB-047 were chosen to study areas. This study depends on the available core and cutting samples to determine the facies analysis, depositional environments, petrographic characteristics and diagenesis processes. Based on the description of the core and the borehole, six types of facies were distinguished in the Nahr Umr Formation, resulting in an intercalated sandstone and shale with a thin layer of siltstone. The petrographic study of the clastic part of the Nahr Umr Formation showed that the sandstone is composed mainly of quartz arenite. Diagenesis processes affecting the Nahr Umr Formation are two types compaction and cementation processes. The sedimentary environment of the Nahr Umr Formation was represented by the deltaic fluvial environments. These environments appear in the central and southwestern parts of the study area. Sequence stratigraphy in the Nahr Umr Formation is beginning by sequence boundary type one (SB1) and sequence boundary type two (SB2) that appeared in the upper part of the formation. The other surfaces were represented by the end of the deposition of the Nahr Umr Formation and the emergence of the transgressive ravinement surface (TRS) within the transgressive system tract (TST)and The maximum flooding surface (MFS) was distinguished in the middle of the formation and each of these surfaces has a lateral extension along the study section.

Estimation of Static Young Modulus for the Third section in Zubair Oil Field :A Comparison Study

A detailed analysis for the mechanical properties of the rock is necessary to maintain a well bore and avoid problems with well bore instability. Static young's modulus is one of the most crucial rock mechanical properties utilized in the drilling process to assess wellbore integrity and setting-up geomechanical earth model. Static young’s modulus change with varies of formation lithology’s which arise the critical need for estimation. The current study, a static young modulus values of different formations in section 12.25“in Zubair oil field are determined by depending on correlation to indicate variations in units and layers. Data of Gamma ray (GR), density log, sonic compression log (DTCO), and sonic shear log (DTSM) logs of well ZA-2 are utilized to estimate this attribute. Four different correlations that are set-up in Techlog 2015 software are used to achieve the aim of this study. These outcomes are calibrated using the fundamental laboratory tests (Brazilin and Triaxle test). The results showed that the modified Morales correlation provides a good matching with Calibration data, while four key data points are poorly correlated with the results of John Fuller's analysis.Also, Morales correlation shows unconformity when porosity reduced by less than 10% at different intervals, and Plumb Bradford correlation produces incorrect findings of static young’s higher than the dynamic. The results declare inversely relationship between porosity and young’s modulus.

Variation of Total Stresses During Field Operation Using Finite Element Technique, Zubair Oil Field.

Most problems in the Zubair oil field are geomechanical which can lead to maximized the non-productive time and some of these troubles are reservoir compaction, porosity change, permeability change, wellbore instability, casing smash, pipe damage, and well kick. This study was to introduce a couple of the reservoir model and geomechanical model using the finite element method. The interesting data (logs, calibration data, drilling, and mud report) were utilized to build one dimensional model (1D) for each well using Techlog 2015 software. The 3D geomechanical model builds by Petrel 2017 software while the finite element technique was built using CMG 2018 program to forecast the stress states during production or injection operation in the field for 10 years. The 1D model results show a good match between mechanical properties and calibration data which is mean a perfect mechanical earth model. Distribution results of all mechanical rock properties in the 3D geomechanical model showed that Shuaiba and Al-Hammar domes were not sufficient for stable wells, especially in the Tanuma formation. While the results of Mishrif formation showed more stability despite production. 3D Finite element model results showed that the total horizontal stress decreases during production and increases in injection wells, this would cause increases in the effective horizontal stress during production and a decrease for injection. The effective vertical stress increased during the production and decreased during injection wells. Based on these results production could cause an increase in the differential stress which leads to rock shear failure.

Crude oil production prediction based on an intelligent hybrid modelling structure generated by using the clustering algorithm in big data

Since the behavior of a complex dynamic system for a large oil field in Iraq is significantly influenced by many nonlinearities, its dependent parameters exhibit non-stationary with a very high delay time. Developing white-box modelling approaches for such dynamic oil well production cannot handle these large data sets with all dependent dimensions and their non-linear effects. Therefore, this study adopts the hybrid model that combines white-box and black-box to address such problems because the model outputs require various variable types to achieve optimal fitness to measured values. The hybrid model structure needs to evolve with changes in the physical parameters (white-box part) and Neural Networks' Weights (black-box part). The model structure of the proposed hybrid network relied on converting fuzzy rules in a Takagi–Sugeno–Kang Fuzzy System (TSK-FS) into a multilayer perceptron network (MLP). The hybrid parameters are formulated concerning six-dimensional dependent variables to describe them in matrix form or layer and by which can quantify total model outputs. After mapping categorical variables to tuples of MLP, the Gauss-Newton regression (GNR) provides an optimal update of the hybrid parameters to get the best fitting of the model outputs with the target of the dataset. The clustering technique and GNR promote predictive performance due to reducing uncertainties in the hybrid parameters. Due to time being the most effective of the independent variables for predicting oil production, datasets are classified into different clusters based on time. The actual field dataset for training and validation is collected from Zubair Oil Field (9 oil wells), which is implemented to build the proposed model. The results of the hybrid model indicate that the development of the proposed structure has achieved the high capability to represent such big data which is the most imperative feature of the proposed model. Furthermore, obtained results show its accuracy far outpacing competitors and achieving a significant improvement in predictive performance.

Vertical Stress Prediction for Zubair Oil Field/ Case Study

Predicting vertical stress was indeed useful for controlling geomechanical issues since it allowed for the computation of pore pressure for the formation and the classification of fault regimes. This study provides an in-depth observation of vertical stress prediction utilizing numerous approaches using the Techlog 2015 software. Gardner's method results in incorrect vertical stress values with a problem that this method doesn't start from the surface and instead relies only on sound log data. Whereas the Amoco, Wendt non-acoustic, Traugott, average technique simply needed density log as input and used a straight line as the observed density, this was incorrect for vertical computing stress. The results of these methods show that extrapolated density measurement used an average for the real density. The gradient of an extrapolated method is much better in shallow depth into the vertical stress calculations. The Miller density method had an excellent fit with the real density in deep depth. It has been crucial to calculate vertical stress for the past 40 years because calculating pore pressure and geomechanical building models have employed vertical stress as input. The strongest predictor of vertical stress may have been bulk density. According to these results, the miller and extrapolated techniques may be the best two methods for determining vertical stress. Still, the gradient of an extrapolated method is much more excellent in shallow depth than the miller method. Extrapolated density approach may produce satisfactory results for vertical stress, while miller values are lower than those obtained by extrapolating. This may be due to the poor gradient of this method at shallow depths. Gardner's approach incorrectly displays minimum values of about 4000 psi at great depths. While other methods provide numbers that are similar because these methods use constant bulk density values that start at the surface and continue to the desired depth, this is incorrect.

Formulating Inhibited Fluids for Stable Drilling Operations into Tanuma and Zubair Shales, Zubair Oilfield, Southern Iraq

Tanuma and Zubair formations are known as the most problematic intervals in Zubair Oilfield, and they cause wellbore instability due to possible shale-fluid interaction. It causes a vast loss of time dealing with various downhole problems (e.g., stuck pipe) which leads to an increase in overall well cost for the consequences (e.g., fishing and sidetrack). This paper aims to test shale samples with various laboratory tests for shale evaluation and drilling muds development. Shale's physical properties are described by using a stereomicroscope and the structures are observed with Scanning Electron Microscope. The shale reactivity and behavior are analyzed by using the cation exchange capacity testing and the capillary suction test is utilized for selecting shale inhibitor base. As a result, four drillings muds are formulated with different additives and approved by using the Linear Swelling Meter and the Hot Rolling experiments. Tanuma’s shale is at higher reactivity level, where it is moderate to high active shale than Zubair’s shale, which is at a low to moderately reactive formation. Microfractures and micropores are excited in both formations and potentially in Tanuma’s shale. The shale stability can be achieved by adding 8 % of KCl for Tanuma’s mud and 4% of KCl for Zubair’s mud. The filtration controls are contributed to seal the shale open structures and adding the Poly amino acid hydration suppressant reduced the risk of shale swelling significantly. The results can be used for designing drilling mud to reduce shale instability issues and the cost.

Estimation of Internal Friction Angle for The Third Section in Zubair Oil Field: A Comparison Study

The One important rock mechanical property in geomechanical investigations is internal friction angle. In order to predict wellbore failure and construct a geomechanical earth model, friction angle is essential. It is usual to estimate the internal friction angle using core tests, but this approach requires more cores and is expensive. Instead, several empirical correlations were found to estimate the internal friction angle from logs data. The goal of this study was to determine the most accurate approach for estimating internal friction angle from log data and to demonstrate how clay volume and porosity influenced this estimate. This property is estimated using three different correlations built within Techlog 2015 software depend on neutron, density, and gamma ray logs data. The findings demonstrate that Weingarten and Perkins Weal correlation is used, especially when water production is actually occurring. The gamma ray technique does not accurately match the core data since it only employs the gamma ray log. With effective porosity and clay volume used as input data for the Plumb Clay Volume and Porosity correlation, the findings demonstrate an excellent match with the core data. The frictional angle exhibits minimal values in the Tanuma formation and at various depths as a result of a decrease in effective porosity and an increase in shale volume. The results showed that for determining frictional angle, the Plumb correlation approach was the most accurate.

Rock-mechanical properties and in situ stress state of the Upper Cenomanian Mishrif limestone reservoir, Zubair oil field, southern Iraq

Rock-mechanical properties and tectonic stress state have crucial implications for reservoir stability, drilling, and production optimization. We have developed the first ever in situ stress analysis focused on the Mishrif reservoir from the supergiant Zubair oil field, southern Iraq. We interpreted an overall hydrostatic pore pressure gradient of 9.8 MPa/km and a vertical stress ([Formula: see text]) gradient of 22.17 MPa/km in the Upper Cenomanian Mishrif limestone formation. We inferred a Poisson’s ratio of 0.27–0.31, a Young’s modulus of 27–38 MPa, a coefficient of internal friction range of 0.66–0.76, and estimated a uniaxial compressive strength (UCS) range of 35–70 MPa with a low cohesive strength of 7.8–9.8 MPa from compressional sonic slowness data. We established fitting relationships between the dynamic and static elastic properties from the core-based triaxial test measurements and used the same for estimating the minimum ([Formula: see text]) and maximum ([Formula: see text]) horizontal stress magnitudes by poroelastic horizontal strain model. We validated the reservoir [Formula: see text] with the mini-frac-based fracture closure pressure and interpreted its gradient as 14.5–15.9 MPa/km, whereas we calculated the [Formula: see text] gradient as 19.5–23 MPa/km. We interpreted C-quality breakouts from four-arm caliper data and deciphered a northeast–southwest [Formula: see text] orientation (N45°E–N70°E). Using the compressive failure criteria, we constrained the reservoir [Formula: see text] by stress polygon analysis against the estimated UCS ranges and concluded a normal fault to strike-slip transitional tectonic stress state ([Formula: see text] ≥ [Formula: see text] > [Formula: see text]) in the Mishrif reservoir.

Integrated 3D Mechanical Earth Modelling to Intensively Investigate the Wellbore Instability of Zubair Oil Field, Southern Iraq

Wellbore instability in the Zubair oil field is the main problem in drilling operations. This instability of the wellbore causes several problems including poor hole cleaning, tight hole, stuck pipe, lost circulation, bad cementing, and well kick or blowout that lead to an increase in the nonproductive time. This article aims to set up an appropriate drilling plan that mitigates these instability issues for further well drilling. Field data from six wells (logs, drilling and geological reports as well as offset well tests) were used to build a one-dimensional mechanical earth modeling for each well. The constructed model of selected wells was combined to construct the three-dimensional mechanical earth modeling, which can enable the distribution of all estimated geomechanical parameters along the field of interest. The results revealed that the strip slip and normal faults are the common fault regimes in Zubair oil field located in carbonate rocks and clastic rocks, respectively. The Mogi-Coulomb failure criterion is conservative in determining the minimum and maximum mud weight, and it agrees with the determination of real failure from the image/and caliper logs. The best direction to drill the deviated and horizontal wells was towards the minimum horizontal stress with 140o azimuth from the north. The recommended ranges of mud weight along the sections of 12.25" and 8.5" of the highly deviated and horizontal wells were (between 1.46 and 1.58 gm/cm3) without any expected wellbore instability problems. Based on the outcomes of 3D model, it is expected that the wellbore instability issues are most likely to occur in the domes of; Shauiba and Hamma and in formations; Tanuma, Khasib and Nahr-Umr. In contrast the less wellbore instability problems are expected to expose in Rafidya dome. The study presents an appropriate mud window that can be used to design to minimize the wellbore stability problems when new wells will be drilled in Zubair oil field.

Chemical and biological compatibility of different injection waters with Zubair Formation Water of Upper Sand Member in Zubair Oil Field, South of Iraq

Water injection by water flooding was used to enhance and increase oil production in Zubair oil field, southern Iraq. Physical-chemical and biological analysis of five water samples from different sources were collected to evaluate its compatibility with formation water using biological experiments and chemical compatibility simulation. The results show that injection water is classified weakly acidic-weakly alkaline and saline water, whereas surface water samples are considered weakly acid-weakly alkaline. The total dissolved solids results show brackish types accept for Formation water which classified weakly acid and Brine water. All the studied water samples contain bacteria colonies of Escherichia coli and Coliform expect for one sample, while Sulfate Reducing Bacteria was founded in all studied samples. Mathematical model of chemical compatibility between studied water samples and Zubair Formation water of the scale prediction model show that there are no needs for any inhibition treatments of all scales except for Geothite and Dolomite that should be treated before water injection. The biological compatibility experiments results show Formation damage about (61%) and (69%) in the studied core samples, while Bactria in water injection caused formation damage about (20%) and (51%).

Present-day stress field and stress path behaviour of the depleted Mishrif reservoir from the super-giant Zubair oilfield, Iraq – A geomechanical case study

Accurate understanding of the reservoir pore pressure and in-situ stress behaviour in response to depletion has critical effects in production optimization and field redevelopment while ensuring reservoir integrity. This work presents the first reservoir geomechanical modeling from the supergiant Zubair oil field, southern Iraq to decipher the pore pressure, in-situ stresses and depletion stress path of the middle Cretaceous Mishrif limestone reservoir. Vertical stress (Sv) has 0.97–0.99 psi/ft gradient. Minimum horizontal stress (Sh) estimated from effective stress ratio and Poisson's ratio-based approaches yielded an about 0.58–0.76 psi/ft gradient and validated with C-category fracture closure pressure data available from mini-frac tests. In absence of image log-based breakout widths, maximum horizontal stress (SH) is estimated from three approaches, which provided a gradient range of 0.89–1.21 psi/feet. Relative stress magnitudes (SH ≥ Sv >Sh) indicate a normal to strike-slip faulting transitional tectonic state in the study area. C-quality wellbore breakouts, interpreted from the oriented four arm caliper data infer a mean N60°E SH orientation, which is parallel to the movement direction of the Arabian plate towards the Eurasian plate. Recent downhole measurements indicate that the primary producer Mishrif limestone is in depleted condition. Stress path, interpreted from the reduction of pore pressure (Δpp) and minimum horizontal stress (ΔSh), yields average values of 0.65 and 0.62 for upper and lower Mishrif, respectively. Deformation analysis in reservoir space indicates stress path in depleted Mishrif formation of Zubair oil field is less than the normal faulting limit (0.71) and depletion-induced faulting is therefore unlikely to occur.

Karst feature in Mishrif Reservoir and effect on drilling and production in Zubair oil field, Southern Iraq

Karst features in the upper part of Mishrif carbonate reservoir can commonly create substantial vugs, pores and fissures and, thus, increasing the porosity and enhancing reservoir connectivity and permeability, this features were formed normally as a result of the carbonate rocks dissolution under the action of meteoric waters during very long periods of emersion spanning from Late Cenomanian to Turonian (about 4.5 Million years). This study deals with the presence of the karst features in Zubair oil field, southern Iraq, where Karst features were observed in the center and northern parts of the field close to the top of Mishrif Formation in most of the cored wells. Characterization of that feature can be achieved from static data at wellbore scale from core, and logs (conventional and non-conventional), and dynamic data such as mud losses, well tests combined with production logs (PLT) in addition to Seismic data. Beside the positive and economic effect of Karst, there is negative effect which includes causing mud losses during drilling in Mishrif Formation because of low reservoir pore pressure, Therefore, acid soluble cement plugs were used to heal that losses to continue drilling.

COMPARATIVE ORGANIC GEOCHEMICAL STUDY OF CRETACEOUS RESERVOIRS OF ZUBAIR OIL FIELD, SOUTHERN IRAQ

Three formations were chosen in the present study, these are Yamamma, Zubair and Mishrif formations, which are considered the main reservoirs at Zubair oilfield southern Iraq, especially during the Cretaceous period. The studied reservoirs are distinguished by different rocks, facies and environmental specifications. Thirteen wells were selected for the present study these are: Zb-44, Zb-202, Zb-10, Zb-294, Zb-81, Zb-233, Zb-329, Zb-49, Zb-9, Zb-156, Zb-8, Zb-256 and Zb-187. To studying the geochemical parameters of crude oils. Geochemical analysis of crude oil was applied. The results of the isotope analyses indicate that the source rock of oil is a mature marine rock that contains a high percentage of sulfur. The American Petroleum Institute values are ​​ranging from 35-20 indicate medium to light hydrocarbons. The results show that the Kerogen type is type II which is derived from marine algae organism. The burial history indicates that the subsidence is high at the late Jurassic - early Cretaceous period and also at the Miocene, and the slow subsidence during the late Cretaceous and moderate subsidence at the Paleogene. The results of the Vitrinite Reflection and Production Index show that the thermal maturity is happened at the early to the main stage, which was represented at the Zb-44 and Zb-202 wells. The transformation ratio of Zubair and Yamamma formations indicates that the possibility of kerogen to yield oil and gas is high in the future with temperature increasing.