Najmah Formation Research Articles

Palaeoenvironment Conditions During Deposition of Sargelu, Naokelekan, and Najmah Formations in Zey Gawara Area, Kurdistan Region, Iraq: Implications from Major and Trace Elements Proportions

Palaeoenvironment conditions during deposition of Jurassic Sargelu, Naokelekan, and Najmah formations were studied based on samples from well Z1 in Zey Gawara area, Kurdistan region, Iraq. For the purpose of determining the palaeoredox, palaeoclimate conditions as well as palaeoweathering, tectonic setting, and provenance indices, the inorganic geochemical analysis to indicate the number and amount of different elements were performed. Based on the occurrence and proportionalities of trace elements e.g., V, Ni, Cu, Sr, the Sargelu Formation was probably accumulated in suboxic to anoxic conditions, while the Naokelekan and Najmah formations were deposited under the suboxic to dysoxic conditions. Relatively semiarid condition have suggested for Sargelu and Naokelekan formations, and relatively semi humid condition for Najmah Formation. The chemical index of alteration and the high plagioclase index of alteration suggest for modest to intense weathering source area for the Sargelu and Naokelekan formations. The index of chemical variability (<1) implies that the Sargelu, Naokelekan, and Najmah formations are probably immature sediments and deposited inside a tectonically active setting. Based on the tectonic indicators, the Sargelu and Naokelekan formations were deposited in an active continental margin, and the Najmah Formation was deposited in an oceanic island arc. The felsic for the Sargelu and Naokelekan and the felsic- intermediate igneous rocks were proposed as plausible source rocks for the Najmah Formation.

Integrating sedimentologic data and stable isotope geochemistry to comprehend the sequence stratigraphic framework of the Middle–Upper Jurassic Sargelu and Najmah formations in NW Kermanshah (Howramanat, Dowdan and Kezi sections)

Integrating sedimentologic data and stable isotope geochemistry to comprehend the sequence stratigraphic framework of the Middle–Upper Jurassic Sargelu and Najmah formations in NW Kermanshah (Howramanat, Dowdan and Kezi sections)

Predicting marine organic-rich deposits using forward stratigraphic modelling: The Jurassic Najmah source rock – Case study

Predicting the distribution and heterogeneity of marine Mesozoic organic-rich rocks is a challenging task that requires multi-disciplinary data integration supported by innovative numerical modelling. This study aims at investigating the factors controlling marine organic matter production, accumulation, and preservation along the eastern Arabian intrashelf basins. Such types of basins exhibit an enhanced lateral and vertical facies variability caused by the interplay between shallow and deeper water sedimentation. The work focuses on characterizing the Mid-Upper Jurassic Najmah Formation that presents source rock, reservoir, as well as seal characteristics along Kuwait. It comprises high maturity levels (>0.6% Ro) and TOC values ranging between 2 and 30%. Forward stratigraphic modelling that includes organic matter production, transport, and preservation (i.e., degradation and dilution) was utilized. Models corresponding to two end-member basin types (restricted and fully open marine) have been simulated to mimic the organic matter deposition within a 3rd-order sequence stratigraphic framework. Simulation outputs provide a qualitative and quantitative assessment of organic matter characteristics (TOC, HI, and OI) at the time of deposition. Results show that (1) a combined variable eustatic sea level change and tectonic subsidence are the main driving mechanisms for sediment accommodation during the Mid-Upper Jurassic time, (2) strong anoxic conditions in the restricted basin simulations promote the preservation of organic-rich deposits in the distal northern sectors while (3) the influence of open marine bottom current oxygenation is not expected to impact the relatively shallower conditions affecting the intrashelf basin (<200 m depth), (4) organic matter dilution can be observed along the southernmost sectors of Kuwait due to high sedimentation rates related to the development of gravity driven deposits. Sensitivity analysis on input parameters (i.e., primary productivity vs water depth) supports the partially restricted basin conditions with occasional connection to the open ocean during eustatic sea level rise. This study contributes to the improvement of basin models in conventional as well as unconventional petroleum system exploration.

Palaeogeographic reconstruction and sequence architecture of the middle‐upper Jurassic successions of Hawraman Basin (NW Iran): Implications for tectono‐depositional processes of the northeastern passive margin of the Arabian Plate

The middle‐upper Jurassic successions (Sargelu, Najmah, and Gotnia formations) of the NW sector of the Zagros Mountain belt are studied to generate a sequence stratigraphic model that helps to understand the Jurassic tectono‐depositional processes and palaeogeographic reconstructions of the northeastern passive margin of the Arabian Plate. The major geological structures observed in northwestern Iran indicate the predominance of an extensional deformation during the Toarcian to Aalenian owing to the eastern Mediterranean rifting. This extensional regime reached its maximum from the Early to Middle Jurassic, causing the fragmentation/drowning of the long‐lived Triassic to Early Jurassic carbonate platform and the development of the Hawraman intrashelf basin (HIB). Associated changes in marine current dynamics, trophic conditions, and degree of substrate oxygenation led to the bloom of oligotypical biota (Bositra limestones) creating unfavourable conditions for the development of other benthic communities (Sargelu Formation). Later, from the Callovian to Oxfordian, changes in carbonate production types (switch from the Bositra filament facies to the stromatolitic‐thrombolitic boundstones) remarkably decreased the dip angle of the depositional profile (Najmah Formation). A second‐order tectono‐eustatic cycle (a major long‐term transgressive/regressive [T/R] cycle) including six minor short‐term T/R facies cycles (TR1–TR6) is identified in the study area. The development of the T/R facies cycles during the Middle Jurassic (TR1–TR3; give‐up sequences; Sargelu Formation) as a result of combined global eustatic transgression and linear subsidence resulted in drowning and backstepping of the platform. In contrast, the short‐term cycles that developed in the upper Jurassic (TR4–TR6; keep‐up sequences; Najmah and Gotnia formations), following a sea‐level fall, resulted in progressive shallowing of the carbonate platform. This study suggests that extensional deformation of the area reached its peak in the transition of the Early to Middle Jurassic, leading to the creation of HIB, but either decreased or played a subordinate role to eustatic sea‐level variations during the deposition of the TR1–TR3 facies cycles (Bajocian‐Callovian). Changes in tectonic regime from an extensional deformation (Early Jurassic) to cooling subsidence (Middle Jurassic), accompanied by a long‐term transgressive sea‐level trend during the Middle Jurassic, were important factors in the development of anoxic trough environments with the deposition of organic‐rich intervals, represented by the TR1–TR3 facies cycles. Tectonic instability (tectonic uplift plus considerable differential subsidence due to the reactivation of faults) in the eastern margin of the Arabian platform during the Late Jurassic led to the development of the TR4–TR6 facies cycles.

Organic Geochemistry Of The Middle To Upper Jurassic Source Rocks Succession In Balad-1 Well, Balad Oil Field, Central Iraq

Thirty four cuttings samples from the Jurassic rock succession in well Balad-1 in the Balad Oil Field, Central Iraq have been collected. Using various organic geochemical techniques, the organic matter’s quantity, quality, maturity, and their source rock’s depositional setting were determined. The samples were evaluated to determine the amount of their organic matter content, type of organic matter, δ13C carbon isotopes abundance for both saturated and aromatic, and molecular properties. The results of organic geochemistry analysis show that Sargelu, Gotnia, and Chia Gara formations contain fair to decent amounts of organic matter. Naokelekan Formation encompasses fair to excellent organic matter, while Najmah Formation comprises very high to exceptional organic matter. The analyzed samples revealed the existence of kerogen types III and II/III mainly within oil window. Thermal maturity related biomarkers are in a good agreement with Rock-Eval parameters, but did not reach equilibrium phase. Source related biomarkers show that these rock units rich in organic matter were mainly deposited in an anoxic marine depositional setting which consists of carbonate influenced by terrestrial input.

Geochemical characteristics of crude oils and basin modelling of the probable source rocks in the Southern Mesopotamian Basin, South Iraq

Bulk property and geochemical analyses were performed on 15 crude oil samples collected from the Early Cretaceous clastic reservoirs in the oilfields of the southern Mesopotamian Basin, South Iraq. Specific gravities ranging between 25.4 and 31.6 API and relatively high sulfur concentrations of up to 1.97 wt % suggest that the samples were generated from kerogen Type II-S at relatively low maturity level. Biomarker ratios and parameters as well as carbon isotope ratios, indicate that the oils were derived from marine carbonate-rich source rocks deposited under reducing environmental conditions during the Jurassic to Early Cretaceous. Furthermore, biomarker maturity suggests two maturity subgroups for the samples based on their sulfur concentrations and specific gravity values. Sub-group (A) is characterized by oils generated from low-mature source rocks from the Late Jurassic-Early Cretaceous Sulaiy Formation; while the relatively high mature oils of sub-group (B) were generated from organic-rich intervals in the Jurassic Sargelu and Najmah formations. Basin model results revealed that these formations reached a peak oil-generation window at different times, expelling various quantities of oil. The basin model results further suggest that the organic-rich interval in the Sulaiy Formation began to expel little amounts of oil, corresponding to transformation ratio (TR) between 50 and 63% since the Late Miocene to present-day, while the organic-rich intervals in the Sargelu and Najmah formations were the earliest to expel significant amounts of oil, with relatively high TR values of 50–90% during the end of the Late Cretaceous to Early Eocene and continued to the present-day. These generated oils subsequently migrated to the shallow stratigraphic reservoir intervals through the vertical pathway of faults, and are currently extracted for use in the oil industry.

Maturity estimation for Type II-S kerogen using Raman spectroscopy – A case study from the Najmah and Makhul Formations in Kuwait

Raman spectroscopy is used to estimate thermal maturity of shale samples collected from seven wells in the Najmah Formation in Kuwait, encompassing a range of maturities between 0.80 and 1.40%VRE. The Najmah Formation contains kerogen Type II-S (S/C > 0.04) and therefore, maturity correlations for Type II are not applicable due to differences in organic material structure, kinetics and thermal transformation pathways. A new correlation is proposed for the purpose of thermal maturity estimation in Type II-S kerogens. The presented correlation is able to predict the thermal maturity, based on Raman spectra, with good accuracy when compared to the reflectance-derived thermal maturity values.

A new tectono sedimentary framework of the Jurassic succession in the Merjan oil field, Central Iraq

In the Merjan oil field (Central Iraq), the previous 2D seismic interpretation of subsurface geometry had been revealed mound facies which was interpreted as large carbonate buildup within carbonate Najmah formation of Jurassic age. The exploration well (Me-1) had been drilled depending on these results, but carbonate buildup did not exist. The current research provides new sight to interpretation of this feature in the Jurassic succession. High accurate 3D seismic reflection data have been used. Two scenarios have been presented to interpretation. Scenario (A) includes tectono stratigraphic study which finds out that mound shape is due to fault system and tectonic activity during the sedimentation which led to the formation of a graben structure and after sedimentation led to tectonic inversion. These events cause the camouflage stratigraphic feature. Scenario (B) relies on conversion of seismic section from time into depth domain using appropriate velocity model. The resultant seismic depth section helps to present another seismic analysis of feature. The proposed structural model according to the depth section interpretation is anticlinal fault-bend fold. Quantitative geometric relationships between fault and fold shapes were established. Both scenarios explain the causes of the formation of the mound seismic facies which was previously interpreted as carbonate buildup. In conclusion, the stratigraphy interpretation based on 2D data might be very risky and 3D seismic is a must for exploration of stratigraphy traps.

Model-based interbed multiple attenuation: a case study of Najmah reservoir, North Kuwait Jurassic

The Late Jurassic Najmah Formation of Northeast Kuwait consists of a fractured tight carbonate reservoir and organic-rich kerogen layer (Figure 1). Development of the Najmah reservoir is contingent on recognizing naturally fractured areas. Prior to seismic reservoir characterization, it is essential to ensure that the data quality is reliable before extracting any information from the seismic data. Noise contamination can obscure the reflections of interest, leading to inaccurate interpretation and the need for further data analysis for reservoir delineation. Data acquired in the study area indicates that P-wave seismic data within the reservoir interval suffers from the interference of coherent noise related to the interbed multiple. The interbed multiple is a serious problem that is common in the land seismic datasets in the Middle East (Al-Khaled et al., 2008; Al-Nahhas et al., 2008; El-Emam et al., 2011, 2005; Lesnikov and Owusu, 2011; Ras et al., 2012; Sonika et al., 2012; Wu et al. 2011). This study utilized a 9 mile2 (23.3 km2) 3D full azimuth P-wave seismic data set provided by Kuwait Oil Company (KOC) to the Reservoir Characterization Project (RCP) at the Colorado School of Mines. The seismic data was a preliminary result of pre-stack time migration from continuing seismic data processing. Short and long period demultiple and interbed demultiples were applied during the original data processing. A synthetic seismogram of well B portrays the reservoir interval with greater amplitude than the seismic data (Figure 2). This anomaly is consistently observed for all seismic well ties of ten available wells within the 3D seismic data. Furthermore, inside-outside corridor stack analysis of zero-offset VSP (Vertical Seismic Profile) depicts similar amplitude difference that indicates the presence of noise interference from multipathing of seismic energy above the reservoir interval. Evidence of the presence of multiples can be observed from the raw stack of down-going wavefields of the zero-offset VSP. Interbed multiple effects on the seismic data can be seen as a periodic repetition of reflection events, either individual events or interference with primaries. In this case, the multiples seem to interfere with the primary reflection. This study reveals interbed multiples related to the first anhydrite layer within the Gotnia Formation above the reservoir layers are the primary cause of the interference. Applying model-based interbed multiple attenuation reduces the noise interference and improves the imaging of the reservoir section. The multiple-attenuated seismic data have a better correlation with synthetics derived from ten control wells used in this study, leading to better seismic inversion accuracy and reservoir characterization.

Mineralogy, geochemistry, and ore formation of Nuwaifa bauxite deposit, western desert of Iraq

Nuwaifa Formation is a part of sequence stratigraphy that belongs to the Jurassic system exposed in the western desert of Iraq. The Jurassic system consists of Ubaid, Hussainiyat, Amij, Muhaiwir, and Najmah formations. Each formation is composed of basal clastic unit overlain by upper carbonate unit. Nuwaifa karst bauxite was developed in fossil karsts within the Ubaid Formation in areas where maximum intersection of fractures and faults exist. This bauxitization process affected the upper surface of the Ubaid limestone formation, which directly underlies the Nuwaifa bauxite Formation. Nuwaifa Formation represents karst-filling deposit that consists of a mixture of allochthonous (sandstone, claystone, and mudstone) and autochthonous lithofacies (bauxite kaolinite, kaolinitic bauxite, iron-rich bauxite, and flint clay). Most bauxite bodies occur within the autochthonous lithofacies and are lenticular in shape with maximum thickness ranges from few meters to 35 m and in some place up to 100 m. Petrographically, the bauxite deposit exhibits collomorphic-fluidal, pisolitic, oolitic, nodular, brecciated, and skeletal textures indicative of authigenic origin. Mineralogy boehmite and gibbsite are the only bauxite minerals; the former is dominant in the upper parts of the bauxite profiles, whereas the latter is dominant throughout the lower and middle part of the bauxite. Kaolinite, hematite, goethite, calcite, and anatase occur to a lesser extent. The study bauxites are mainly composed of Al2O3 (33–69.6 wt.%), SiO2 (8.4–42 wt.%), Fe2O3 (0.5–15.9 wt.%), and TiO2 (0.7–6.1 wt.%) with LOI ranging from 13.5 to 19.1 wt.%. Geochemical investigations indicate that the immobile elements like Al2O3, TiO2, Cr, Zr, and Ni were obviously enriched, while SiO2, Fe2O3, CaO, MgO, Zn, Co, Ba, Mn, Cu, and Sr were depleted during bauxitization process. The results of this study strongly suggest that the bauxite deposits of the Nuwaifa Formation are derived from the kaolinite of the Lower Hussainiyat Formation.

Geochemical characterization of source rocks and oils from northern Iraq: Insights from biomarker and stable carbon isotope investigations

The origin of oil in the Cretaceous–Tertiary reservoirs of northern Iraq remains controversial. Consequently, a detailed geochemical study, including Rock-Eval pyrolysis, gas chromatography–mass spectrometry, and carbon isotope analysis was conducted on 65 source rock core and cuttings samples from the Najmah, Chia Gara, and Balambo formations, northern Iraq. Twenty-four crude oils were also analyzed to provide new insights into source organic matter, depositional environment, and correlations between crude oils and their probable source rocks.Bulk geochemical analysis of candidate source rock samples indicates that the Najmah Formation has more petroleum potential than the Chia Gara and Balambo formations. These three candidate source rocks were deposited in slightly different environments and have different organic matter types as determined by screening analysis and biomarker data. The Najmah Formation contains organic-rich (TOC up to 4.2 wt%) oil-prone type I-II kerogen deposited under anoxic conditions. The Chia Gara and Balambo formations contain mixed type II-III and type-III kerogens, oil- and gas-prone, deposited under less reducing conditions with more terrigenous organic matter input. Unfortunately, most of the rock samples contain less than 2.5 wt% TOC, which likely excludes them as source rocks, but more deeply buried equivalents may contain more organic matter.Biomarker and stable carbon isotope compositions suggest that Cretaceous- and Miocene-reservoired oils from the northern Iraq basins, although predominantly derived from marine carbonate organic matter, constitute five genetically distinct oil families. Based on age-specific biomarkers, the oils were probably derived from three active petroleum systems in northern Iraq: Jurassic, Jurassic–Early Cretaceous, and Early to mid-Cretaceous. Biomarker maturity parameters indicate that most of the oils were generated in the early oil window, similar to the extracts.

Application of prestack orthotropic AVAz inversion for fracture characterization of a deep carbonate reservoir in northern Kuwait

Prestack anisotropic (orthotropic symmetry) AVAz inversion of full-azimuth 3D seismic data for fracture characterization of the upper Najmah Formation in northern Kuwait was conducted. This formation comprises a thin, tight, naturally fractured carbonate overlying a kerogen-rich shale source rock. The inversion methodology uses an orthotropic system of symmetry that is appropriate for analysis of vertical transverse isotropic (VTI) anisotropic rock layers containing swarms of aligned vertical fractures. Measured seismic amplitudes show good agreement with synthetic models created at different well locations, indicating reliable representation of fractures in amplitude data. Inversion results show good agreement with well and drilling data. Rock-physics modeling shows a relationship between elastic properties and seismic anisotropy and between elastic properties and reservoir properties, such as porosity and fracture density.

Calcareous nannofossil zonation and sequence stratigraphy of the Jurassic System, onshore Kuwait

ABSTRACTThis paper presents the calcareous nannofossil zonation of the Middle and Upper Jurassic of onshore Kuwait and formalizes current stratigraphic nomenclature. It also interprets the positions of the Jurassic Arabian Plate maximum flooding surfaces (MFS J10 to J110 of Sharland et al., 2001) and sequence boundaries in Kuwait, and correlates them to those in central Saudi Arabia outcrops. This study integrates data from about 400 core samples from 11 wells representing a nearly complete Middle to Upper Jurassic stratigraphic succession. Forty-two nannofossil species were identified using optical microscope techniques. The assemblage contains Tethyan nannofossil markers, which allow application of the Jurassic Tethyan nannofossil biozones. Six zones and five subzones, ranging in age from Middle Aalenian to Kimmeridgian, are established using first and last occurrence events of diagnostic calcareous nannofossil species. A chronostratigraphy of the studied formations is presented, using the revised formal stratigraphic nomenclature.The Marrat Formation is barren of nannofossils. Based on previous studies it is dated as Late Sinemurian–Early Aalenian and contains Middle Toarcian MFS J10. The overlying Dhruma Formation is Middle or Late Aalenian (Zone NJT 8c) or older, to Late Bajocian (Subzone NJT 10a), and contains Lower Bajocian MFS J20. The overlying Sargelu Formation consists of the Late Bajocian (Subzone NJT 10b) Sargelu-Dhruma Transition, and mostly barren Sargelu Limestone in which we place Lower Bathonian MFS J30 near its base. The lower part of the overlying Najmah Formation consists of the Najmah Shale, which is subdivided into three subunits: (1) barren Najmah-Sargelu Transition, (2) Late Bathonian to Middle Callovian (lower Zone NJT 12) Lower Najmah Shale, and (3) Middle Callovian to Middle Oxfordian (upper Zone NJT 12 to NJT 13b) Upper Najmah Shale. Middle Callovian MFS J40 and Middle Oxfordian MFS J50 are positioned near the base and top of the Upper Najmah Shale. The upper part of the Najmah Formation is represented by the Late Oxfordian (Subzone NJT 13b) Najmah Limestone, and is overlain by the Kimmeridgian (Zone NJT 14) Jubaila Formation. Early Kimmeridgian MFS J60 and Late Kimmeridgian MFS J70 are positioned near the base and top of the Jubaila Formation. The positions of Late Jurassic MFS J80, J90 and J100 are not constrained by our biostratigraphic data and are positioned in the Gotnia Formation. The Upper Tithonian MFS J110 and the Jurassic/Cretaceous boundary are positioned in the Makhul Formation.

Geochemical characterisation, volumetric assessment and shale-oil/gas potential of the Middle Jurassic–Lower Cretaceous source rocks of NE Arabian Plate

ABSTRACT The Middle Jurassic–Lower Cretaceous strata of the NE Arabian Plate contain several prolific source rocks providing the charge to some of the largest world-class petroleum systems. They are located within the Zagros Fold Belt and Mesopotamian Foreland Basins covering the northern, central and southeastern parts of Iraq, Kuwait and western and southwestern Iran, particularly the Lurestan and Khuzestan provinces. These source rocks include the Bajocian–Bathonian Sargelu, the Callovian–Lower Kimmeridgian Naokelekan and the Upper Tithonian–Lower Berriasian Chia Gara formations of Iraq and their chronostratigraphic equivalents in Kuwait and Iran. They have charged the main Cretaceous and Cenozoic (Tertiary) reservoirs throughout Iraq, Kuwait and Iran with more than 250 billion barrels of proven recoverable hydrocarbons. These formations represent the transgressive system tracts of sequences deposited within deep basinal settings and anoxic environments. They are dominated by black shales and bituminous marly limestones, with high total organic carbon (TOC) contents (ranging from 1–18 wt%), and by marine Type IIS kerogen. Their Rock-Eval S2 yields may reach up to 60 mg HC/g Rock, particularly along the depocentre of the Mesopotamian Foreland Basin. The immature hydrogen index (HI) values might have been up to 700 mg HC/g TOC, whereas the present-day observed values vary depending on the location within the basin and the present-day maturity. The Source-Potential Index (SPI; i.e. mass of hydrocarbons in tons, which could be generated from an area of 1 sq m in case of 100% transformation ratio) averages around 2–3, but can even reach up to 14–16 along the basins’ centres. The Sargelu and the overlying Naokelekan-basinal Najmah formations (and their equivalents) could represent the best potential shale-gas/shale-oil plays in Iraq, Kuwait and Iran, due to their organic richness, favourable maturity and the presence of regional upper and lower seals. The estimated oil-in-place for the potential Sargelu shale-oil play in Iraq only is around 1,300–2,500 billion barrel oil-equivalent (BBOE) and in Kuwait is about 7–150 BBOE.

Challenges in Testing and Completion of an HP/HT Reservoir in Kuwait

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 167647, ’Challenges in Testing and Completion of Highly Sour HP/HT Reservoirs in the State of Kuwait,’ by Y. Al-Salali, H. Al-Bader, V. Duggirala, M. Ayyavoo, P. Subban, A.R. Al-Ibrahim, and A. Rajkhowa, Kuwait Oil Company, prepared for the 2013 SPE Kuwait Oil and Gas Show and Conference, Mishref, Kuwait, 7-10 October. The paper has not been peer reviewed. Recently, a new structure located in northwestern Kuwait was drilled and tested with a deep-drilling rig. Formations in this deep structure have very low porosity and permeability. During short-term testing, wells produced oil and gas with, unexpectedly, the highest hydrogen sulfide (H2S) content of any reservoir so far discovered in Kuwait. Because the reservoir was rated as high-pressure/ high-temperature (HP/HT) and sour, it was a challenge to test the wells safely and successfully. Introduction Jurassic carbonate reservoirs in Kuwait fields are deep, low-permeability, and HP/HT and generally contain sour volatile oil/condensate and gas. Formations containing these deep reservoirs are tight and naturally fractured carbonates. Najmah (NJ), Sargelu (SR), Upper Marrat, Middle Marrat, and Lower Marrat are the five formations proved to hold hydrocarbon- bearing reservoirs. NJ and SR formations are tighter than the three Marrat formations. The NJ formation consists of limestone at the upper part and shale at the bottom. The SR formation consists mainly of shale and limestone. Both are naturally fractured reservoirs, so fractures play an important role in production. The Marrat formations consist mainly of limestone, occasionally with streaks of anhydrite at the top and with streaks of shale at the bottom. The core analysis of the Middle Marrat formation shows that dolomitization plays a major role for matrix-porosity enhancement, which is responsible for good reservoir character. As part of an exploration strategy, a new structure in northwestern Kuwait was drilled to deep Jurassic formations. This new structure is a northwest/southeast- trending doubly plunging anticline and is intersected by a number of longitudinal- and transverse-fault sets. Earlier, a significant discovery of light oil and gas from the new deeper structure was made from an unsuccessful Paleozoic well through re-entry. After the discovery of light oil and gas, three more exploration/delineation wells were drilled to confirm the reserves potential of this structure. Some of the wells are located near the crest of the structure, whereas some are 1.5 km from the crest (Fig. 1).

Oil seeps affinity and basin modeling used for hydrocarbon discoveries in the Kifle, Merjan, and Ekheither fields, West Iraq

Calculated C28/C29 sterane ratio, δC13 ‰ isotopes and plots of tricyclic terpane (C22/C21, C24/C23, and C26/C25 and hopane (C35S/C34S, C29/C30, and C31R/C30) of Najaf and Karbala Governorates oil seeps are used to assess the Jurassic Butmah, Sargelu, and Lower unit of Najmah formations in Kifle, West Kifle, Merjan, and Ekhaither fields western part of the Euphrates rivers. Plots of Roch-Eval pyrolysis analysis data of 334 source rock samples on van Krevlen, production index, and petroleum potential diagrams as well as 1D PetroMod software basin modeling of timing and extent of the hydrocarbon generation have confirmed the petroleum generation timing and their transformation ratios from these source rocks. The generated oil have possessed lateral migration along the upper unit of the Najmah Formation to the site of the Abo Jir major fault where it is changed to vertical migration along the fault that intersect with the ground surface. During the vertical migration path, impregnations of all the successive layers with heavy oil and asphalt have occurred. Main oil reservoirs of Cretaceous Nahr Umr and Harth formations and the Tertiary Dammam–Euphrates Formation were charged from this vertical migration path. The accumulated oils are of 15–38 API with 1.45–5.9 sulfur content in the Hartha Formation reservoir and 20–35 API with 0.9–3.7 sulfur content in the Nahr Umr Formation reservoir.

A new look on the Jurassic formations of the western part of Iraq

The Jurassic succession of western Iraq includes the Ubaid, Hussainiyat, Amij, Muhaiwir, and Najmah formations. Each formation is composed of basal clastic unit overlain by upper carbonate unit. Extensive and huge erosional unconformity occurred at the Triassic–Jurassic boundary due to marked shifting of structural (E–W) strike of the Triassic (Rhaetic) Zor Hauran Formation to (NE–SW) Jurassic formations. Sea level falling (lowstand system tracts) would result in the progradation of the land on the expense of the sea forming the recognizable progradation of the fluvial and deltaic deposits of the lower clastic units of the Jurassic formations, whereas sea level rising (highstand system tract), i.e., sea prograding, causes deposition of the carbonate units of the Jurassic formations system. This progradation resulted to various carbonate environments of deposition ranging from subtidal, intertidal, to supratidal. The main target on most of the exploration blocks in the western part of Iraq focused on the lower Paleozoic successions, whereas prospects in Triassic, Jurassic, and lower Cretaceous targets are less extensive but may have significant potential on certain blocks in both stratigraphic and structural traps. The western part of Iraq was subjected to intermittent pulses of uplifting (sea regression) and subsidence (sea transgression) to form the Jurassic basin system in the area. The Jurassic formations lack the characteristics of petroleum systems. In contrast, in the central and northern parts of Iraq, the Jurassic formations (Najmah and Gotnia formations) were deposited in subsiding basins in which the reservoir and sealed evaporitic rocks existed. In turn, in the western desert of Iraq, the Jurassic formations lack these petroleum system characteristics. Hence, it can be proposed that the petroleum–nonpetroleum inflection could be proposed in the east of area km 160.

New Jurassic Play Concepts in the Mesopotamian Basin and the Western Desert of Iraq

Intensive tectonic activity during Zagros Orogeny (Late Cretaceous-Late Tertiary) and the final uplift in Holocene, reactivated folds and faults and eroded parts of the Lower Cretaceous, Jurassic and earlier sequences in western Iraq. Subsequently, within the Study area, the post Jurassic tectonics and the Upper Jurassic paleo-depositional environments left potential carbonate reservoirs at the basin margin (the Najmah Formation) and a potential evaporite cap rock (the Gotnia Formation) basin wards. Such relationship sealed the generated hydrocarbons from the source rocks formations (the Sargelu Formation) but it unlikely sealed the laterally migrated oil into the Najmah Formation. The major N-S fault planes of the Khleisia uplift that extended from northwestern part of Iraq to the southwestern Desert forming a barrier to laterally migrated oil and putting an end to the potential migration pathways from the Basinal Sargelu Formation in ENE Iraq. At the same time the prolonged thermal activity may have convert the organic matters within Paleozoic layers to Thermogenic gases. Total petroleum system enabled us to better understand the provenance of oil accumulated in the Najmah Formation’s layers another rock packages. According to 1-dimention model the migrated oil can pass through the Khleisia fault planes into the Middle Jurassic Muhaiwir Carbonate Formation (marginal equivalent to basinal Sargelu Formation) to form potential oil reservoir sealed by Gotnia evaporates and Kimmeridgian anhydrite .The equivalent formations located in western desert (Ubaid, Hussiniyat and Amij) Formations to the formations located in the central part (Mesopotamian Basin) which is (Butmah, Adaya, Mus and Alan Anhydrite) may consider as a new Jurassic reservoirs. So far no oil wells yet are available to cover the study of the whole area.

Organic matter identification in source and reservoir carbonate in the Lower Cretaceous Mauddud Formation in Kuwait

ABSTRACT The purpose of this study is to identify the source rock, reservoirs and nonproductive zones in the Lower Cretaceous Mauddud Formation in Kuwait, using geochemical methods. This formation is one of the major Cretaceous oil reservoirs. It is composed mainly of calcarenitic limestone interbedded with marl and glauconitic sands. Its thickness ranges from almost zero in the south to about 100 m (328 ft) in the north. A total of 99 core samples were collected from six oil fields in Kuwait: Raudhatain, Sabiriyah and Bahra in the north, and from the Burgan, Ahmadi and Magwa in the south. Well logs from these fields (gamma ray GR, sonic, resistivity, density) were correlated and used in the study. The core samples were screened for the amount and nature of the organic matter by Rock-Eval 6 pyrolysis (RE6) using reservoir mode. A set of samples was selected to study the properties of the organic matter including the soluble and insoluble organic parts. The geochemical characterisation was performed using different methods. After organic solvent extraction of rock samples, the solvent soluble organic matter or bitumen was characterised in terms of saturates, aromatics and heavy compounds (resins and asphaltenes). Then the hydrocarbon distribution of saturates was studied using gas chromatography (GC/FID) and gas chromatography-mass spectrometry (GC/MS) for tentative oil-source rock correlation. After mineral matrix destruction of previously extracted rocks, insoluble organic matter or kerogen was analysed for its elemental composition to identify kerogen type. The geology and the analytical results show similarities between the wells in the southern fields and the wells in the northern fields. Average Total Organic Matter (TOC) in the carbonate facies is 2.5 wt.% and the highest values (8.0 wt.%) are in the northern fields. The clastic intervals in the northern fields show higher total organic matter (1.3 wt.%) relative to the southern fields (0.6 wt.%). The total Production Index is higher in the carbonate (0.6) than the clastic section (0.3). This reflects the amount of extractable hydrocarbons, which are usually associated with the carbonate section in this formation, representing its reservoir section. Although the carbonate rocks are dominated by richer total organic matter, there are some intervals, with low total organic matter values (0.07 wt.%), representing its poor reservoir sections. The kerogen type varies between type II-III and III in the shales with a slightly better quality in the carbonate section. It is immature in almost all the studied fields. The composition of the rock extract has no relation with the rock type. Some sandstone show similar extract composition to the carbonate rocks in the reservoir intervals. The extracts from these intervals show different genetic nature than those in the shales. The maturity level in the reservoir extract is much higher than in the shale intervals. Thus, the oil accumulated in the reservoir might be largely related to migrated oil from a more mature source rock deposited in a clearly different environment than the associated shaly intervals. The best candidates being a more deeply buried Early Cretaceous Sulaiy Formation and Upper Jurassic Najmah Formation.

A PRELIMINARY EVALUATION OF JURASSIC SOURCE ROCK POTENTIAL IN KUWAIT

Jurassic sedimentary rocks in Kuwait are generally assigned to the Marrat, Dhruma, Sargelu and Najmah Formations (mainly limestones and calcareous shales) and the overlying Hith and Gotnia Formations which are composed of anhydrites and evaporites. This paper reports the results of organic‐geochemical analyses of Jurassic carbonate and shale samples recovered from ten wells in Kuwait. Analytical techniques included TOC analysis, elemental analyses of kerogen, density separation and petrographic analyses. The thermal history of Kuwait was modelled and calibrated with maturity indicators from the studied wells.The analytical results point to the presence of marine kerogen between Types II and III. Generally, the formations show amorphous rich sapropelic organic matter with high H/C ratios and low densities. Biodegradation of some samples resulted in elevated O/C ratios. The results of maturity studies indicate that most of the Jurassic Succession is mature, maturity differences being due to depth variations. Oil generation began in Late Cretaceous to Eocene time when structural traps had already been formed. Jurassic source rocks may therefore have supplied reservoir units in Kuwait. In particular, the Najmah Formation includes well‐preserved amorphous marine algal type organic matter. The high TOC values and thermal maturity of this formation make it one of the most important potential sources of oil in Kuwait.