An Integrative Modeling Approach to Estimating Volume of Generated Oil from Lower Cretaceous Sulaiy Formation in Southern Iraq

Thermal maturity and petroleum generation capacity of the source rocks of Lower Cretaceous Sulaiy Formation were studied and evaluated via modeling the thermal maturation of ten wells in this study by using the Arrhenius equation for chemical reactions or (time-temperature index of thermal maturation). A thermal maturation map of the Sulaiy Formation shows that it a high maturity level increasing eastward; also, the geochemical analysis indicated suitable organic content in quantity and quality to generate hydrocarbons. The onset of oil migration from the Sulaiy Formation is calculated by this study to be at the end of Lower Eocene time (53 million years ago), and as it is a late hydrocarbon generation and migration; thus indicating mainly vertical migration paths. According to quantitative estimation, the oil generated and migrated vertically from the Sulaiy Formation towards the Lower Cretaceous reservoirs in southern Iraq estimated at 42.6 x 109 bbl. This study elevated the status of the Sulaiy Formation to a major source of oil tapped in the Lower Cetaceous reservoirs of southern Iraq.

Microfacies Architecture and Stratigraphic Development of the Yamama Formation, Southern Iraq

The Yamama Formation belongs to the late Berriasian-Aptian succession, which was deposited during the Lower Cretaceous period within the main shallow marine depositional environment. Petrographic study and microfacies analysis enabled the recognition of six main microfacies for three association facies. These are the Semi-restricted, Shallow open marine and Shoal environments. The study succession represents deposition of three third order cycles, these cycles where deposited during successive episodes of relative sea level rises and still stand. The presence of shoal association facies (oolitic packstone microfaces) between the Sulaiy and Yamama formations refer to continue the deposition during the same stage, and may suggest the end of Sulaiy Formation was maximum flooding surface (mfs). The first stage started with occurrence of the shallow open marine association facies underlain by semi-restricted association and then shoal association facies. There are three cycles of this sequence consistently in the south of the study area, so that it continues to the lower part of the Ratawi Formation to be the upper contact of the Yamama Formation of a conformable and continuous in sedimentation To the north of the study area (near of Rf-1 and Hf-5 wells) the shoal association was only shown once at the bottom of the Yamama Formation and these cycles to became unclear. This suggest that the paleo-high was developed to the south of studied area, while the open sea was characterized the northern part.

Facies analysis and sequence stratigraphy of the uppermost Jurassic– Lower Cretaceous Sulaiy Formation in outcrops of central Saudi Arabia

ABSTRACT Uppermost Jurassic–Lower Cretaceous carbonates of the Sulaiy Formation are well exposed at the type locality Dahal Hit, and along the entire natural escarpment near Ar Riyad, the capital of the Kingdom of Saudi Arabia. This study provides a facies and sequence-stratigraphic analysis based on detailed sedimentological and gamma-ray logging of 12 outcrop sections. The sections represent the Sulaiy Formation along a 60 km-long outcrop belt, including the Hith-Sulaiy transition in a large solution cavity named Dahal Hit, situated south of Ar Riyad. The latter section is studied in detail because it is the only locality in Saudi Arabia where the Hith Anhyrite (Hith Formation in this study) to the Sulaiy Formation transition crops out. Ten lithofacies types were identified for the Sulaiy Formation including potential reservoirs such as oolitic cross-bedded grainstones, biostromal boundstones, and bioclast-rich, graded pack-to-grainstones. Lithofacies types are grouped into five facies associations: (1) offshoal, (2) transition zone, foreshoal, (4) shoal margin, and (5) shoal, distributed along a carbonate ramp. Their vertical stacking pattern revealed a systematic hierarchy of cyclicity consisting of small-scale cycles, medium-scale cycle sets and two large-scale sequences for the Sulaiy Formation. Four cycle motifs, with an average thickness of 2–4 m, are present: (1) offshoal to transition zone cycle motif, (2) offshoal to foreshoal cycle motif, (3) transition zone to shoal margin cycle motif, and foreshoal to shoal margin cycle motif. A total of 15 cycle sets, ranging between 8 and 12 m in thickness each, were interpreted. They were correlated, where possible, across the study area. Three types of medium-scale cycle sets are observed: (1) offshoal to shoal cycle set motif, (2) offshoal to foreshoal cycle set motif, and (3) shoal margin to offshoal cycle set motif. The Lower Sulaiy Sequence consists of twelve cycle sets and is interpreted to contain two Arabian Plate maximum flooding surfaces (MFS): (1) Upper Tithonian MFS J110 (147 Ma) in its lowermost part, which is interpreted to be the time-equivalent of the Manifa reservoir in subsurface Arabia. (2) Lower Berriasian MFS K10 (144 Ma) in the seventh-up cycle set. The Upper Sulaiy Sequence is only represented in the Wadi Nisah Section and is believed to be incomplete because the Sulaiy/Yamama Formation boundary was not included in our study. It is presumed to contain Upper Berriasian MFS K20 (141 Ma).

Petroleum system analysis of the Mishrif reservoir in the Ratawi, Zubair, North and South Rumaila oil fields, southern Iraq

ABSTRACT Five oil samples reservoired in the Cretaceous Mishrif Formation from the Ratawi, Zubair, Rumaila North and Rumaila South fields have been analysed using Gas Chromatography – Mass Spectroscopy (GC-MS). In addition, fifteen core samples from the Mishrif Formation and 81 core samples from the Lower Cretaceous and Upper Jurassic have been subjected to source rock analysis and palynological and petrographic description. These observations have been integrated with electric wireline log response. The reservoirs of the Mishrif Formation show measured porosities up to 28% and the oils are interpreted as being sourced from: (1) Type II carbonate rocks interbedded with shales and deposited in a reducing marine environment with low salinity based on biomarkers and isotopic analysis; (2) Upper Jurassic to Lower Cretaceous age based on sterane ratios, analysis of isoprenoids and isotopes, and biomarkers, and (3) Thermally mature source rocks, based on the biomarker analysis. The geochemical analysis suggests that the Mishrif oils may have been sourced from the Upper Jurassic Najma or Sargelu formations or the Lower Cretaceous Sulaiy Formation. Visual kerogen assessment and source rock analysis show the Sulaiy Formation to be a good quality source rock with high total organic carbon (up to 8 wt% TOC) and rich in amorphogen. The Lower Cretaceous source rocks were deposited in a suboxic-anoxic basin and show good hydrogen indices. They are buried at depths in excess of 5,000 m and are likely to have charged Mishrif reservoirs during the Miocene. The migration from the source rock is likely to be largely vertical and possibly along faults before reaching the vuggy, highly permeable reservoirs of the Mishrif Formation. Structural traps in the Mishrif Formation reservoir are likely to have formed in the Late Cretaceous.

Update to Late Triassic – Jurassic stratigraphy of Saudi Arabia for the Middle East Geologic Time Scale

ABSTRACT This Note presents a formal update to the Middle East Geologic Time Scale 2008 (ME GTS) for the Late Triassic and Jurassic rock units of Saudi Arabia. It reviews their lithostratigraphic nomenclature, ranks and stage assignments, and proposes names for third-order chrono-sequences as compiled and/or interpreted from the published literature. The review starts with the Late Triassic (Late Norian – Rhaetian) Minjur Sandstone of the Buraydah Group, with the Triassic – Jurassic (TrJ) boundary positioned at its top. The Minjur Sandstone consists of two units, here ranked as members, which are interpreted as the Late Norian – ?Early Rhaetian Lower Minjur Sequence (Lower Minjur Member) and ?Late Norian – Rhaetian Upper Minjur Sequence (Upper Minjur Member). The Early Jurassic Unconformity - Hiatus (Hettangian, Sinemurian and Pliensbachian stages) separates the Minjur Sandstone from the Toarcian Marrat Formation of the Jurassic Shaqra’ Group. The Shaqra’ Group consists of seven formations, from base-up: (1) Marrat Formation consisting of the Lower, Middle and Upper units, here ranked as members. (2) Dhruma Formation consisting of eight units (D1–D5, Wadi ad Dawasir “delta”, D6 and D7). The Dhruma units have been named in the literature as Balum Member (D1 and lower part of D2 units), Dhibi Limestone Member (upper part of D2 unit), Uwaynid Member (D3), Barrah Member (D4), Mishraq Member (D5), ‘Atash and Hisyan members (D7); the D6 and Wadi ad Dawasir “delta” units are not formally named as members. (3) Tuwaiq Mountain Limestone consisting of the Baladiyah (T1 unit), Mysiyah (T2 unit) and Daddiyah (T3 unit) members. (4) Hanifa Formation consisting of the Hawtah and Ulayyah members. (5) Jubaila Limestone with J1 and J2 units. (6) Arab Formation consisting of D to A members. (7) Hith Anhydrite consisting of Main Hith Anhydrite Member and Manifa Reservoir/Member. The seven formations of the Jurassic Shaqra’ Group are interpreted as 11 third-order chrono-sequences: (1) Early Toarcian Marrat Sequence B (Lower and Middle Marrat members) and Mid- to ?Late Toarcian Marrat Sequence A (Upper Marrat Member), the latter containing Arabian Plate maximum flooding surface MFS J10. (2) Bajocian Lower Dhruma Sequence (Balum Member and Dhibi Limestone), formed by the Balum and Dhibi subsequences, the former containing MFS J20. (3) Late Bajocian – Mid-Bathonian Dhruma Sequence B (Uwaynid, Barrah, Mishraq members, and Wadi ad Dawasir “delta” unit), with the Mishraq containing MFS J30. (4) Late Bathonian – early Mid-Callovian Dhruma Sequence A (unit D6, ‘Atash and Hisyan members), with the Hisyan containing MFS J40. (5) Mid- to Late Callovian Tuwaiq Sequence (Tuwaiq Mountain Limestone) containing an undesignated MFS at the base the Daddiyah Member (T3 unit). (6) Early and Mid-Oxfordian Hawtah Sequence (Hawtah Member of Hanifa Formation) containing MFS J50. (7) Late Oxfordian – ?Early Kimmeridgian Ulayyah Sequence (Ulayyah Member of Hanifa Formation) containing MFS J60. (8) Kimmeridgian Jubaila Sequence (Jubaila Limestone) containing MFS J70. (9) Arab-D Sequence (Arab-D Member inclusive of the Arab-D Anhydrite) containing an undesignated MFS in the Arab-D carbonate. (10) ?Kimmeridgian – ?Tithonian Arab-C and B Sequence (Arab C and B members) containing fourth-order MFS J80 and J90 in the lower carbonates of the members; and (11) ?Kimmeridgian – Tithonian Arab-A - Main Hith Sequence (Arab-A Member and Main Hith Anhydrite below the Manifa Reservoir/Member) containing fourth-order MFS J100 in the Arab-A Member. The Tithonian Manifa Member (upper part of the Hith Anhydrite Formation) is interpreted as heralding a transgression (fourth-order MFS J110), which deposited the Late Jurassic – Early Cretaceous Sulaiy Formation of the Thamama Group. The Jurassic – Cretaceous (JK) boundary is placed in the Sulaiy Formation by stratigraphic position. Based on the Geologic Time Scale GTS 2004 and Arabian Orbital Stratigraphy (AROS), the ages in million years before present of the sequence boundaries and maximum flooding surfaces are estimated for these chrono-sequences.

Jurassic Sequence Stratigraphy of the Western and Southern Arabian Gulf

ABSTRACT The Jurassic sequence stratigraphic scheme for Central Saudi Arabia is extrapolated to the formations of the western and southern Arabian Gulf region resulting in a tentative chronostratigraphic framework. The framework is tentaively constrained as follows: (1) Upper Triassic-?Lower Jurassic continental clastics (Minjur and equivalents) and the subsequent pre-Toarcian unconformity indicate regional erosion and non-deposition over the Arabian platform. (2) A Toarcian sequence (Marrat and equivalents) provides a basal Jurassic regional datum, except in Oman. (3) The late Toarcian and Aalenian correspond to a substantial sea- level lowstand and a regional depositional hiatus. (4) The Middle Jurassic Dhruma Formation corresponds to four different sequences with a major intervening hiatus. The Upper Dhruma Member, together with the Tuwaiq Mountain form the topmost sequence. The correlation between the Dhruma, Tuwaiq Mountain, Hanifa and Jubaila formations, to their equivalents in other Arabian Gulf countries, requires clearer definitions. (5) The Arab and Hith Anhydrite formations are Tithonian based on their sequence assignment, while the Sulaiy Formation is Berriasian and straddles the Jurassic-Cretaceous boundary. (6) The four Tithonian Arab carbonates may have been deposited as transgressive and early highstand deposits. The Tithonian Arab, Gotnia and Hith anhydrites may be late highstand deposits which overstep inland “salinas” (Gotnia and western Rub’ Al-Khali). Each carbonate and overlying anhydrite sequence appear to correspond to a complete third-order cycle. (7) The equivalents to the Kimmeridgian Jubaila Formation and Tithonian Arab carbonates are absent by non-deposition in Kuwait. In Oman, the Arab and Hith Anhydrite formations are absent by erosion. (8) The Tithonian Hith Anhydrite provides a final Jurassic regional, stratigraphic datum, except in Oman and eastern United Arab Emirates.

Thermal History of the Lower and Middle Cretaceous Source Rocks in Kuwait

ABSTRACT The thermal history of Kuwait was reconstructed with Shell’s “Cauldron” basin modeling software and calibrated with maturity indicators from four wells. Maturity measurements were carried out on source rock samples from wells in the Ash-Shaham, Minagish, Raudhatain and Riqua (offshore) fields. The analytical data indicate that ‘top oil window’ is located at a depth of around 2,500 meters (8,250 feet). Due to the low structural dips of the sequences in Kuwait, thermal modeling in one-dimension was found to be adequate. The Cretaceous source rocks subsided without major anomalies in burial rate or heatflow. Modeling results indicate that the Cretaceous Sulaiy, Minagish and Zubair formations entered the oil window during the Late Cretaceous and Early Tertiary, whereas oil expulsion occurred throughout Tertiary time. Quantification of expelled oil volumes suggests that the Sulaiy Formation is the most productive source rock in Kuwait, whereas the Upper Cretaceous Burgan and Maudud formations are presently at the top of the oil window.