Fourth EAGE Workshop on Arabian Plate Geology

The Oxfordian to Valanginian succession on the Arabian Plate is dominated by carbonate shelves and intrashelf basin deposits. Evaporites are a major component of Kimmeridgian to Tithonian deposition in both shelf and basinal locations. Siliciclastics are less important, though they increase in the Early Cretaceous. Important petroleum systems reflect the presence of numerous source, reservoir and seal combinations. Local stratigraphy is well understood where there are major petroleum systems but there are considerable stratigraphic, depositional and preservational variations across the Arabian Plate and correlations between more distant areas are not well documented. There is a dearth of comprehensively documented age control. Nevertheless careful sequence stratigraphic analysis means that many correlations can be established, and that a plate-wide sequence stratigraphic framework can be applied. Sharland et al (2001) identified ten plate-wide maximum flooding surfaces in this succession. Further work continues to validate these surfaces world-wide. We use them to discuss the sequence stratigraphy of the interval across the entire Arabian Plate and its importance for future exploration and detailed reservoir description. Our methodology facilitates high resolution Gross Depositional Environment mapping and detailed play fairway analysis. This is fundamental to identifying new stratigraphic trap and continuous play potential across the Arabian Plate.

A new record of calcareous nannofossil datum markers from Middle Jurassic to Early Cretaceous (Aalenian to Valanginian) strata of onshore North Kuwait has been calibrated with nannofossil marker species of the global Jurassic to Early Cretaceous nannofossil biostratigraphy schemes and compared with calcareous nannoplankton assemblages recorded from Lower to Middle Cretaceous strata offshore Kuwait. The studied sections embrace, in ascending order, the Dhruma, Sargelu, Najmah, Gotnia, Hith, Makhul, Minagish and Ratawi formations and comprise argillaceous limestones, grainstones, packstones, bituminous packstones, wackestones, dolomite, anhydrite, laminated bituminous calcareous mudstone and calcareous shales all representative of a variety of environments from marginal marine (sabkha) and shallow hypersaline (salina) to fully marine mid to outer shelf settings.

What are arguably the most prolific petroleum systems in the world are driven by multiple world-class source rocks. These highly organic-rich rocks account for most of the discovered oil and gas on the Arabian Plate. The source rocks were deposited in widespread shallow-marine intrashelf basins and deep-marine basins. They differ in age between basins and are not correlative with global occurrences of organic enrichment. Burial under Mesozoic and Cenozoic strata has caused the source rocks to generate oil and gas. Timing of generation differs mainly by variations in sedimentary load; differences in the kinetics of generation and regional heat flow play a lesser role. In the west of the Arabian Plate, generation occurred under Cretaceous and Paleogene sedimentary load, whereas in and near the Zagros Foldbelt generation continues under recent syntectonic sedimentary load. The most organic-rich source rocks are carbonates containing mainly marine (Type-I/II) organic matter that has generated sulfur-bearing oil and gas. The gas in some carbonate reservoirs is further enriched in H2S derived from thermal-sulfate reduction. Clay-rich source rocks contain a mixture of marine and land-plant (Type II/III) organic matter and have generated a mixture of low-sulfur oil and gas. Gas is more common also where the source rocks are deeply buried and more thermally mature. Low-angle structural dip of highly continuous strata allowed oil and gas to migrate laterally for greater than 150 km from areas of generation to traps, limited only by the extent of structural dip and continuous seal. Intervals of widespread evaporite and shale acted as barriers to vertical migration. Oil and gas migrated to younger rocks through gaps in migration barriers caused by non-deposition or erosion, including localized karst dissolution, or by faults that pierce these migration barriers, commonly where they are thin. Oil and gas generated from Jurassic and Early Cretaceous source rocks are found in many Jurassic, Cretaceous and Cenozoic reservoirs, but rarely occur in older rocks. The Arabian Plate is reservoir rich, with multiple reservoir-seal pairs for oil and gas accumulations. Most reservoir rocks are shallow-marine carbonates and paralic sandstones, but nonmarine and deep-marine sedimentary rocks and even fractured basement are reservoirs in some basins. Shallow burial and limited cementation in the subsurface have favored preservation of porosity and some superb reservoir quality. Most traps are structural. Compressional thrusted anticlines dominate the Zagros Foldbelt, whereas compressional anticlines with reactivated basement-involved faults occur on the Arabian Platform. Halokinesis of deeply buried Cambrian-Precambrian salt forms traps, especially in the offshore Gulf and in Oman. Extensional faulted anticlines form traps in rift basins, notably in Syria and Yemen. The few discovered stratigraphic traps include truncation pinchouts and reefal buildups.

The Middle-Upper Jurassic and Lower Cretaceous strata of the NE Arabian Plate contain several prolific world-class source rocks for some of the largest petroleum systems globally. They are located within the Zagros and Mesopotamian Foreland Basins covering north, central and south-eastern parts of Iraq together with western and south-western parts of Iran, particularly the Lurestan and Khuzestan provinces. These source rocks principally include the Bajocian–Bathonian Sargelu, the Callovian–Early Kimmeridgian Naokelekan and the Late Tithonian–Early Berriasian Chia Gara Formations of Iraq and their chronostratigraphic equivalents in Iran. They have charged the main Cretaceous and Tertiary reservoirs throughout Iraq and Iran (in various trap types and sizes) with more than 250 billion barrels of proven recoverable hydrocarbons.

A new stratigraphic model for the Upper Jurassic to Lower Cretaceous has been constructed for the eastern Arabian plate based on well correlations calibrated with seismic.

The sedimentary fill of the Marib-Jawf Basin is Jurassic and Lower Cretaceous. Initial marine transgression occurred in the Middle Jurassic, with the Kohlan Formation unconformably overlying Paleozoic sedimentary rocks and Precambrian basement. These basal fluvial to nearshore-marine sandstones and shales are conformably overlain by intertidal to shallow-marine carbonates of the Saba Formation of Callovian-Oxfordian to Kimmeridgian age. Extensional faulting formed grabens that deepened during the Kimmeridgian and Tithonian. Up to 6000 feet (2000 meters) of deep-marine sediments unconformably overlie and lap onto high margins of the basin. At the base are outer-shelf/slope argillaceous carbonates and shales of the Arwa Formation. These pass upwards into slope and basinal shales with minor limestones and sandstones of the Meem Formation, which in turn are overlain by shales with interbedded sandstones and limestones of the Lam Formation. The sandstones were deposited by turbidity currents from the margins of the basin, where submarine fan deposits are present. Shales of the Meem and Lam formations are organic rich and are the source rocks for most of the oil and gas in the basin. A drop in relative sea level occurred in the middle to late Tithonian with restriction of the basin from the open ocean. Three progradational sequences (Yah, Sean and Alif Members of the Alif Formation, from base to top) of fluvial-alluvial to deltaic-marine siliciclastic sediments were deposited down the axis of the basin, passing into offshore-marine shales and basinal evaporites to the east. The sandstones are the main reservoirs for oil and gas in the basin. A transgressive shale at the top of the Alif Formation records marine flooding of the basin, followed by deposition of five sequences of basin-filling evaporites of the Safer Formation. Thick halite beds were deposited subaqueously when the basin was filled with hypersaline water and have thin anhydrite beds at top and base. These evaporites are separated by lowstand fluvial to shallow-marine siliciclastics, including thin, organic-rich shales that are minor source rocks for oil. The evaporites are the primary seal for the reservoirs of the Alif Formation. Berriasian-Valanginian shelfal-marine shales and limestones of the Azal Formation unconformably overlie the Safer evaporites. The lower part of the Azal Formation is dominantly limestone and shale, the middle is shale with few thin limestones, and the upper unit has limestone, shale and minor sandstone. These are overlain by the Tawilah Group, possibly as old as Valanginian-lower Hauterivian and at least as young as Aptian in parts of the basin, equivalent to the Qishn Formation to the east. The Tawilah Group is dominated by nonmarine to shallow-marine sandstones and shales with minor thin dolomite and skeletal-limestone beds. Regional erosion truncates the sedimentary fill of the basin, progressively cutting out more strata toward the west and in places exposing Middle Jurassic rocks at the surface.

The upper interval of the Arab Formation is a secondary reservoir objective in a major gas field currently under development in onshore Abu Dhabi, United Arab Emirates (UAE). This presentation will illustrate the impact of integrating core sedimentological descriptions, log response, borehole image and pressure data to develop a sequence-stratigraphic correlation of thin reservoir targets in the upper interval of the Arab Formation.

The Jurassic carbonates host significant oil reserves and produce from giant and super giant oilfields. The Upper Jurassic (Oxfordian) Hanifa Formation and Middle Jurassic (Late Callovian) Tuwaiq Mountain Formation contain world-class hydrocarbon source rocks and carbonate reservoirs. The grainy carbonates of the Arab A, B, C, and D members are among the best Jurassic carbonates reservoirs. The Hith Formation evaporites (Tithonian) provide excellent regional seal of the exceptionally prolific Jurassic petroleum systems. Understanding the regional stratigraphic framework by integrating core, well-logs, and 2-D/3-D seismic interpretations is essential for unlocking the hydrocarbon exploration potential and new exploration opportunities. The Hanifa reservoirs (skeletal/oolitic limestones and packstones) were deposited as prograding clinoforms during the highstand of the Hanifa third-order composite sequence along the margins of the Hanifa intrashelf basin, which was inherited from the Tuwaiq Mountain intrashelf basin. The Hanifa grainstone shoaling complexes shifted further basinward as a result of continuous carbonate growth and progradation, progressively infilling most of the accommodation space that was created after Bathonian time. The organic-rich mudstones of both the Tuwaiq Mountain and Hanifa formations were deposited in the intrashelf basins of the Jurassic Arabian carbonate platform interior. An early Kimmeridgian base-level fall, as revealed from core and 3-D seismic chronostratigraphic analysis, terminated the Hanifa carbonate platform and resulted in substantial subaerial exposure of previous grainstone shoaling complexes and proximal areas. A renewed regional transgression occurred post-Hanifa time, leading to deposition of the Jubaila Formation and infilling the remaining accommodation space. Widespread deposition of packstones and grainstones of the Arab-D Member represents the maximum flooding and the beginning of the late Kimmeridgian sea level fall, which resulted in the deposition of Arab-A, B, C members, and pervasive Hith anhydrite deposits across the region, marking the turning point from a predominantly carbonate factory to an overall evaporite factory. Gross depositional environment (GDE) maps of the Upper Jurassic formations have been reconstructed by integrating core/drill cutting sedimentology, well-log electrofacies, and seismic attributes. These GDE maps depict evolving depositional environments that span the Hanifa intrashelf basin, Arab Formation carbonate platform, and finally the Hith evaporite system. A variety of new exploration concepts has emerged through building a robust stratigraphic framework and reconstruction of gross depositional environments.

Structural and stratigraphic traps in the Partitioned/Divided Zone (PZ) between the Kingdom of Saudi Arabia and Kuwait suggest minimal tectonics during carbonate build-ups within the Late Jurassic to Early Cretaceous (Figure 1). Today, these same reservoirs are structurally compartmentalized as determined from pressure data, wells and the incorporation of an array of 2-D/3-D seismic data (Figure 2).

Middle Minagish (Tithonian-Berriasian) and Ratawi (Valanginian) Formations, are oil bearing carbonate reservoirs in the Umm Gudair field of Kuwait. Reservoir property varies widely between these two, due to significant variation in the interplay of sedimentation, depositional architecture, and diagenesis. In order to constrain reservoir properties and predicting their distribution, this study integrates data pertaining to sedimentology, sequence stratigraphy, high resolution petrography, of 3 cored wells (900ft of cores) from the Minagish, and 2 cored wells (350ft of cores) from the Ratawi Formations.

The Upper Jurassic Arab-C Reservoir in the Dukhan Field is the product of a stable epicontinental shelf subjected to continuous relative sea-level variations, with the preserved sediments representing a complex of syndepositional carbonate lithologies and textures deposited in subtidal, intertidal and sabkha environments. Though the depositional units are spatially correlated within a sequencestratigraphic context, petrophysical observations do not conform to the primary depositional fabric, and a cross-cutting relationship is observed.

Diverse diagenetic processes have been proposed to explain the reservoir properties of the Arab C and D reservoirs in the Middle East: from early diagenesis (e.g. early cementation, dissolution and/or dolomitization) to burial and late burial diagenesis (e.g. compaction, cementation, burial dolomitization, leaching, hydrothermal/fault-related dolomitization). This contribution examines the Arab C and D reservoirs from an onshore field in Saudi Arabia with specific focus on: (1) late diagenetic evolution; (2) timing of burial diagenesis with respect to charge; and (3) their impact on reservoir rock and fluid properties.

The Jurassic sequences on the Arabian Platform contain widespread carbonate reservoirs, source rocks and seals, which contribute to a world class petroleum system. Since the Triassic period, the Platform was subjected to alternating periods of transgression and regression and differentiation with intrashelf basins. One key variation in sediment deposition was the formation of evaporite seal beds associated with these intrashelf basins. To elucidate the basin depositional history near the flank of the Gothnia Basin margin, a 3D seismic chronostratigraphy technique and attribute assessment were adopted to assess the evaporite distribution during the Late Jurassic. Seismic chronostratigraphy cubes and Wheeler transform models were generated and interactively assessed for structural controls to deposition and assessment of transgression and regression, unconformity and hiatus events at the basin margin. The seismic chronostratigraphy model, which represents dense semi-automatically generated horizons chronostratigraphic order, was derived from mapping samples in the seismic trace. This allowed the interactive assessment of the Jurassic basin margin depositional history in both time and space, and use of horizon sets for attribute and geobody assessment. The techniques applied in this study area proved beneficial in accurately defining the limits of the evaporite salt beds and basin depositional history during the Late Jurassic

Detailed core description of multiple wells within a giant UAE gas field has enhanced understanding of the depositional make-up and possible controls on depositional style for the Arab Formation. In this field, the Arab Formation differs from other areas such as Qatar in that anhydrite and dolomite are scarce in the lower Arab Formation units and recognition of any sequence stratigraphic boundaries is limited. In summary the limestone-dominated reservoir units combine to reflect a large-scale shallowing-upwards trend from basinal/outer, mid-ramp to inner ramp depositional settings, based on core-derived facies association analysis. The facies associations are defined according to sedimentological and faunal characteristics and form genetically related, larger-scale units reflecting a low-energy, mid to outer ramp depositional setting, which grades sharply into an inner ramp shoal and shoal complex depositional setting. Within the low-energy units of the lowest Arab Formation reservoir, mudstones are interbedded with bioclast-rich accumulations of variable thickness and an uncertain origin. The vertical spacing of these units may be a function of random/non-random cyclicity (?Milankovitch) and reflect variation in depositional slope angle and hence slump/debrite deposits or deeper water faunal communities. The transition from the distal depositional setting to the proximal depositional setting occurs over a relatively narrow zone into oolitic grainstones as the system progrades. The grainstones form a thick sequence of bedded units reflecting both shoal and intershoal areas within an inner ramp depositional setting, but cyclicity is not apparent in this interval. The small-scale cyclicity present in the lower Arab Formation units may potentially be equal to those that are defined by dolomite-anhydrite cycles in Qatar, but in this area of Abu Dhabi, the overall distal setting has negated the development of these mixed evaporite-carbonate cycles. The implications of this include a lack of intraformational seals and a uniformity of lithofacies (i.e. limited variation), which is also expressed in the more proximal inner ramp deposits and provides for good reservoir potential.

Development of predictive geological rules is vital for successful exploration and development. The “sweet spotting” concept is widely used to predict areas with better-than-average porosity and permeability. However the definition of sweet spots in unconventional plays is still an emerging concept. Empirical observations from a decade of unconventional development in North America suggest better-than-average production is governed by a combination of favorable matrix permeability, presence of natural fractures and source-rock richness and maturity.

The poster illustrates how an Upper Kimmeridgian analogue exposed in NE Spain has been used to characterise the internal heterogeneities of a low-angle carbonate ramp that includes the development of reef build-ups formed by colonial forms (corals, stromatoporoids), and microbial crusts with associated encrusting organisms. Results from this outcrop analogue have been applied for improving oil recovery from the Arab D reservoirs of a mature field in UAE. The well-exposed outcrops around the Jabaloyas village (Eastern Spain) have been used for detailed facies and sequence stratigraphic reconstructions within a 16–22 m thick series. These outcrops show strike and non-strike sections across a 12 km2 area (i.e. 4 x 3 km). Seventeen stratigraphic profiles were carried out to control vertical and lateral facies distribution. The facies follows an overall retrogradational-progradational trend with the development of low-energy peloidal-skeletal wackestone-packstone in the middle part of the sequence. Different types of grain-supported facies (ooidal, peloidal, intraclastic, skeletal) are found both in high-energy mid-ramp domains and inner ramp areas. The studied series is bounded by discontinuities that are traceable across a total of 17.5 km linear distance and encompasses coral-microbial build-ups 5 to 15 m high developed in mid-ramp setting during the stages of maximum accommodation gain. A total of 274 reefs have been mapped across the different reconstructed 2D transects. Most of them have pinnacle or conical geometry. Well-cemented discontinuity surfaces were used to identify four stages of sedimentary evolution, two of them including individual episode of reef growth. Some trends on the spatial density and on the fabric within each stage have been recorded. The overall distribution of the main facies were included in a full-field model (20 x 20 m grid increment) while the geometry, size and distribution of the reefs were better adjusted in sector models (1 x 1 m). These models assess the distribution of reservoir bodies and their connectivity. They are now used as a template for diagenesis modelling and constitute the geological input for simulation models.

Much previous exploration in the Minagish Field of Kuwait has concentrated upon prospectivity of the principle structures facilitated by 3-D seismic with much less attention directed toward stratigraphic studies. Therefore, questions concerning the seismic imagery of carbonate reservoir quality and seal potential owing to facies changes accompanying allocyclic (global tectono-eustatic) and autocyclic (depositional) processes with a paragenetic overprint (diagenetic-fracture history) have been only modestly studied. The focus of this investigation is upon the carbonate seismic facies placed within a sequence-stratigraphic framework.

The Late Jurassic to Early Cretaceous sequences of the Arabian Platform form a key petroleum province with super-giant carbonate reservoirs. Despite this economic importance and an excellent database the stratigraphy still contains ambiguities with possible alternative interpretations of far-reaching implications.