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= BUHASA OIL FIELD = The Bu Hasa Oil Field is an oil field in Abu Dhabi. It was discovered in 1962 and developed by Abu Dhabi Petroleum Company. The oil field is owned by Abu Dhabi National Oil Company and operated by Abu Dhabi Company for Onshore Oil Operations. The total proven reserves of the Bu Hasa oil field are around 6.52 billion barrels (1450×106tonnes), and production is centered on 600,000 barrels per day (95,000 m3/d).

GEOLOGICAL FRAMEWORK

The Early Cretaceous, from Berriasian to Late Barremian, was a time of tectonic quiescence and stability on the eastern Arabian Plate. The carbonate formations deposited during this relatively long time interval (20 million years) are rather monotonous and laterally continuous over long distances. In contrast, during the deposition of the Aptian Shu’aiba Formation, intra-shelf basins developed and were filled in successive stages (Murris, 1980).

The Arabian Plate formed part of the Afro-Arabian continent in the Early Cretaceous, with a vast epeiric seaway facing east towards the Neo-Tethys Ocean from Oman in the south to Iraq and southern Turkey in the north However, at various times these gave way to differentiated shelf systems with discrete intra-platform basins. By Early Aptian, the Bab Basin in the southern Arabian Gulf and the Kazhdumi Basin (Murris 1980; van Buchem et al., 2010b) in southwest Iran had become well established.

RESERVOIR TRAP
The Source rock is Limestone and the Reservoir rock is the Carbonate which described in detail under section "Facies in Reservoir", and the Seal rock is Shale from

Bab Basin and Nahr Umr Formation; the type of the trap is Stratigraphic trap.

Shu’aiba Sequence 1: Zone A
The Hawar Member is covered, without transition, by very shallow-marine carbonates of Thamama Zone “A” or Zone “1” in Abu Dhabi (Granier, 2000), representing the start of a new transgression and the base of the Shu’aiba Formation. Shu’aiba Sequence 1 corresponds to this zone and to Sequence 1b of Yose et al. (2010) and the upper part of Arabian Plate Sequence 1 of van Buchem et al. (2010a) Zone “A” shows a thickness variation similar to the Hawar Member: it is thinner (10–15 m or 33−49 ft thick) in the area of the Bab Basin and thicker (25–35 m or 82−115 ft thick) where the Shu’aiba Platform would later develop.

Shu’aiba Sequences 2 to 4
The most dramatic differentiation between Shu’aiba shelf and Bab Basin occurred after the deposition of Shu’aiba Sequence 1 (Zone “A”). Microbial/algal layers formed on the higher grounds while the deeper areas gradually drowned and were covered with fine mudstone. Shu’aiba sequences 2–4 were deposited during the transgressive and the highstand phases of the sequence set that led to the development of a broad and thick carbonate platform bounding a relatively deep intra-platform sea, the Bab Basin, presumably connected to the Neo-Tethys Ocean to the east (Figure 14). Shu’aiba sequences 2 to 4 comprise platform or shelf deposits, both aggradational (sequences 2 and 3) and progradational (Sequence 4) as well as deep and shallow basinal deposits. During the transgressive and highstand phases of the Shu’aiba, the Bab Basin was a gradually deepening intra-shelf basin, about 600 km along its NW-SE axis and 400–600 km wide from north to south, extending from northern Oman to the east, to Qatar to the west, to Central Abu Dhabi to the south and to Iran to the north.

Shu’aiba Sequence 5
A significant sea-level drop of as much as 40–60 m (131−197 ft) marked the end of Shu’aiba Sequence 4. The entire Shu’aiba shelf was exposed, except for the deeper platform lagoon, while the Bab Basin, which had reached a maximum depth in excess of 140 m (459 ft) at its centre, as suggested by thickness differences of the Shu’aiba in wells, remained entirely submerged but became a large, relatively shallow inland sea. The sudden surge of terrigenous clay that followed the sea-level drop at the end of Sequence 4 is reflected by an abrupt change in the composition of basin sediments. Clay that was virtually absent during deposition of sequences 1–4 is dominant in Shu’aiba Sequence 5.

FACIES IN RESERVOIR
‘Good’ and ‘very good reservoir’ refer to permeability values of >2 mD and >10 mD, respectively. Based on the average estimated formation pressure of the cored Shu’aiba reservoirs all rocks with a permeability of <0.1 mD are considered as seal.

Interbedded Limestone-Marl Facies
This facies is characterised by a dm-scale alternation of carbonate mudstone and argillaceous mudstone/ marl with varying percentage of clay and different degrees of bioturbation. Argillaceous mudstone with high clay content is dark grey to grey, whereas mudstone with lower clay content is lighter. The beds have sharp to gradual boundaries, depending on the degree of bioturbation. In intervals that are not bioturbated, this facies appears with well-identifiable, mm-scale laminations, sharp bed boundaries and occasional pyrite concentrations. In contrast, more bioturbated intervals show gradual bed boundaries and only rarely pyrite. In some intervals strong bioturbation partly destroyed the alternations and formed nodules and lenses of light carbonate mudstone bedded into a matrix of dark grey mudstone to marl.

Peloidal-Foraminiferal Packstone
This facies appears in core as grey wackestone commonly with a nodular fabric and dm-scale bedding. Thin sections reveal that the sediment is dominated by peloids in a grain-supported texture with occasional occurrence of orbitolinid foraminifera. The peloids are very small (≤ 100 micron), have the same colour as the mud and are therefore hard to recognise. The rock texture and bioturbation suggest that this facies was deposited in a well-oxygenated, moderate to high-energy environment.

Bioclastic Rudstone and Floatstone
This facies contains large bioclasts and bioclastic debris, which are either packed or floating in a beige to grey, wacke- to grainstone matrix. Rudists are the most abundant fossils in rudstone with diametres of up to 8 cm; both broken and intact rudist shells are present. Other fossils include thickshelled bivalves, Lithocodium and occasionally corals. Rudists and Lithocodium are often filled with sparry calcite. Floatstone often interfingers with orbitolina-bearing wackestone. The porosity in this facies can be as high as 27%, with average core and log porosities of 20%.

Wacke- and Packstone with Conical Orbitolinids
The name of this facies is based on the abundance of cone-shaped orbitolinids that occur as wackestone and packstone textures. These grey-to-beige limestones form dm-scale beds and can be up to 10 m thick. The argillaceous content is minor but, where present, records the thorough bioturbation that is otherwise less visible. Orbitolinids vary in size but are generally highly conical and well-identifiable visually as they are darker than the surrounding matrix. Other bioclasts include thin-shelled bivalves and crinoid fragments. These sediments interfinger with a variety of other facies but are most commonly associated with wacke- to packstones with discoidal orbitolinids and bioclastic float- to rudstones.

Miliolid Packstone and Grainstone
These massive, light grey to brown limestones are bioturbated but lack other sedimentary structures. Miliolids, which are only found in this facies, and peloids are abundant but only well recognisable in thin section. Orbitolinids are easily identifiable in cores of this facies (Figure 3j). These packstone and grainstone often show concave-convex and sutured contacts pointing at increased compaction. Although cementation is locally present in this facies, most of the primary interparticle porosity is preserved, leading to residual porosities of up to 25%. The grain-supported textures, the abundance of bioclasts and the scarcity of mud indicate that this facies was deposited in a well-oxygenated, moderate to high-energy environment.

Wacke- and Packstone with Discoidal Orbitolinids
The occurrence of discoidal orbitolinids is the key identifying feature of this facies. Usually grey to dark-grey these sediments show cm- to dm-scale bedding. Thin-shelled bivalves and crinoids are occasionally associated with the orbitolinids in thin section. Bioturbation is common and well-identifiable. In intervals with higher argillaceous content cores are partially broken up. These limestones interfinger with wackestone with Thalassinoides burrows, wackestone with conical orbitolinids and miliolid pack- to grainstone.

Wackestone with Thalassinoides Burrows
Thalassinoides and Glossifungites-type burrows are the key identifying feature of this facies. The burrows are up to 1 cm in diametre and are easily recognisable because of their darker colour relative to the surrounding sediment. Burrow interiors are generally dark grey and are concentrated in the upper part of the beds. Bed thickness is generally greater than 15 cm. The microfacies consists of wackestone with small bivalve shells and occasionally foraminifera. Dolomitisation is common and probably responsible for the frequent brownish discolouration of the limestones. Core porosities are predominantly below 15%. This facies interfingers with mottled mudstone, dark grey mudstone and shale as well as wackestone with discoidal orbitolinids. These carbonates were deposited in a relatively oxygen-rich environment where there was significant burrowing and bioturbation.

Mottled Mud- and Wackestone
This densely cemented, dark-grey mottled facies is composed of highly burrowed mud- and wackestone. Burrows in the darker more argillaceous matrix are infilled with a younger, more limestone-rich mudwackestone. Burrows are generally not compacted. Bed boundaries are often gradual. Bed thickness ranges from 6 to 12 cm. Fossils are rare and consist of filaments and smaller foraminifera. The study of thin sections revealed strong cementation of the matrix and some pre-existing vuggy porosity as well as pyritisation in some places. These carbonates interfinger with argillaceous mudstone and wackestone with Thalassinoides burrows. This facies resembles the argillaceous mudstone facies with the difference that the rock is less fissile (probably due to higher carbonate content).

Argillaceous Mudstone
This facies is characterised by bioturbated, dark grey to black mudstone with variable argillaceous content. Microfossils are limited to few bivalve shells. Bedding is indicated by horizons with less argillaceous material. More argillaceous intervals of these facies are broken-up and can be quite fissile. Better bioturbated intervals are more carbonate-rich. The microfacies is dominated by mudstone with rare small bivalve shells and forams. This facies interfingers with wackestone with Thalassinoides burrows and mottled mudstone. The core of Well A1 that penetrates this facies is very fissile containing numerous black mudstone discs. Log-based porosities are extremely low in this facies whereas gamma-ray logs through these mudstones record the highest radiation among all facies.