Linking diagenetic history to depositional attributes in a high-frequency sequence stratigraphic framework: A case from upper Jurassic Arab formation in the central Persian Gulf

Journal of African Earth Sciences 153 (2019) 91–110

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Linking diagenetic history to depositional attributes in a high-frequency sequence stratigraphic framework: A case from upper Jurassic Arab formation in the central Persian Gulf

T

Masoud Sharifi-Yazdia, Hossain Rahimpour-Bonaba,∗, Vahid Tavakolia, Maziyar Nazemia, Mohammad Reza Kamalib a b

School of Geology, College of Science, University of Tehran, Tehran, Iran Faculty of Science and Engineering, University of Curtin, Bentley, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: Late Jurassic Depositional environments Diagenetic history Sequence stratigraphy Paleoclimate

The carbonate Arab Formation (Kimmeridgian-Tithonian) is one of the most prolific hydrocarbon reservoirs in the world. Petrographic studies of this unit in the central Persian Gulf led to identification of eleven sedimentary facies, and facies are grouped into 5 shore-parallel facies belts (supratidal, tidal flat, lagoon, shoal and midramp). Evidences such as the absence of barrier reefs and the low diversity of facies reveals that these facies formed in a ramp-like platform. Postdepositional processes (diagenetic evolution) considerably influenced reservoir properties of sedimentary facies. Depositional and diagenetic features in this formation are strongly controlled by the interplay of glacio-eustasy and climate leading to sea-level fluctuations during the Late Jurassic. Based on the petrographic analyses and previous studies, four third-order sequences are recognized and all sequence boundaries are capped by evaporites. The main diagenetic processes impacting the Arab Formation occurred under marine and hypersaline conditions and burial realms. The hypersaline conditions correspond to sea-level fall and development of brines and are characterized by dolomitization and moldic dissolution in the Upper Arab Formation. Vuggy dissolution is another major reservoir improving factor. Generally, vuggy dissolution along with anhydrite cementation, occurred extensively in the Upper Arab Formation, while in the Lower Arab Formation, these processes are limited to just beneath of the sequence boundary (nodular anhydrite form). Diagenetic features (eogenetic and mesogenetic environments) appear to correlate to depositional environments in the Arab Formation and are controlled by sea-level changes. Consequently, diagenetic processes and reservoir properties can be predicted within a sequence stratigraphic framework.

1. Introduction Depositional and diagenetic processes are the main factors controlling reservoir properties (Lucia, 2007; Ahr, 2008; Beigi et al., 2017). Carbonate platform geometry dictates lateral arrangements of depositional facies and hence primary porosity (Lucia, 2007). While diagenetic alterations modify limestone depositional properties and lead to porosity redistribution. Successive modifications and pore rearrangements of carbonate reservoirs can only be understood by studying the diagenetic history of carbonates (Gaupp et al., 1993; Mazzullo, 1994; Hiatt and Kyser, 2000; Janjuhah at al., 2017). Spatial trends of diagenesis and its impact on reservoir porosity-permeability patterns, have been shown to relate, at least indirectly, to relative sea-level (RSL) change and sequence stratigraphic framework (Taghavi et al., 2006;

∗

Rahimpour-Bonab et al., 2012; Kordi et al., 2016). Consequently, it should be possible to predict intra-formational reservoir quality using sequence stratigraphy. In this study, we reconstruct the architecture of the (carbonate) Arab Formation, in the Iranian Persian Gulf, and propose a sequence stratigraphic framework for these strata. We then determine controls on reservoir properties, resolve the diagenetic history of the Arab Formation and its impact on reservoir quality and evolution, as well as evaluating the linkage between depositional/diagenetic processes and RSL changes. Finally, we consider how reservoir quality can be predicted using the proposed sequence stratigraphic framework. The Kimmeridgian-Tithonian (Late Jurassic) carbonate Arab Formation is regarded as one of the most prolific hydrocarbon-bearing units in the world (Beydoun, 1988; Alsharhan and Magara, 1995; Lindsay, 2006). This formation hosts giant hydrocarbon reservoirs in

Corresponding author. E-mail address: [email protected] (H. Rahimpour-Bonab).

https://doi.org/10.1016/j.jafrearsci.2019.02.006 Received 2 August 2018; Received in revised form 3 February 2019; Accepted 5 February 2019 Available online 20 February 2019 1464-343X/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. (a) Generalized stratigraphy of Upper Jurassic lithostratigraphic rock units at the Ghawar Field in eastern Saudi Arabia (modified after Cantrell et al., 2001). The Upper Arab Formation contains the members A to C and the Lower Arab Formation contains the member D member. (b) Location map of the studied area in the central of the Persian Gulf. The studied oil field and the location of wells is seen.

during the Tithonian, led to evaporite deposition overlying the Arab and the Hith platforms (Alsharhan and Magara, 1995). The Arab Formation is equivalent to the upper Surmeh Formation (Khami Group) in the stratigraphic nomenclature of Iran. This formation is subdivided into the four members A to D. Each member is covered by an anhydrite layer, and the uppermost member (A) is overlain by the anhydrite of the Hith Formation (Powers, 1962, 1968; AlHusseini, 1997). Generally, the D member is known as the “Lower Arab Formation” and the members A to C are called the “Upper Arab Formation” (Lapointe, 1991) (Fig. 1a). The study area is located in the central Persian Gulf (Fig. 1b) in the eastern part of the South Pars Field. This area is controlled by two kinds of structural phenomena including the Qatar Arch and salt tectonics (Alsharhan and Nairn, 1997) (Fig. 3). The Qatar Arch formed through basement faulting in the Late Precambrian and was active during the Late Jurassic (Murris, 1980; Alsharhan and Nairn, 1997; Pollastro, 2003). Regarding the deposition of the Arab Formation in the Rub AlKhali basin in the central Persian Gulf during the Late Jurassic (Handford et al., 2002; Sfidari et al., 2018a), the Qatar Arch as a paleohigh was a main syndepositional structure by establishment of Rub Al-Khali basin. As well, postdepositional movement of the Qatar Arch influenced the distribution and formation of hydrocarbon reservoirs in the study area (Alsharhan and Nairn, 1997). The Late Precambrian/ Early Cambrian Hormuz Salt also played a major role in the formation of hydrocarbon fields, mainly through salt migration (Stöcklin, 1968; Edgell, 1996; Perotti et al., 2016; Beigi et al., 2017; Jafarian et al., 2017). In general, the active salt movement was significant during the Permian-Triassic, Cretaceous, Eocene-Oligocene and Neogene (Perroti et al., 2016); however, more stable conditions dominated the Jurassic in the Arabian Plate. In spite of this general salt tectonic limitation, the salt dome in the studied area had important local impact on the depositional system of the Arab Formation (Fig. 3b). This formation is considered to be the most important reservoir zone in the study area and is capped by the impermeable evaporite of the Hith Formation (Alsharhan and Magara, 1995; Clark et al., 2004; Assadi et al., 2018). As shown, the studied exploration wells were drilled on the flank of the salt structure (Ghazban, 2007) (Fig. 3b).

many parts of the Arabian Plate, including the Persian Gulf, Abu Dhabi, offshore UAE, Saudi Arabia, Bahrain, and Qatar (Alsharhan and Kendall, 1986; Alsharhan and Nairn, 1997). In the Iranian waters of the Persian Gulf, several offshore fields such as Balal, Reshadat and Salman fields produce from the Arab Formation (IOOC Unpublished Report) (Fig. 1a and b). Apparently, the frequent RSL fluctuations during the Late Jurassic forced changes in the carbonate factory and diagenetic patterns both spatially and temporally in the Arab Formation. As a result, RSL changes indirectly controlled depositional and diagenetic features of this carbonate evaporite succession (Clark et al., 2004; Al-Saad and Sadooni, 2011; Morad et al., 2012; Al-Awwad and Collins, 2013; Nader et al., 2013; Daraei et al., 2014; Beigi et al., 2017; Marchionda et al., 2018). However, linking between various variables including facies distribution, diagenetic history and sequence stratigraphy have been less considered (Morad et al., 2012; Beigi et al., 2017). Herein, we evaluate the Arab Formation with a focus on understanding RSL controls on reservoir characteristics.

2. Geological setting and stratigraphy During the Late Permian and, eastern Gondwana's disintegration, the Neotethys Ocean opened (Al-Husseini, 2000; Sharland et al., 2001; Ziegler, 2001). Following rifting, passive margin platforms developed around the margins of Neotethys during the Early to Late Jurassic (Fig. 2), along with the contemporaneous Gotnia, Arabia and Rub AlKhali intrashelf basins (Ayres et al., 1982a,b; Ziegler, 2001; Pollastro, 2003). This occurrence was concurrent with multiple RSL changes in the Arabian Platform. During RSL rise in the Early to Late Jurassic, source rock formed as a result of organic matter accumulations (Powers, 1962, 1968; Alsharhan and Magara, 1995; Alsharhan and Nairn, 1997; Ziegler, 2001; Handford et al., 2002). This was followed by deposition of shallowing-upward carbonate cycles of the Arab Formation, and then under the influence of RSL fall, Hith Anhydrite was precipitated during the Late Jurassic. The Hith Formation also acts as a cap rock (Ziegler, 2001; Haq and Al-Ghahtani, 2005). During deposition of the Arab Formation, periodic RSL fall coupled with an arid climate related to the intertropical convergence zone (Sellwood et al., 2000) 92

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Fig. 2. Paleofacies map of the Arab Formation during the Kimmeridgian and Tithonian (Compiled with some modification after Al-Hussaini, 1997).

3. Material and methods

impregnated with blue-dyed epoxy resin. According to microscopic examinations, thin section descriptions include texture, size and type of allochems and diagenetic features. Rock were classified using the limestone classifications of Dunham (1962) and Embry and Klovan (1971). Facies interpretations are based on rock characteristics, comparison of successions with well-established models (Wilson, 1975; Buxton and Pedley, 1989; Flügel, 2010). Special emphasis was put on identifying standard ramp microfacies types (RMF) according to Flügel (2010). For evaluation of the energy index of sedimentary facies, Plumley et al. (1962) classification was used. In addition to thin section evaluation, oxygen and carbon stable isotope data (36 samples) were acquired from dolomites in well B-01.

In order to construct a depositional model and to decipher the diagenetic history of the Arab Formation more than 350 thin sections were made from samples taken from two wells (B-01 and B-02) from an oil field, located in the central Persian Gulf (Fig. 1). Thin section samples were taken at 0.5 m intervals through the cored intervals. The core from well B-01 intersects members A to D of the Arab Formation, with a cored interval of 137 m. The core from well B-02 intersects members A to C with a 100 m cored interval. Among the > 350 thin sections, 150 were treated with Alizarin Red-S and potassium ferrocyanide, following Dickson's (1965) procedure, and 100 were

Fig. 3. (a) A cross-section of Phanerozoic successions on the Arabian Plate (modified after Afifi, 2005). The situation of the study area is shown. Seemingly, this area was geologically under influence of the Qatar-Fars Arch and the regional salt tectonic. (b) Two-dimensional seismic profile of the study area. The influence of the salt dome on the Arab Formation of the study area and locations of the two drilled wells (B-01 and B-02) on the flank of the salt structure, are shown. 93

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2/5 16/2 17/3 20/2 18/4 18/6 17/1 20/5 23/5 18/1 20 Supratidal Intertidal Intertidal Intertidal Lagoon Shoal Shoal Shoal Shoal Mid. ramp Mid. ramp

The porosity of sedimentary facies was determined using Helium porosity on 430 core plug samples. Finally, to identify and correlate systems tracts, maximum flooding surfaces and sequence boundaries, petrographic analyses were correlated to gamma-ray (GR) and density logs (RHOB). 4. Results and interpretation 4.1. Microfacies

Low Low Low Medium Low Medium High High High High to medium Poor

Porosity(%)

Detailed paleoenvironmental reconstruction and lithostratigraphic variations of the Arab Formation throughout the Middle East have been described in detail in several studies including Alsharhan and Kendall (1986), Alsharhan and Magara (1995), Alsharhan and Whittle (1995), Al-Sharhan and Nairn (1997) and Daraei et al. (2014). In the study area, 11 facies are defined in the Arab Formation (Table 1). While some of them show analogous components, they mostly have different fabrics and textural attributes, resulting in various petrophysical characteristics. The 11 facies define five facies belts, including sabkha, tidal flat, lagoon, shoal and mid-ramp. The main allochems in the Upper Arab Formation facies include ooids, peloids, oncoids, and mollusca. Important allochems in the Lower Arab Formation include benthic foraminifera, green algae, mollusca, stromatoporoids and corals as well as non-skeletal fragments (oncoids, peloids and intraclasts). Ooids are rare.

_ _ Poorly sorted Well sorted Poorly sorted Moderately sorted Very well sorted Moderately sorted Moderately sorted Moderately sorted Fine _ _ Peloid Peloid, ooid, gastropod, bivalve Gastropod, bivalve, peloid, oncoid Peloid, oncoid, gastropod, bivalve, ooid Ooid, gastropod, oncoid, bivalve, peloid Oncoid, gastropod, bivalve, intraclast Benthic foraminifera, oyster, coral, stromatoporoid, green algae, peloid, intraclast Benthic foraminifera, green algae, oyster, stromatoporoid Peloid, echinoderm, sponge spicules, ostracoda Anhydrite Dolomudstone Peloidal stromatolite boundstone Peloidal ooid packstone-wackestone Bioclast peloid wackestone-packstone Peloidal oncoid bioclast packstone Ooid oncoid grainstone Oncoid bioclast intraclast grainstone Bioclast peloid intraclast grainstone Bioclast peloid intraclast packstone-wackestone Peloidal bioclast wackestone-mudstone F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11

_ _ Poor Good Poor Medium Good Medium Medium Medium Poor

Sorting Components Facies name Facies code

Table 1 Facies and facies belts of the Arab Formation in the studied wells. Texture and dominant components are also been shown.

Roundness

Energy level

Facies belt

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4.1.1. Facies 1 (F1): Anhydrite This is the most abundant facies in the Upper Arab Formation. It consists mainly of ∼80% anhydrite associated with micrite and fine crystalline dolomite (< 20 micron) (Fig. 4a). It appears as massive, layered and nodular (chicken wire) in the core. The chicken wire structure shows syndepositional, interstitial growth of displacive anhydrite that expanded in a sabkha (supratidal) setting (Kasprzyk, 2003; Aleali et al., 2013). This facies corresponds to RMF 25 (peritidal) (Flügel, 2010), 1 (Peritidal) (Buxton and Pedley, 1989) and 9 (evaporative flats) Wilson (1975). It is equivalent with the Type І energy index (Plumley et al., 1962). 4.1.2. Facies 2 (F2): Fine crystalline dolomudstone This facies predominantly consists of dolomicrite that replaced the lime mud (Fig. 4b). In some cases, anhydrite nodules appear in F2, suggesting deposition during periods of sea-level fall in the arid climate. Compared with similar facies, it could be ascribed to the intertidal zone of hypersaline lagoon (Warren, 2006). This facies corresponds to RMF 22 (peritidal) (Flügel, 2010) and 1 (peritidal) (Buxton and Pedley, 1989) and 8 (tidal flats) Wilson (1975). It corresponds to Type І energy index (Plumley et al., 1962). 4.1.3. Facies 3 (F3): Peloidal stromatolite boundstone This tidal flat facies includes microbial colonies, containing algal filaments (∼40%), peloids (∼20%), fine intraclast (∼10%) and some impurities such as organic matter (∼13%) (Fig. 4c). Mud cracks and bird's-eyes textures are not present in this facies which could be ascribed to their deposition in the lower intertidal setting (Clark et al., 2004). Seemingly, due to vuggy porosity development, some of the fenestral pores have been destroyed. This facies is similar to RMF 22 (peritidal) (Flügel, 2010), 1 (peritidal) (Buxton and Pedley, 1989) and 8 (tidal flats) Wilson (1975). It shows Type І energy index (Plumley et al., 1962). 4.1.4. Facies 4 (F4): Peloidal ooid packstone/wackestone This facies consists of fine-grained peloids and ooids (∼40%) with 1 mm in diameter associated by micrite (20–40%). Due to fabric inversion it is difficult to distinguish peloids from ooids (Fig. 4d). Handford et al. (2002) and Clark et al. (2004) assigned this facies to an intertidal to supratidal environment. This facies is compatible with RMF 94

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4.1.5. Facies 5 (F5): Bioclastic peloid wackestone/packstone This facies is characterized by wackestone/packstone and muddy matrix (∼42%) associated with debris of peloids (∼20%), bivalves (∼10%), gastropods (∼10%), ostracods (∼10%). This facies indicates a low-energy setting in a restricted lagoon (back shoal) (Wanas and Sallam, 2016) (Fig. 4e). This facies is similar to RMF 20 (lagoon) (Flügel, 2010), 2 (inner restricted lagoon) (Buxton and Pedley, 1989) and 7 (shelf lagoon) Wilson (1975). Type III of Plumley et al. (1962) classification is equivalent to the energy level of this facies. 4.1.6. Facies 6 (F6): Peloidal oncoid bioclastic packstone Its main allochems include peloids (∼30%), oncoids (∼20%), bioclasts (gastropods and bivalves) and ooids (∼10%). Allochems are moderately sorted and rounded along with some micrite (∼22%) that indicate a leeward shoal environment located between shoal and lagoon (Assadi et al., 2018; Sfidari et al., 2018b; Flügel, 2010). Peloids might have reworked from a low-energy environment to leeward shoal. This facies represents a transition from low-energy lagoon to high-energy shoal setting (Fig. 4f) (e.g., Al-Saad and Sadooni, 2011). This facies is equivalent to RMF 27 (carbonate sand shoals and banks) (Flügel, 2010), 3 (beach facies) (Buxton and Pedley, 1989) and 6 (winnowed edge sands) Wilson (1975). Its energy index is Type ІV of Plumley et al. (1962). 4.1.7. Facies 7 (F7): Ooidal oncoid grainstone The significant components of this facies include micritized ooids (∼50%), oncoids (∼15%) associated with some peloids (∼10%) and gastropods (∼10%) (Fig. 4g). The grains are well sorted signifying deposition in a high-energy central shoal environment (Schulze et al., 2005; Blomeier et al., 2009; Cantrell, 2006). This facies corresponds to RMF 30 (carbonate sand shoals and banks) (Flügel, 2010), 3 (beach facies) (Buxton and Pedley, 1989) and 6 (winnowed edge sands) Wilson (1975). Its energy index is Type V of Plumley et al. (1962). 4.1.8. Facies 8 (F8): Oncoid bioclastic intraclast grainstone This facies consists primarily of micritized allochems including oncoids (∼40%), gastropods (∼20%), intraclasts (∼15%) and bivalves (∼10%) (Fig. 4h). Moderately sorted and rounded grains intraclasts represent seaward shoal environment (Blomeier et al., 2009). This facies corresponds to RMF 27 (carbonate sand shoals and banks) (Flügel, 2010), 3 (beach facies) Buxton and Pedley (1989) and 6 (winnowed edge sands) Wilson (1975). Type V of Plumley et al. (1962) classification of energy index could be correlatable to this facies. 4.1.9. Facies 9 (F9): Bioclastic peloid intraclast grainstone Small benthic foraminifera (∼20%), oysters (∼10%), corals (∼10%), stromatoporoids (∼10%), peloids (∼10%), intraclasts (∼10%) and green algae (Clypeina, Salpingoporella and Thaumatoporella) (∼5%) are components of this facies (Fig. 4i). It is only developed in the Lower Arab Formation. The lack of micrite and moderate sorting, accompanied with intraclasts suggest a high-energy, seaward shoal environment. This facies corresponds to RMF 27 (carbonate sand shoals and banks) (Flügel, 2010), 3 (beach facies) (Buxton and Pedley, 1989) and 6 (winnowed edge sands) Wilson (1975). Type V of Plumley et al. (1962) classification of energy index corresponds to this facies.

Fig. 4. Thin section photomicrographs, illustrating main facies from the Arab Formation in the center of the Persian Gulf. (a) Facies 1: anhydrite with chicken wire fabric, (b) Facies 2: dolomudstone, missing bioclast, (c) Facies 3: stromatolite boundstone, (d) Facies 4: ploid ooid packstone/wackestone, accompanied with anhydrite nodule, (e) Facies 5: bioclastic peloid wackestone/ packstone, (f) Facies 6: peloidal oncoid bioclastic packstone, (g) Facies 7: ooidal oncoid grainstone, (h) Facies 8: oncoid bioclastic intraclast grainstone, (i) Facies 9: bioclastic peloid intraclast grainstone. Stromatoporoids (cladocoropsis) are present in this facies. (j) Facies 10: bioclastic oncoid intraclast packstone, (k) Facies 11: peloid bioclastic wackestone/mudstone.

4.1.10. Facies 10 (F10): Bioclastic peloid intraclast packstone/wackestone Facies 10 consists of micritized skeletal and non-skeletal grains including small benthic foraminifera (∼20%), peloids (∼20%), intraclast fragments (15%), echinoderms (∼10%) and green algae (∼10%). The concurrent presence of high-energy fragments (intraclasts and peloids) along with micrite (∼20%), indicates textural inversion (Fig. 4j) and a depositional environment of a proximal mid-ramp near the fair-weather wave base (Flügel, 2010). This facies is merely observed in the Lower Arab Formation. This facies is equivalent to RMF 9 (mid-ramp) (Flügel,

24 (peritidal) (Flügel, 2010), 1 (peritidal) (Buxton and Pedley, 1989) and 8 (tidal flats) Wilson (1975). It shows Type ІV energy index (Plumley et al., 1962). 95

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micro-organisms (Riding and Liang, 2005) had an important effect on the Arab Formation (Fig. 7a and b). Micritic envelopes are observed in facies F3 to F8. This feature has no direct effect on reservoir quality, but may preserve allochems from further alteration by diagenetic processes.

2010), 4 (seagrass community) (Buxton and Pedley, 1989) and 3 (deep shelf margin) Wilson (1975). Type ІV of energy index is suggested for this facies. 4.1.11. Facies 11 (F11): Peloidal bioclastic wackestone/mudstone In this facies, peloids (∼20%) are associated with echinoderms (∼10%), bivalves (∼10%) and green algae (∼10%) fragments and calcareous sponge spicules in a dark muddy matrix (∼30%) representing a low-energy environment similar to a distal (outer) ramp to proximal basin setting (Flügel, 2010) (Fig. 4k). This facies is, similar to F10, but only present in the Lower Arab Formation. It is known to be associated with sedimentary structures such as firm-ground and burrows in the Ghawar Field in Saudi Arabia (Handford et al., 2002; Swart et al., 2005). This facies corresponds to RMF 3 (mid-ramp) (Flügel, 2010), 4 (seagrass community) (Buxton and Pedley, 1989) and 3 (deep shelf margin) Wilson (1975). Its energy index is types III. Detailed petrographic studies reveal several general trends from the Lower Arab Formation to the Upper Arab Formation that represent, in part, increasing climate aridity. At the beginning of the Upper Arab Formation increasing aridity led to deposition of evaporite deposits and alteration of depositional facies (Alsharhan and Magara, 1995). Regarding the nature of constituent particles of the Arab Formation, faunal diversity (F8 and F9) decreases from the Lower Arab Formation toward the Upper Arab Formation and non-skeletal fragments such as ooids, oncoids and peloids (F6 and F10) increase in frequency (Fig. 5). Absence of siliciclastic components in this formation suggests that the depositional setting was relatively stable (Alsharhan and Nairn, 1997) during the Late Jurassic.

4.3.1.2. Marine cementation. In the shallow marine high-energy setting facies, marine calcite cement with LMC (low magnesium calcite) mineralogy (Stanley and Hardie, 1998) formed as isopachous acicular rim cements around various allochems (Fig. 7a and b). This cement is only reported from the Lower Arab Formation and due to pervasive dolomitization of the Upper Arab Formation, no traces of it are found. This cement led to early lithification and prevented compaction during later burial and may have helped partial preservation of the interparticle porosity. Furthermore, this porosity may have been partly preserved due to overpressure conditions, which in turn, resulted from sealing by overlying evaporite (Daraei et al., 2014). Marine cements are abundant in F9 but rarely seen in F10. 4.3.1.3. Early dolomitization. Early dolomite was identified by euhedral to subhedral crystal dolomite that pervasively formed in the Arab Formation. Dolomite crystals are ranging in size from 20 to 100 microns in F1 to F5 and 100 to 400 microns in F6 to F8, both showing fabric retentive and fabric destructive fabrics. (Fig. 7e, f, g). This dolomite shows mesogenetic vuggy dissolution and anhydrite cementation suggesting its formation in eogenetic realm. 4.3.1.4. Moldic dissolution. Moldic pores in the F4, F5 and occasionally in F6 and F7 are seen, which are associated with evaporite facies and evaporite nodules in the Upper Arab Formation (Fig. 7c and d). Microcrystalline dolomite and anhydrite cement occluded moldic pores. Petrographic evidence show that fabric selective dissolution (moldic) led to increasing porosity in the Arab carbonates. Moldic porosity dramatically improved of the reservoir in the Arab Platform (Alsharhan and Magara, 1995; Morad et al., 2012; Daraei et al., 2014; Beigi et al., 2017).

4.2. Field-scale conceptual depositional model In this study, the lateral distribution of facies belts along the carbonate ramp is reconstructed (Fig. 5). Facies belts formed parallel to the shoreline, and extended from peritidal to mid-ramp positions in an open marine setting. A ramp-like platform supported for the Arab Formation based in the absence of barrier reefs, the low variability of facies types and the development of ooidal shoal close to the shoreline (Flügel, 1982; Burchette and Wright, 1992; Avrell et al., 1998; Wanas, 2008). This interpretation is similar to that of other studies on the Arabian Platform (Handford et al., 2002; Lindsay, 2006 and Morad et al., 2012; Daraei et al., 2014; Beigi et al., 2017; Marchionda et al., 2018). Indeed, arid climate at the time of construction of the Arab Platform inhibited the biological success of the reef-building organisms and led to expansion of shoal grainstone along with evaporite deposits. In the Lower Arab Formation, reef buildups are present as coral patch reefs and patch reefs of organisms with stromatoporoid skeletal morphology.

4.3.2. Mesogenesis 4.3.2.1. Compaction. Features of mechanical compaction are visible in some grain-dominated facies (Fig. 7h), but apparently it partially retarded in the Arab Formation due to the early marine cementation and overpressure (Daraei et al., 2014) (Fig. 7i and j). In most cases, stylolites cross-cut previous features such as dolomite and anhydrite cements, suggesting their late diagenetic origin. Facies associations of F1 to F5 and rarely F6 demonstrate pressure-solution features. 4.3.2.2. Dolomite cementation. Dolomite cement appears as euhedral crystals which formed in shallow burial realms (Choquette and Hiatt, 2008). This type of cement is exclusively observed in the Upper Arab Formation, growing over coarse crystalline dolomite (such as seen in F6, F7 and F8), leading to occlusion of intercrystalline porosity and pore throats (Clark et al., 2004) (Fig. 7k). In some cases, dolomite crystal cores are dissolved (skeletal dolomite formation), but the dolomite cement remained intact (Fig. 7l).

4.3. Diagenetic processes The diagenetic processes that overprinted the Arab Formation have been previously discussed by several workers (Alsharhan and Magara, 1995; Alsharhan and Whittle, 1995; Al-Saad and Sadooni, 2011; Clark et al., 2004; Morad et al., 2012; Daraei et al., 2014). Diagenetic events had significant effects on the evolution of pores and final reservoir characteristics (Hollis, 2011; Hosseini et al., 2018; Nazemi et al., 2018, 2019). The diagenetic history of the Arab Formation begins with the stratigraphic architecture of depositional environments that shifted position as relative sea-level rose and fell, and was followed by the hypersaline realm and then burial stage (Fig. 6). The lack of evidence for meteoric diagenesis in the Arab Formation, led to the suggestion that this platform had no important subaerial exposure during its diagenetic history (Wilson, 1981; Mitchell et al., 1988; Cantrell and Hagerty, 1999). Main diagenetic stages include:

4.3.2.3. Vuggy dissolution. The non-fabric selective vuggy dissolution is an important diagenetic process in the Upper Arab Formation. It plays a major role in reservoir improvement in the dolostone units (Fig. 7m). Vuggy pores occur in the early dolomites and dolomite cements. They are frequently filled by poikilotopic and pervasive anhydrite cements in the Upper Arab Formation. Vuggy dissolution took place in deep burial settings. 4.3.2.4. Anhydrite cementation. Anhydrite cements occur commonly with pore-filling, poikilotopic and pervasive textures in the Upper Arab Formation (Fig. 7c, l and m). Dissolution of overlying anhydrite resulted in development of this cement (Warren, 2006). The pore-filling

4.3.1. Eogenesis 4.3.1.1. Micritization. During the Late Jurassic, intensive boring by 96

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Fig. 5. (a) Field-scale facies conceptual depositional model, reconstructed for the Jurassic Arab Formation of the study area. Eleven facies types are recognized in this environment that deposited along the peritidal to mid-ramp settings. Within the shallower part of this carbonate ramp, eight main facies belts can be distinguished. b) Also shown are the idealized shallowing-upward sequences for this environment.

Arab Formation is widely developed, while this diagenetic process only developed in the uppermost dolomite of the Lower Arab Formation. In comparison, the presence of anhydrite cement in the uppermost of the Lower Arab Formation conforms to observations in the Arab Formation

type formed during hypersaline diagenesis that resulted in decreasing porosity. The presence of dolomite inclusions, filling of vuggy and intracrystalline voids by anhydrite cement reflects its late burial origin, just before pressure-solution initiation. Anhydrite cements in the Upper 97

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Fig. 6. Diagram showing the general paragenetic sequence of diagenetic events in the studied section of the Arab Formation. Diagenesis occurred in three diagenetic environments including marine, hypersaline and burial. Influences of each diagenetic events on reservoir properties are illustrated as well. Generally, marine cementation, dolomitization and dissolution resulted in improving reservoir quality.

of the United Arab Emirates (Kawaguchi, 1991) and Saudi Arabia (Swart et al., 2005). Generally, anhydrite cements acted as a negative factor in reservoir quality in the Arab Formation of the Arabian Platform (Alsharhan and Magara, 1995; Morad et al., 2012; Nader et al., 2013).

burial diagenetic realm in the Arab Formation (Alsharhan and Magara, 1995; Alsharhan and Whittle, 1995; Morad et al., 2012). This type of dolomite is observed in two populations with varying sizes including medium crystal size (100 microns) and mainly in form of coarse crystal size (300–250 microns) (Fig. 7n).

4.3.2.5. Burial dolomitization. This type of dolomite appears as sucrosic crystals with euhedral shapes in the limestone interval (F9 and F10) of the Lower Arab Formation (Fig. 7n and o). This dolomite postdates compactional (mechanical and chemical) features and created intercrystalline porosity in the mud-dominated facies of the mid-ramp in form of partial dolomitization. Seemingly, it has developed in the

4.4. Sequence stratigraphy. While the vertical stacking patterns of the sedimentary facies are controlled by sea-level fluctuations, their lateral distribution is affiliated with the depositional environment (Schlager, 2005). Sequence stratigraphy displays the position of sedimentary bodies in fluctuating settings between major stratigraphic boundaries. In the present study, the common transgressive systems tract (TST)98

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Fig. 7. Diagenetic processes in the Arab Formation. a and b) micritization of allochems and marine cement around allochems, c and d) dissolution of aragonite components and creation of moldic porosity, e) sabkha dolomitization caused dolomite crystals less than 20 micron, f) Reflux dolomites. Proximity of hypersaline brines created fine crystalline dolomite. (g) Coarse crystalline dolomite caused by seepagereflux mechanism. These dolomites formed due to distance of hypersaline brines in high-energy facies. (h) Mechanical compaction precluded precipitation of marine cement and abnormal pressure (i and j) stylolitization in dolomudstone facies. Notice open stylolite and sill stylolite. (k) Dolomite cement in form of overgrowth. (l) Skeletal dolomite, filled by anhydrite cement. (m) Vuggy pores and filling with anhydrite cement. Notice dissolution occurred in two stages before and after anhydrite cementation (n) stylolite-related dolomite in medium crystalline size along with recrystallized coarse crystalline dolomite (o) coarse crystalline dolomite which formed by recrystallization. (p) A core sample of moldic dissolution belonging to B-02 well. (q) A core sample of vugs, belonging to B-01 well. (r) Core sample with vugs, filled by anhydrite cement belonging to B-01 well. (s) Core sample with stylolite, belonging to B-01 well.

late TST and early HST and were formed during seal-level rise. Deepening-upward facies has a retrogradational to aggradational stacking pattern in TST, and shallowing-upward facies has an aggradational to progradational stacking pattern in the HST. Sequence boundaries characterized by evaporite sediments (anhydrite layers) which developed in the sabkha environment, caused by sea-level fall. Some of the studies (McGuire et al., 1993; Handford et al., 2002) reported sequence

regressive systems tract (RST) or highstand systems tract (HST) model (Embry, 1993) is used to define the sequences. Integration of petrographic data and petrophysical logs (gamma-ray and density) led to identification of various systems tracts and sequence surfaces (maximum flooding surfaces (MFS) and sequence boundaries). Generally, early HST and late TST are composed of low-energy facies deposited during sea-level fall. Whereas high-energy facies generally occur in the 99

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Fig. 8. Sedimentological log of the Arab Formation in the studied Field (B-01 well). The main stratigraphic subdivisions of the Arab Formation are also depicted along with the lithology, petrophysical logs, microfacies, facies belts, important diagenetic alterations, depositional sequences, anhydrite content and main pore types (color legend as Fig. 5). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

the top of the thick anhydrite layers were considered as sequence boundaries that were deposited during the late HST. Due to continuous depositional conditions of the Arab formation during the Late Jurassic evidence of subaerial exposure and erosion are absent. On the other

boundaries are placed at the below the evaporite deposits while the other studies (Le Nindre et al., 1990; Al-Husseini, 1997; Daraei et al., 2014; Beigi et al., 2017; Sfidari et al., 2018a) indicated that sequence boundaries are placed at the top of the evaporite deposits. In this study, 100

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hand, arid climate conditions prevented widespread freshwater features along the sequence boundaries. Based on this study, glacio-eustasy (Emery and Meyers, 1996; Catuneanu, 2006) has played a significant role in the development of depositional sequences (Le Nindre et al., 1990; Sharland et al., 2001; Haq and Al-Qahtani, 2005). Moreover, arid climate domination influenced the regional Late Jurassic carbonate factory (Alsharhan and Magara, 1995). Generally, the Arab Formation consists of four shallowing-upward cycles, and each cycle includes a third-order sequence (Figs. 7 and 8). Due to influence of the eustasy as the main control over sea-level changes, the number of third-order sequences are correlated with the other researches in the Arabian Platform (Fig. 10) (Le Nindre et al., 1990; McGuire et al., 1993; Azer and Peebles, 1995, 1998; Daraei et al., 2014; Beigi et al., 2017). Each sequence contains multiple fourth-order sequences, known as high-frequency sequences (HFS) (Catuneanu, 2006). In general, HFS's are control by glacio-eustasy accompanied by local tectonic deformation (syndepositional structures) (Miall, 2000). Determination of HFS can help the precise identification of reservoir heterogeneity in exploration strategy. This illustrates detailed variations of reservoir characteristics (sedimentological and diagenetic features) in basin scale influenced by sea-level fluctuations (Awwad and Pomar, 2015). 4.4.1. Sequence D This sequence is about 60 m thick. It is only drilled in the well B-01 with 34 m of available core. It consists of a HST and contains two fourth-order sequences (Fig. 8). Because of the lack of core data, TST and MFS are not recognized in this sequence. The sequence D is mainly composed of offshore facies (such as seaward shoal and mid-ramp) with an aggradational stacking pattern that is sharply capped by the supratidal facies. Diverse allochems including oncoids, intraclasts, mollusca, algae, foraminifera, stromatoporoids and coral debris constitute the major components of this sequence. The sequence boundary is characterized by nodular anhydrite (Handford et al., 2002; Meyer and Price, 1993) representing the thinnest observed boundary. Meanwhile, RHOB logs show an increase and the GR logs display a decreasing trend at the upper sequence boundary (Fig. 8). Two fourth-order sequence boundaries were identified in this sequence. The lower sequence boundary was highlighted by grainstone facies (F9) as the shallowest facies. Facies from F9 to F11 with gradual alteration are observed beneath this boundary. At the top of it, nodular anhydrite is known as the shallowest facies throughout sequence D (Handford et al., 2002; Swart et al., 2005). A gradual facies transition from facies F9 to F11 is evident near the lower boundary while an abrupt facies change occurs from facies F1 to F11 is seen at near the upper boundary. Eogenetic alterations in this sequence include micritization, marine cementation and dolomitization. The interparticle porosity, as a characteristic feature, contributes to the distinction of this sequence from the other sequences. The dolomitic units are observed in the limestone successions in this sequence and are composed of coarse crystalline dolomite with intercrystalline porosity, and unlike other sequences of the Arab Formation, the vuggy dissolution has not been observed. Other mesogenetic diagenetic features, including stylolitization and burial dolomitization, are recorded in the mid-ramp facies. Vuggy porosity is seen in the uppermost interval of the HST in dolomite lithology of this sequence which is occluded by poikilotopic anhydrite cement.

Fig. 9. Sedimentological log of the Arab Formation in the studied Field (B-02 well). The main stratigraphic subdivisions of the Arab Formation are also depicted along with the lithology, petrophysical logs, microfacies, facies belts, important diagenetic alterations, depositional sequences, anhydrite content and main pore types (color legend as Fig. 5). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

4.4.2. Sequence C This sequence is placed at the base of the Upper Arab Formation with a total thickness of 33–36 m (Figs. 8 and 9). This sequence is subdivided into three fourth-order sequences (Figs. 8 and 9). Sequence C begins with a TST, represented by high-energy shoal facies. High rate of dolomitization led to destruction of primary fabrics. Most sedimentary facies are assigned to the late TST and early HST. The MFS is characterized by seaward shoal facies, equivalent to the J80 according to Sharland et al. (2001). The late HST is associated with the deposition of evaporites, and the sequence boundary is identified by a remarkable

rise in RHOB and decreasing GR values (Figs. 8 and 9). This sequence is bounded by two sequence boundaries marked by nodular anhydrite (lower boundary) and an anhydrite layer (upper boundary). In contrast to sequence D, sedimentary facies (peritidal to shoal facies) gradually shift to the upper sequence boundary. In the early TST, intertidal facies (F2) (the shallowest facies) is considered as fourth-order sequence boundary. In the late HST, evaporite deposits are known as fourth101

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Fig. 10. Sequence stratigraphic correlation between wells (B-01 and B-02) (color legend as Fig. 5). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

sequence.

sequence boundary. Diagenetic processes such as reflux dolomitization, vuggy dissolution and anhydrite cementation in several systems tracts are visible. Stylolites and solution seams are seen in early TST and late HST. Dolomite cementation abundantly occurred in different positions of this

4.4.3. Sequence B The thickness of this sequence measures 31–45 m. This sequence is composed of two fourth-order sequences (Figs. 8 and 9). The TST is 102

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composed of tidal flat and lagoon facies. The MFS is distinguished by seaward shoal facies, containing bioclasts, oncoids and intraclasts. The maximum flooding surface can be correlated with the J90 of Sharland et al. (2001) in other areas in the Arabian Platform. The HST interval is recognized by the central shoal facies which displays an aggradational stacking patterns. Close to the sequence boundary, the pattern transforms into a lagoonal and intertidal facies with a progradational geometry. The sequence boundary is characterized by an anhydrite layer belonging to a sabkha environment which is associated with reduced GR and increased RHOB logs (Figs. 8 and 9). This sequence is bounded by two anhydrite layers at the top and bottom. Two fourth-order sequence boundaries were recognized in the early TST and late HST in which facies (F1 to F8) changed together gradually. The main diagenetic alterations affecting this sequence include: micritic envelopes in shoal and lagoon facies, reflux dolomitization with various crystal sizes, moldic pores in the tidal flat and leeward shoal facies, dolomite cementation in the grain-dominated facies, stylolitization in the mud-dominated facies, vuggy dissolution and anhydrite cementation in various facies.

history and sequence stratigraphy, a model is suggested to predict reservoir properties controlling factors in a temporal and spatial framework. Various depositional and postdepositional processes occurred in the studied unit as successive phases (marine, hypersaline and shallow to deep burial) and some of them were controlled by the sea-level fluctuations. Generally, marine and hypersaline diagenetic processes (calcite marine cementation, early dolomitization and moldic dissolution) are syndepositional, sensitive to sea-level changes, and follow depositional patterns (e.g. grain size, fabric, composition). But burial diagenetic modifications, that occurred during the late postdepositional conditions, are not directly connected to sea-level fluctuations. However, among diagenetic events, mesogenetic alterations follow facies geometry and so show some relationship with the sequence distributions. 5.2.1. Linking facies distribution, diagenetic history and sequence stratigraphy in normal marine conditions In marine conditions, the sedimentary facies assigned to various sub-environments dynamically accumulated in response to sea-level changes (Fig. 12). In the syndepositional diagenetic realm, early diagenetic processes influenced by sea-level fluctuations, developed in the Arab carbonate platform. The transgressive systems tract (TST) is accompanied by retrogradational geometry. Owing to the increase of the accommodation space, facies aggradationally repeated in the late TST and early HST. It is composed of grain-dominated facies (F9) along with marine calcite cement which is only found in the Lower Arab Formation. Periods of low sedimentation rate resulted in marine calcite cementation under the arid climate. High-energy shoal complexes (F9) are associated with circumgranular isopachous calcite cement as the first generation of cement, suggesting that high-energy conditions were dominant in the Arab Formation (Handford et al., 2002; Daraei et al., 2014). This cement shows a decreasing trend in volume toward below the fair-weather wave base (FWWB) in the mid-ramp facies (F10 and F1) because of a reducing hydrodynamic energy level. Poor early cementation allowed for further compaction in the mud-dominated facies. This cement has partially preserved interparticle porosity and noticeably enhanced the reservoir quality in the grain-dominated facies (F9) in the limestone intervals of the Lower Arab Formation. At the same time, low sedimentation rate had different effects on the mud-dominated facies and resulted in development of bioturbation in suboxic conditions of the lagoon (F5) and distal mid-ramp (F11) settings. Micritic envelopes frequently occur in the shallow facies such as intertidal (F4) and lagoon (F5) environments at the early TST and late HST (Fig. 12a 12d,). These environments have high a potential for activity of micro-organisms. In comparison, high-energy environments such as shoal were less subjected to micritization.

4.4.4. Sequence A The thickness of this sequence ranges from 20 to 29 m and contains two fourth-order sequences (Figs. 8 and 9). The TST begins with an anhydrite facies, followed by stromatolite boundstone, lagoon and shoal facies in a retrogradational geometry. The MFS is characterized by seaward shoal facies which corresponds to the J100 of Sharland et al. (2001) in other parts of the Arabian Platform. The early HST facies are dominated by central shoal facies, which accumulated with aggradational geometry, passing upward into supratidal facies that includes the Hith anhydrite (as known sequence boundary). By approaching to this boundary, RHOB increases but GR shows a decreasing trend (Figs. 8 and 9). Alternative evaporite deposits formed two fourthorder sequence boundaries (in the early TST and the late HST) that include F1 to F8 facies. Gradual alteration among these facies is similar to that of the other sequences. Micritization is frequently seen in the shallow facies of this sequence. Early dolomitization and moldic dissolution are prevalent in this sequence. Vugs and anhydrite cement similar to that of sequence C and B cross-cuts different facies. Solution seams postdate previous features (early dolomite, dolomite cement, dissolution and anhydrite cement) in the mud-dominated facies. The dolomite cement similar to that of the sequence C, is found in the early TST. 5. Discussion 5.1. Sequence model Our proposed conceptual dynamic depositional model for the Arab Formation in the studied sections represents facies variations from A to D members as a response to sea-level fluctuations (Fig. 11). In the Lower Arab Formation (Member D), the facies sharply transformed from mid-ramp to peritidal, but in the Upper Arab Formation (members A, B and C) facies are transitionally transformed from peritidal to shoal facies. As previously mentioned, two parameters including sea-level fall and arid climate during late HST controlled the development of evaporite deposits at the top of the Lower Arab Formation and within the Upper Arab Formation (in form of inter-layers). According to the Burchette and Wright (1992) model, supratidal environments surrounded the intrashelf basin. This suggests that regression occurred rapidly between the Lower Arab Formation and the Upper Arab Formation while this transformation is gradually within the sequences of the Upper Arab Formation (A to C members).

5.2.2. Linking facies distribution, diagenetic history and sequence stratigraphy in hypersaline conditions Increasing aridity during the late Jurassic led to prevalence of hypersaline diagenesis. The relative sea-level fall (late HST) resulted in development of hypersaline diagenesis, involving evaporite precipitation, dolomitization and fabric selective dissolution. Overlying evaporite deposits represent sequence boundaries with variable thickness. Percolation of Mg-rich hypersaline fluids through underlying successions caused seepage-reflux dolomitization. The main mechanisms suggested for dolomitization in the Arab Formation are sabkha and seepage-reflux (Alsharhan and Whittle, 1995; Cantrell et al., 2001; Swart et al., 2005; Lu and Cantrell, 2016). These two mechanisms are hydrologically and hydrochemically linked (Machel, 2004). Reflux dolomitization is observed throughout the Upper Arab Formation and shows pure dolostone (Swart et al., 2005) in the Lower Arab Formation through lime successions. To estimate dolomitization temperatures, stable oxygen isotope data are employed using Land's (1985) equation:

5.2. Linking facies distribution, diagenetic history and sequence stratigraphy By an integration of sedimentary facies distribution, digenetic 103

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Fig. 11. A conceptual dynamic depositional model for the Arab Formation in the studied section. This model represents facies variations from A to D members as response to sea-level fluctuations (color legend as Fig. 5). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

seawater (SMOW). Accordingly, low temperature of dolomite successions of the Upper Arab Formation (35 °C–41 °C) revealed that these dolomites formed near the surface (considering −1.2 SMOW of Jurassic seawater (Sellwood et al., 2000)) by evaporative fluids, as well as

T °C = 16.4 – 4.3 ([δ18Odolomite − 3.8] − δ water ) + 0.14

([δ18Odolomite

− 3.8] − δ water

)2

Where: T: temperature (°C); δ18Odolomite (PDB); δwater of Jurassic 104

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Fig. 12. Model for carbonate ramp in response to sea-level fluctuation, (a) normal marine conditions during deposition of the Lower Arab Formation led to widespread growth of coral and stromatoporoid patch reefs, micritization, marine cementation and bioturbation (in lagoon and basin settings). (b) Hypersaline diagenetic realm in the Lower Arab Formation. Hypersaline fluids circulated through permeable deposits and created dolostones in the limestone successions. (c) Burial diagenetic realm in the Lower Arab Formation led to formation of stylolite, burial dolomite and vugs. (d) Normal marine conditions during deposition of the Upper Arab Formation are similar to the Lower Arab Formation. (e) Hypersaline diagenetic realm caused sabkha and reflux dolomitization in the Upper Arab Formation. Fine crystalline dolomite formed in vicinity of hypersaline brines, and coarse crystalline dolomite formed farther away from the source of the hypersaline brines. Moldic porosity was also formed by undersaturated hypersaline brines in proximity of hypersaline brines. (f) Burial diagenetic realm in the Upper Arab Formation. Vuggy dissolution, anhydrite cementation, dolomite cementation and stylolitization formed in the burial diagenetic stage.

isolated this zone from the Upper Arab Formation. Recrystallization in the burial realm led to δ18O depletion (−4.5 to −4.9) in dolomite (Cantrell et al., 2001; Swart et al., 2005). Heavy carbon isotopes (1.2–3.1 δ13C) throughout the dolomites of the Arab Formation also

elevating salinity. These can result in a slight enrichment of oxygen isotopes (Swart et al., 2005) (Fig. 13a). Depleted δ18O of dolomite in the Lower Arab Formation could be the result of recrystallization during burial when evaporite deposits 105

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In fourth-order depositional sequences, fine crystalline dolomite (mud dominated facies including F1 to F5) formed in a peritidal area at the vicinity of the sequence boundary due to abundant nucleation, whereas in grain-dominated facies (F6, F7 and F8) (late TST and early HST) coarse crystalline dolomite was formed. It can be concluded that the size of dolomite crystal is related to the distance from the source of hypersaline fluids, as well as sedimentary texture. In other words, the saturation of magnesium decreases as a result of downward percolation of hypersaline brines into the underlying deposits which led to the formation of coarse crystalline dolomite in the grain-dominated facies (Lucia, 1999; Warren, 2000). Increasing size of dolomite plays a significant role in the improving reservoir quality of the Arab Formation (Fig. 12b and e). Moldic pores occur below the evaporite facies that are known as sequence boundaries of fourth-order and third-order sequences. Commonly, the interplay of freshwater and mineralogically metastable components (aragonite) leads to development of moldic porosity (Tucker and Wright, 1990; Moore, 2001). Although the Arab carbonate formed in the Calcite Sea (Husinec and Read, 2007) aragonitic fragments such as ooids and mollusca were abundant in the Upper Arab Formation (Alsharhan and Whittle, 1995; Cantrell and Hagerty, 1999; Cantrell, 2006; Hollis et al., 2017). This may have been caused by increasing salinity during the late Jurassic. Domination of the arid climate during deposition of the Upper Arab Formation is reflected by the presence of evaporite layers and absence of hyposaline diagenetic products (Wilson, 1981; Mitchell et al., 1988; Cantrell and Hagerty, 1999). The association of moldic pores with evaporite deposits, implies porosity formation in a hypersaline regime, as suggested by Sun and Esteban (1994). Seemingly, dissolution of some aragonitic fragments by brines was followed by precipitation of fine crystalline dolomite resulted in porosity inversion. This denotes that dolomitization occurred coeval with dissolution of allochems (Alsharhan and Whittle, 1995). Similar mechanism have been found in the Permian-Triassic Kangan and Dalan formations in the central Persian Gulf as reported by Mehrabi et al. (2014). Later pore-filling anhydrite cements occluded some of this porosity during the burial stages. Undersaturated hypersaline brines (with respect to aragonite) resulted in dissolution of aragonite fragments (Fig. 12e.). Significant dissolution in the form of moldic porosity is observed in F4, F5 and F6 due to proximity to hypersaline brines. Shoal facies (F6 and F7) show lesser moldic dissolution because of distance to the source of the hypersaline brines. The seaward shoal facies (F8) shows a lesser degree of dissolution. Generally, frequency and volume of this type of porosity decrease downward, through the early HST and late TST, owing to increasing normal salinity of seawater. Later diagenetic processes such as anhydrite cementation plugged moldic porosity during the hypersaline stage destroying reservoir quality. Moldic pores are absent in the sequence D and C owing to development of reflux dolomitization (before dissolution) which prevented dissolution of allochems. Moldic porosity development in ooid fragments beneath the sequence boundary of the Lower Arab Formation has been described by Cantrell and Hagerty (1999), Swart et al. (2005) and Cantrell (2006). They suggest that it could be due to leaching of allochems during subaerial conditions in the calcite lithology. In contrast to the present study, dolomite was not seen in the uppermost interval of the Lower Arab Formation in the studies.

Fig. 13. (a) δ18O values of the Arab succession. This figure depicts heavy δ18O values of the Lower Arab Formation and lighter δ18O values of the Upper Arab Formation.(b) Cross-plot of δ18O andδ13C values of samples from the Arab Formation. Based on geochemical data, dolomites of the Upper Arab Formation formed as early dolomite near the surface from the Jurassic seawater but the Lower Arab dolomites recrystallized in higher temperatures. Red points are burial dolomites of the Lower Arab Formation that show negative δ18O values. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

5.2.3. Linking facies distribution, diagenetic history and sequence stratigraphy under burial conditions In the Upper Arab Formation, dolomite cement occurs as overgrowth in shallow to medium burial diagenetic conditions (Fig. 12f.) predating burial dissolution events. This dolomite cement is mostly observed as overgrowth for the coarse dolomite crystals. The graindominated facies (F6 to F8) in the late TST and the early HST of the fourth-order sequences show larger dolomite crystals than the muddominated facies. Among sequences of the Upper Arab Formation, widespread dolomite cementation mainly occurred in the early TST of

indicate a lack of meteoric diagenesis (Swart et al., 2005) (Fig. 13b). Along with evaporation, as a main factor for reflux dolomite formation, anaerobic bacteria also facilitated dolomitization by sulfate ion elimination (by sulfate reduction mechanism) in greenhouse periods. Burns et al. (2000) demonstrated that the activity of anaerobic microbes could result in development of dolomites in periods of decreased oxygen levels such as the Late Jurassic. Apparently, during the Late Jurassic, integration of micro-organism activity (Riding and Liang, 2005) and evaporative pumping led to early dolomitization in the Arab Formation. 106

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Fig. 14. Correlation between sedimentological logs of this study with other areas of Arabian Platform, including Daraei et al. (2014) (central Persian Gulf), Azer and Peebles (1995) (UAE) and Al-Awwad and Collins (2013) (Saudi Arabia).

pores and anhydrite cement reflects the complexity of the diagenetic history. Accordingly, Mazzullo (1994) pointed out that mesogenetic dissolution cannot be placed in the sequence stratigraphic framework. Nevertheless, a relatively logical relationship is seen between the thickness of evaporite deposit and volume of vuggy pore space. It could

the A and C sequences. This is due to the presence of shoal facies and coarse crystalline dolomites in this position immediately above tidal facies. Vuggy dissolution and anhydrite cementation are not limited to special facies or system tracts. Genetically, the relationship of vuggy 107

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be related to high sensitivity of dolomite crystals to Ca2+ increase that occurred by dissolving overlying Ca2+-bearing evaporites (dedolomitization). Most commonly, dedolomitization is a process which concerns with dissolution of dolomite at first, and then this follows by calcite cementation of pore spaces. Depending on fluid saturation, vuggy pores could be filled by anhydrite cement in carbonate-evaporite successions (Baker and Kastner, 1981; Morad et al., 2012). Where cement is absent, there is a potential for the development of vuggy porosity (Raines and Dewers, 1997). Basinal fluids by dissolution of evaporite sediments provide an important source for Ca2+-rich fluids that may lead to dissolution of dolomite successions (in burial realm). The observed late anhydrite cement, occluding the vuggy porosity, indicates dedolomitizing fluids (rich in Ca2+), which originated from sulfate-rich fluids in the burial diagenetic environment. Thus, the evaporite sediments (equivalent to the sequence boundaries) significantly induced and enhanced dedolomitizing in the Arab Formation. Incidentally, there is a direct relationship between abundance and thickness of overlying evaporite and propagation of this late burial (mesogenetic) vuggy porosity. Therefore, frequent evaporite layers in the Upper Arab Formation led to creation of vuggy dissolution and reservoir quality improvement. On the contrary, vuggy dissolution is limited to below of the sequence boundary (beneath of the nodular anhydrite) in the Lower Arab Formation (due to low thickness of evaporites related to the sequence boundary in the form of nodular anhydrite). Pervasive and poikilotopic anhydrite cements, and vugs, show a clear relationship with the low thickness of evaporite in the sequence boundaries. Hence, the maximum anhydrite cement is observed in the Upper Arab Formation, whereas in the Lower Arab Formation, due to lower evaporite content, anhydrite cement occurs just beneath the evaporites of the sequence boundary. Intertidal (F2 to F4), lagoon (F5) and mid-ramp (F10 and F11) facies are dominated by stylolites and solution seams. These facies are located close to the sequence boundaries of fourth-order sequences (early TST and late HST), and stylolites diminish toward the MFS facies (shoal facies) in the Upper Arab Formation. Conversely, stylolitization increases toward the MFS of the fourth-order sequence, belonging to the mid-ramp facies in the Lower Arab Formation. Low hydrodynamic energy governing depositional setting, resulted in development of muddominated facies, and further compaction led to accumulation of impurities (Fig. 12d). Development of the sucrosic burial dolomites in the deeper facies (F10 and F11) of the Lower Arab Formation, associated with the stylolites, implies that Mg-rich clay minerals were available for stylolite formation in the deeper facies which supplied enough magnesium for dolomitization (Anan and Wanas, 2015) under burial conditions (Fig. 12c). This process is chiefly found at the near MFS of the fourthorder in sequence D. The burial dolomites occur as crystals with a bimodal grain sizes distribution. Seemingly, medium crystalline dolomite (100 microns), associated with stylolites, formed during burial, and later recrystallization increased crystal sizes from 300 to 250 micron. δ18O values of these dolomites are negative, similar to that of reflux dolomites in the Lower Arab Formation which may be due to recrystallization processes. Recrystallization generated coarse crystalline dolomite and also resulted in increasing porosity.

processes (diagenetic evolution) considerably influenced sedimentary facies and reservoir characteristics. Based on the petrographic analyses and other studies (Le Nindre et al., 1990; McGuire et al., 1993; Azer and Peebles, 1995, 1998; Daraei et al., 2014), four third-order sequences are recognized in this formation (Fig. 14), and all sequence boundaries are characterized by evaporite capping. Diagenetic processes in marine, hypersaline and burial settings overprinted the carbonates of the Arab Formation. Diagenetic evolution modified depositional features in various phases. The development of marine calcite cement and micritization were concurrent with establishment of a carbonate factory in the early HST. Sea-level fall resulted in hypersaline conditions and prevalence of hypersaline brines. Although this phase is associated with deposition of low-energy facies, hypersaline brines intensely affected the underlying successions, as well. The hypersaline diagenetic regime is characterized by dolomitization and moldic dissolution in the Upper Arab Formation. Just underneath of the sequence boundary, moldic porosity and fine crystalline dolomite formed. Farther away from the sequence boundary, moldic pores are fading but intercrystalline porosity was enhanced by coarse crystalline dolomite. Dolomite cementation occurred in the shallow to medium burial environment. This event was found close to the MFS facies (containing high-energy facies) due to the presence of intercrystalline porosity in the coarse crystal dolomite. Vuggy dissolution, caused by destabilization of dolomite in response to fluids rich in Ca2+, crossed through various facies, system tracts and sequence surfaces in the Upper Arab Formation. Anhydrite cement has the same sequence position as the vuggy dissolution in the Upper Arab Formation. Nevertheless, these diagenetic features indicate a clear relationship with the sequence boundaries by linking diagenesis to development of evaporite deposits (F1). In this regard, the frequency and thickness of the evaporite overlaying in the Upper Arab Formation are higher than in the Lower Arab Formation. Consequently, vuggy dissolution (mesogenetic) along with anhydrite cementation, occur extensively in the Upper Arab Formation, while these processes are limited to just beneath of the sequence boundary (nodular anhydrite form) in the Lower Arab Formation. In deep burial conditions, stylolitization and burial dolomitization occurred in the mud-dominated facies in the Lower Arab Formation. These facies developed during maximum flooding surfaces and belong to the Lower Arab Formation. They also occurred in the early TST and late HST of the Upper Arab Formation. In summary, this study reveals some important diagenetic features (eogenetic and mesogenetic environments) which have sensible relationships with the depositional processes in the Arab Formation and hence can be correlated within the sequence stratigraphic framework. This is evident in the relationships between facies characteristics (e.g. grain size, fabric, composition) and diagenetic alterations. Furthermore, identification of these features within this framework would enable us to create a basic conceptual 3D model for the reservoir characteristics and its spatial and temporal variations. Acknowledgments The authors are grateful to the University of Tehran provided facilities for this study. We also thank the IOOC (Iranian Offshore Oil Company) for data preparation. The authors also thank SH Dashtgard and AH Enayati for invaluable suggestion and comments. We also greatly appreciate reviews and important comments and corrections by H. Wanas and an anonymous reviewer.

6. Conclusions The Arab Formation was deposited under arid conditions. Its depositional and diagenetic features are controlled by the effect of glacioeustasy and climate. Petrographic studies of the Arab Formation in the central Persian Gulf led to the recognition of 11 sedimentary facies were deposited in five facies zones, including supratidal, tidal flat, lagoon, shoal and mid-ramp belts. Analyses of sedimentary facies preliminarily indicate that the Arab Formation was deposited in the ramptype platform. This is evident by the lack of barrier reefs, the low diversity of facies types and expansion of ooidal shoal. Postdepositional

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