Diagenetic control on porosity types of carbonate rocks of Late Triassic Baluti Formation in Northern Thrust Zone, Iraqi Kurdistan region, Northern Iraq

The carbonate rocks of Late Triassic Baluti Formation in Sararu outcrop, Northern Thrust Zone, Northern Iraqi Kurdistan region consist of brecciated marly limestone and sandy dolomitic limestone interbedded with shale that characterized by complex heterogeneity. The microscopic description of the porosity types and diagenetic processes controlled it were studied in current work. The petrographic study of 12 thin section of Baluti carbonate displayed that the main skeletal grains are shallow marine derivative fossils and non-skeletal grains are only monocrystalline quartz. The main matrix is micrite which sometimes changed to microspar and sparry calcite. Seven pore types were distinguished in the studied rocks of the formation from both primary and secondary stages. The primary porosity are interparticle and intraparticle pores. While, the secondary types were moldic, intercrystalline, fracture, vuggy and stylolitic porosities. The formation in studied outcrop was undergone in several diagenetic processes, that enhanced or reduced the porosity of the Baluti carbonates. Early micritization protect the primary pores from destroying by compaction. While, solution was assists in creating different secondary pores in different diagenetic stage of the formation particularly the dominant moldic porosity. Calcite cements aid in reducing all pore types in the studied formation. Late dolomitization contribute to formation intercrystalline and dolomoldic porosities. stylolites with dissolution helps to creating stylolitic porosity. Neomorphism, silicification and pyritization generally reduced porosity in the formation.

The porosity had a significant consideration in carbonate rocks because about half of the hydrocarbon reservoirs in the world formed from carbonates which most of them deposited in shallow marine settings [1].The porosity in carbonate rocks is complex due to difference in type, shape, size and grain components which known as heterogeneity.The reason of this heterogeneity is variance in depositional environment and later modification caused by diagenesis [2].On the other hand, the main factor controlling the primary pore types and subsequent formation of secondary pores in carbonate rocks is diagenesis processes [3].It is defined as a sedimentary manner include all physical and chemical changes, that take place subsequent to deposition and before the metamorphism [4].It is led to changes in the primary depositional shapes [3].Generally, dolomitization and dissolution increase reservoir quality of carbonate rocks and cementation decrease it [5].
The Triassic units in northern Arabian Plate had considerable interest due to recently hydrocarbon exploration in Kurdistan region of Iraq in last two decade and they contribute in the petroleum system formation.The Upper Triassic Baluti Formation regarded as source, reservoir and seal rocks in the Triassic petroleum system of Iraq [6,7].Wetzel in 1950 and Morton 1951 was first who introduced and amended the Baluti Formation in its type locality in Baluti village, southern Amadiya, in the Chia Gara anticline [8].In type locality, it consists of green and grey shales, dolomitic limestone, intercalated with thin-bedded limestones, silicified limestones and solution-recrystallization breccias.
There are several works concerned to sedimentological, paleontological and petrophysical studies of the formation in northern Iraq but no one study a relationship between porosity and diagenesis of the formation in the studied outcrop.The interested studies are the following: Buday [9], Jassim et al. [10], Hanna [11], Aqrawi et al. [6], Shingaly [12], Asaad and Omer [13], Lunn et al. [14], Azo et al. [15], Asaad and Omer [16], Asaad et al. [17] and Mohialdeen et al. [18].The main goal of the current paper is clarifying porosity types formed on the Baluti Formation in the Sararu area and diagenetic control of them.

Geological setting
The Baluti outcrops are restricted in limited anticlines of the high folded, imbricated and northern thrust zone of Iraq [19].Whereas, in the drilled wells, it occurred in all tectonic zones involving the wells: Bakirman-1, Bjeel-1,2,3,7, Bekhma-1, Gulak-1, Jabal Kand-l, Khlesia-l, Atshan-l, and Wadi Kifl-l [9,10,20].The studied section of Baluti Formation is situated in Sararu village approximately 10 km south of the Iraqi border with Turkey, about 6 km northeast Qumri village and 1 km north Aden village on the paved way of Barwari Bala in Sararu ravine, on latitude 37° 14′ 59″ N and longitude 43° 17′ 52″ E (figure 1a and c).Structurally, Sararu section sited on the southern limb of Ora anticline in the Northern Thrust Zone.The zone formed a slim domain (10-15 km) had East-West direction in parallel with Iraqi-Turkish borderline in Dohuk governorate, northern Iraqi Kurdistan region (figure 1a).The Diari region and the Hizl River marked its boundary from the east and the west respectively [21].It farther extends toward the south, creating Kiri-Diari Mountain series, where the suture zone is situated inside the Turkish country.The studied region is subjected into two main thrust faults which are called the southern thrust and northern thrust which both displacemented to the south and exhibit east-west tendency [22,23].Stratigraphically, the Mesozoic age units occupied the southern limb of Ora anticline.The anticline is characterized by intensive normal and reverse faulting [21,22,26] which cause overturns of its stratigraphic units and older formations set over the above younger ones (figure 1b).
Lithologically, the Baluti Formation in Sararu section had of 90.5 m thick (figures 2a and b) the lower 42.5 m consist of the alternation of fractured thin to medium of yellow to grey sandy dolomitic limestone bearing bivalves and laminated dark grey shale and blue shale (figure 2c).Whereas, the middle part comprises of 36 m of thick to very thick bedded of grey, yellow to orange sandy dolomitic limestone intercalated with grey shale (figure 2d).Its carbonate characterized by fractures, joints and veins stuffed by iron oxide and calcite cement.Whilst, the upper part had 12m of medium to thick bedded yellow to grey brecciated marly limestone characterized by fracturing (figure 2e).The lower contact is conformable and gradational with the Upper Triassic Kurra Chine Formation and marked over the massive saccharoidal dolomite of Kurra Chine rocks.Moreover, the upper contact is conformable with Sarki Formation and taken between brecciated marly limestone of Baluti Formation and thick to massive dolomite beds of Sarki Formation (figure 2b).

Methods and Materials
The field works was obtained by describing and measuring the studied surface section in details in addition documenting the lithology, determining the mineralogy, grain size, and macrofossils.The common lithology in Sararu outcrop is marly limestone, dolomitic sandy limestone, and shale.The sum of 12 samples were collected from the carbonate rocks of the Baluti Formation.Furthermore, several samples were collected from underlying and overlaying formations to determine accurate position and nature of their boundary with Baluti Formation.The collected samples were taken in every change in lithology (random sampling) and lower and upper surface of each sample was marked.
The same numbers of thin section (one thin section for each sample) were intended in the Earth Sciences and Petroleum department workshop in Salahaddin University-Erbil.The slabs of the samples were cut perpendicular to bedding plane and then made to thin sections.The alizarin red solution (ARS) was used for staining the thin sections following the Friedman [27] procedure to discriminating between calcite and dolomite.The microscopic study was done by polarized microscope and the study of porosity types were depended on the Choquette and Pray [28] classification scheme.

Diagenetic impact
Carbonate rocks of the Baluti Formation in Sararu outcrop affected by several diagenetic processes in different time stages (figure 4).The earlier diagenetic processes in studied rock are micritization which are the product of borer organisms (endolithic algae or fungi) [29] in stagnant marine phreatic environment [30].It common in the lower part of studied formation and occurred as rims along foraminifera tests (figure 3a) and algae shells (figure 3d).Both types of dolomitization early and late were seen in the carbonate rocks of the studied formation.Early dolomitization it is also called syngenetic dolomitization and formed during deposition or immediately after limestone forms through a few days, weeks, or months [31].It common in upper part and restricted samples in lower part of the Sararu section and had very fine to fine dolomite crystals scattered within the micrite groundmass of the Baluti Formation (figure 3i).Whereas, late dolomitization usually have coarser crystals dominant in the middle part of the studied outcrop (figure 5a).It thought to be formed after sediment lithification by Mg-rich solutions influence and generally demolished the rocks original texture [32].It usually takes place after years or even millions of years and called epigenetic dolomitization [31].The common cement types of the Baluti carbonate in the studied outcrop are granular calcite cement (figure 3g) with less blocky calcite cement (figure 5b) and rare syntaxial overgrowths cement (figure 3j) which each of them indicating to the certain diagenetic environment.Granular calcite is initiated in marine vadose environment.While, blocky calcite and syntaxial overgrowths cements created in meteoric phreatic environment [4].The physical compaction found in different parts of the studied section represented by over close packing (figure 5c), breaking of and deformation of grains (figure 5c) in addition to fractures that occasionally are filled by sparry calcite cement (figure 3f).Chemical compaction (stylolite's) which is common in the middle part of the Sararu outcrop and in the form of non-sutured seam stylolite anastomosing (figure 5d) and parallel set (figure 5e) according to Wanless [33] classification.
The other important diagenetic processes which supposed to be formed in early and continuous to the middle stage of diagenesis is solution.It is observed in the different portions of the studied outcrop and created by solution of metastable minerals especially high-Mg calcite and aragonite [34].Neomorphism also prevalent in the whole studied section of the Baluti Formation and it obliterated the preserved fossils in Baluti carbonates (figure 5f).formed stylolitic porosity.SB6, X.N. f) Skeletal grain possibly is foraminifera highly subjected to neomorphism (red arrow), BS.9., P.P.
Silicification which regarded as replacement of carbonate by silica or silica cement precipitation in pores [4] is obvious in different part of studied section (figure 6a).Glauconitization process also have minor role in a paragenetic sequence of the studied formation, particularly in the lower and upper part (figure 6b).It is referred to marine setting, relatively shallow environment and slow sedimentation [4].Pyritization is probably enhanced by decay of organic matter, which is caused by anaerobic bacteria, or sulfate solution by reducing bacteria and creating different forms of pyrite [35,36].It is frequent in the studied section in the forms of small cubic pyrite (figure 5f), framboidal pyrite (figure 6b) and moldic or fill pyrite (filled the different fossils) (figure 6a).Tectonic related fractures which are result of uplift in the late diagenetic stage is observed in some samples of the studied section and mainly filled with calcite cements (figure 6c).

Porosity types
Several pore types from both primary and secondary formation were recognized in the studied rocks of the Baluti Formation in Sararu section.The recognized porosity types mainly depended on the Choquette and Pray [28] classification.The discriminated primary pore types which usually created before or during the depositional phase [4] are interparticle and intraparticle pores.The first is fabric selective and formed in the spaces or voids between particles before diagenesis and sometimes during diagenesis by dissolution of cement and matrix [37].It is occurred in the lower part of Sararu outcrop (figure 6d).The second is also fabric selective pores formed within the allochems that not filled subsequent diagenetic processes and usually with solution of the soft organic portion from the carbonate skeleton [38] (figure 6d).
The secondary porosity includes moldic, intercrystalline, fracture, vuggy and stylolitic porosities in the Baluti carbonates.It is ordinarily formed by diagenetic processes and after deposition [4].The moldic pores which are dominant pore types in the studied rocks of Baluti Formation are usually created by the selective elimination of primary components from sediments or rocks and they are either fossils molds like those of mollusks and calcareous algae (figure 6d) or dolomite molds or dolomolds under the impact of fresh water on their principal susceptible minerals (figure5a).While, Intercrystalline porosity is avoids that formed between dolomite crystals and is dominant in the middle part of Sararu outcrop (figure 5a).Fracture pores are nonfabric selective forming due to tectonic activity or compaction.This kind of porosity is observed in several thin sections of the studied formation (figure 6a) which most of them filled by blocky and granular calcite cements (figure 5b).Other porosity type is Vuggy pores, which are created by the expansion of fabric-selective pores particularly moldic and intraparticle pores [39] (figure 5a).They are common in the upper part of Sararu outcrop.Stylolitic porosity is commonly created by stylolites and they form some pores around their sutures when they passed by of CO2-rich solution [40] (figure 5e) which is restricted in the middle part of section.

5.Discussion
The carbonate rocks of Upper Triassic Baluti Formation in Sararu section undergone different diagenetic processes, particularly it was located in the high tectonic activity zone (Northern Thrust Zone).These processes may enhance or diminish the porosity of the Baluti carbonates (figure 7).
Primary porosity (interparticle and intraparticle) are usually created before diagenesis during deposition.Interparticle porosity is usually occur within medium-to high-energy depositional settings in carbonate rocks [37].This indicated by the depositional condition of the lower part of the Baluti Formation in studied section which deposited in highly agitated water shoal environment composed of grainstone microfacies [16].The interparticle pores in Baluti carbonate exhibit in patchy distribution that indicate it formed by selective dissolution of interparticle matrix and cement during diagenesis, and patchy cementation of both primary and secondary interparticle pores [37].The intraparticle pores formed by solution of the soft organic parts from the skeletal wall of preserved fossils.The early micritization may save primary pores from reducing by later compactions [41] and indicated by occurrence with these pores in the Baluti Formation in the studied section (figure 4).On the other hand, mechanical compaction particularly overclose packing of grains reduced primary porosity in the studied formation.Granular calcite cement which started created during eogenesis in marine vadose environment and formed on the skeletal walls and fractures also reduced the intraparticle, moldic and fracture porosities.Blocky calcite cement which formed during mesogenesis to telogenesis also reduced secondary porosity.Late dolomitization which characterized by coarse dolomite crystals and intrercrystalline pores formed between them in addition to dolomolds due to the effect of freshwater on their principal susceptible mineralogy [13].It usually enhances reservoir properties of the carbonate rocks [42].It took place during mesogenetic stage [43].The important enhanced porosity diagenesis type of Baluti carbonate studied outcrop is solution.It takes place in predominantly in high-Mg calcite and aragonite [44] particularly in the presence of fresh water.It contributes to forming the moldic and vuggy porosity.Stylolites had a role in the formation of stylolitic porosity when they enlarged by passing CO2-rich solution.Fracture porosity formed during diagenetic stage either by physical compaction or during uplift by tectonic activity.Neomorphism, silicification and pyritization had a negative role in porosity evolution of the Baluti Formation and reduced pores particularly secondary types.

6.Conclusions
The petrographic components of carbonate rocks of Upper Triassic Baluti Formation in Sararu section include skeletal grains that mainly shallow marine derivative faunas and consist of benthonic foraminifera, dasycladacean green algae, calcareous red algae, ostracods, brachiopods, bivalves, calcispheres, echinoderms, ammonoids and bioclasts.Whereas, non-skeletal grains are only extraclasts and mainly monocrystalline quartz.The main groundmass of the of studied carbonate rocks is micrite which occasionally transformed to microspar and sparry calcite.Seven pore types were discriminated in the Baluti carbonates in studied outcrop from both primary and secondary origin.The primary porosity includes both interparticle and intraparticle kinds.Whereas, the secondary types were moldic, intercrystalline, fracture, vuggy and stylolitic porosities.The formation in studied outcrop was undergone in different diagenetic processes, that enhanced or

Fig. 2 .
Fig.2.a) Lithological column of the Baluti Formation in Sararu area.b) Field photograph showing the Baluti succussion in studied outcrop with lower and upper contacts.c) Field photograph of fractured thin to medium sandy dolomitic limestone interbedded with laminated grey shale.d) Field photograph of thick bedded sandy dolomitic limestone intercalated with shale.e) Field photograph of brecciated marly limestone.

Fig. 7 .
Fig.7.Schematic diagrams show the stages of diageneses influenced the porosity evolution in carbonate rocks of Late Triassic Baluti Formation in Sararu section.