Microfacies Analysis and Diagenetic Effects of Middle Jurassic Succession in Rania and Sargelu Sections, Northern Iraq

Abstract


Introduction
The Sargule Formation (Middle Jurassic) represents one of the most important source rocks in Iraq and surrounding areas.It cropped out with other Jurassic formations in northwestern and northern Iraq as small scattered patches within a narrow belt that trending from (Halabja) in the SE to (Duhok) in the NW such as Halabja, Hanjira village northwest Rania town, Gara mountain in Gali-Zewki south of Amadia, Naokelekan village near Rownduz and Banki village north east of Zakho.Wetzel (Bellen et al., 1959) described the lithology of the Sargelu Formation in Surdash anticline.The type section is 115 m composed of thin-bedded black bituminous limestone, dolomitic limestone and black papery shales with streaks of thin black chert bands.The Sargelu Formation was deposited euxinic marine environment, with some inlayers showing shallowing or aerated condition.The shallower and better aerated environment occurred mainly on the stable area (Jassim and Goff, 2006).This study concentrates on two of the outcrops, namely.Sargelu and Rania sections, it represents an attempt to exploit the information gathered from petrographic and microfacies analyses for the studied succession in deciphering the depositional and diagntical characters.
The studied area covers two outcrops they are located within the Sulaimaniya in northeastern of Iraq, the first section is bounded by latitude (35°52′2.0″)North, and longitude (45°09′ 49.0″) East near Surdash village 180 m thick while the second with thickness of 40 m lies within latitude (36°17′11.8″)North, and longitude (44°51′ 17.2″) East near Hanjira village two kms north west of Rania town (Fig. 1).The previous studies by many researchers concentrated on stratigraphy, sedimentology, palynofacies and microfacies analysis of the Sargelu Formation.This research tries to provide a petrographic descriptions of the associated sediments of the Sargelu Formation better understanding of the depositional and diagentical history, so as to reach.
The contact between Sargelu and Naokelekan formations is sharp and conformable and characterized by black thin bedded, highly bituminous shales with thin bands of black chert at the top of the Sargelu Formation, abruptly followed by well bedded grey limestone rich in Stromatolites of the Naokelekan Formation (Al-Fraiji, 2008).This succession is containing abundant oil-prone organic matters, and they concluded the generation and expulsion of the hydrocarbon from the Sargelu Formation began and ended in the late Miocene succession (Al-Ameri et al. 2009).The study aims to analyze microfacies related to depositional environment and diagenetic effects of the Sargelu Formation in Rania and Sargelu areas.

Materials and Methods
The current study was completed in two stages: • Field work covered two sections of the Sargelu Formation and it involved both description and sample collection from Rania and Sargelu Sections.The area of study was divided into two sites (42) samples were collected from Sargelu Section covers a thickness of 38 from Rania Section sampling was conducted to cover the major lithologies when include limestone, chert, shale and dolomitic limestone of these sites and in a line perpendicular to the strike of the exposed beds.• Laboratory stage; during this stage the samples were chosen and made thin sections.The petrographic study and diagenetic features determination are based on the study of thin sections of selected samples in studied sections.

Geological and Tectonic Settings
The Sargelu Formation was described as the first time by Wetzel in 1984(Bellen et al., 1959) at the Surdash structure within the High Folded Zone in Northeast of Iraq.The lithologic of this succession is consist of thinly bedded limestone, black bituminous, dolomitic limestone, and black laminated shale with thin bands of black chert in the upper part (Bellen et al., 1959).
The same lithological composition occurs along the whole area of outcrops as well as in the wells.The thickness of this succession in the type locality is 115 m while in the subsurface section on the Low Folded Zone and Mesopotamian Zone of Unstable shelf, the thickness is considerably higher and varies between 250-500 m, such as described by Ditmar et al. (1958) in Jassim and Goff (2006).
According to the evidence of fossils, the age of the formation has been determined as upper most liassic at the lower part and Bathonian at the upper part (Bellen et al., 1959).The depositional environment of succession is euxinic with some inlayers showing either shallowing of better aerated condations.The shallower and better aerated environment occurred mainly on the stable shelf area (Jassim and Goff, 2006).The boundaries of the Sargelu Formation are conformable surface and gradational as underlying Sehkanian Formation and overlying Naokelekan Formation.(Figs. 2 and 3).(Aqrawi et al., 2010).Fouad (2015) divided Iraq into several tectonic zones (Table 1).The Jurassic rocks are commonly exposed is isolated patches at some eroded parts of cores and limbs of anticlines in the High Folded, Imbricated, and Thrust zones of northern Iraq.The sections are as follow: • Sargelu Section: this is name of Sargelu village that is located within Unstable Shelf, High Folded Zone, including Surdash anticline.• Rania Section: this is name of Rania City near Hanjira village that is located within Unstable Shelf, High Folded Zone, including Shaweri anticline.Baban and Ahmed (2021) and Mamaseni (2020) they are studied the source rocks potentiality for the Sargelu Formation in Taq Taq Oilfield.They was suggested that succession is characterized by three microfacies are; foraminiferal packstone, grainstone microfacies, fossiliferous packstone microfacies, and foraminiferal wackestone which were deposited in middle and outer carbonate ramp.

Microfacies Analysis
The term "Microfacies" was suggested by Brown (1943in Flugel, 2010).It is the total of all the paleontologic and sedimentologic criterias which can be identified by thin section, peels and polished slabs (Flugel, 2010).Microfacies are one important way which reflects the environmental and depositional conditions.In this study, the classification of Dunham (1962) is used.In the Sargelu Formation, three types of microfacies are recognized in the two sections.

Lime Mudstone Microfacies
This microfacies consists of microspar; psendospar only with rare fossils content, generally less than 10%.This microfacies is found almost in all studied sections.In this microfacies there are some diagenetical features such as fractures, stylolite and framboidal pyrite or as cubic pyrite that found in Sehkaniyan Formation.This microfacies is concentrated in the upper part of Sargelu Section (Plate 1A).

Lime Wackestone Microfacies
This facies lies in common facies range between more than 10% and then the Lime Mudstone framework grains in a micritic matrix.These grains include the shells of pelagic pelecypod (Posidonia) and calcicsheres.Wackestone can be subdivided into two submicrofacies.

Calcisphere-posidonia lime wackestone submicrofacies
This submicrofacies is identified in the Sehkaniyan Formation which is rich in calcisphere and Posodonia.The calcisphere may be of radiolarian origin because there is remnants of radiolarian shells; these shells might have undergone replacement by calicite resulting in calcisphere, (Plate 1B).

Posidonia lime wackestone submicrofacies
This submicrofacies is recognized in Rania Section only, the main component of this microfacies is Posidonia that occasionally show sometimes cementation by calcite, (Plate 1C).

Lime Packstone Microfacies
This micriofacies is related common and appeared in thin sections as skeletal grains reaching up to 60% leaving minor micrite in between grain supported limestones.Two submicrofacies are recognized.

Posidonia lime packstone submicrofacies
This submicrofacies is observed in the upper part of Sargelu Section and lower part of Rania Section.Rich in thin shells of pelagic Posidonia in a decreasing order cementation by calcite (Plate 1D).

Posidonia-ostacoda lime packstone submicrofacies
This subdivision is found in both Sargelu and Rania Section.This submicrofacies consists, mainly of Posidona and Ostracoda.Shells and dolomite grains are noted in this facies, compaction and dolomitization have affected these beds (Plate 1E).

Lime Crystalline Microfacies
This microfacies is recognized in the middle part of Sargelu Section and in the upper part of Rania Section.It may be a diadentical submicrofacies in which calcite and to a lesser extent dolomite crystals are observed.Dolomitization may be an effective process (Plate 1F).

The Contacts of the Sargelu Formation
The upper contact is apparently conformable and gradational; also the upper part of the Sargelu succession is characterized by rich in posidonia and Ammonite.The first bed of the Naokelekan Formation is contented Stromatolites and extinct posidonia and Ammonite.The stromatolites are produced by algal and/or cyanobacterial mats, which deposited in the restricted circulation platform and tidal flats, in addition to sabkha and salinas (Wilson, 1975).The planar stromatolites are formed in the intertidal environment.(Plate 2A,B,C,D,E and F) This boundary is recognized by disappear posidonia and Ammonite at the first bed of Sehkaniyan Formation and observe algae and Haurania from benthonic foraminifer.Haurania was used by Al-Fraiji (2008)

Diagenesis
Diagenesis of the carbonate rock is more varing than that of clastic rocks particularly, because of the metastable nature of carbonate minerals (Bathurst, 1975).The solution passing through pores and cavities of carbonate rocks promotes alteration in mineral composition (Wood and Hewett, 1987).Diagenetic processes, as known from texts, which including physical, chemical and biological changes affecting upon the sediments since their deposition, and then lithification and emergence on ground surface under the normal pressure and/or heat (Larsen and Chilingar, 1979).Diagenetic process effects on the Sargelu succession are: Micritization, Cementation, Neomorphism, Recrystallization, Dolomitization, Silicification, Dissolution and Compaction.

Dolomitization
Dolomite is commonly recognized in the rocks during all ages by its typically rhombic crystal form.However, in many ancient rocks, dolomites can often be found as mosaic of anhedral crystal with irregular intercrystalline boundaries and undulatory extinction.Dolomite has also been recognized as replacement minerals mineral (Blatt et al., 1980).Dolomitization requires rise in Mg/Ca ratio in saline water that enter pores and interstices of carbonate rocks (Folk and Land, 1975).Dolomitization can hardly precipitation directly from seawater, because it requires water of salinity greater than of seawater (Bathurst, 1975).And the provision of Mg+2ions remains the main factor for the process (Longman, 1980).This model is interpretation dolomite formation of late-stage in the all sections; its evidence was percent in the samples, the presence of dolomite from clay is relevant to hydrocarbon exploration and production in that both the dolomite and hydrocarbon may originate from the same shale.(Plate 4A, B & C).

Cementation
Cementation is a chemical depositions of calcium carbonate from the saturated solution, between or inside grains or in the pores and cracks which resulted by solution , all leading to growth of spary calcite in these cavities (Larsen and Chilingar, 1979).In cases of allochthous limestone, cementation causes lithification, while authochthonous carbonate rocks may merely undergo decrease in porosity and permeability (Chilingar et al., 1967).Most cementation in carbonate occurnce at relatively shallow interval in one of four major diagenetic environments; the vadose, meteroric, mixing and marine phreatic zones (Longman, 1980).Cementation in Sargelu Formation in not a dominant process in all sections, however there are many types of cement are noted in the sections such as; • Drusy cement that identified by Flugel (2010) as consists of anhedral to subhedral calcite crystals, crystal size increase from wall of pores to center of cavities.Drusy cement found commonly in sections that filled the outside chambers of Psoidonia (Plate 4D).• Granular cement consists of nearly equidimensional, anhedral to subhedral calcite crystals, always forming after lithification and compaction of the sediments and after formations of cement (Bathurst, 1975).This type of cement observed filling Ammonite and Ostracoda (Plate 4E).• Blocky cement refers to a mosaic of crystals in which the individuals have roughly the same diameter in all direction (Blatt et al., 1980).This type is not commonly in the sections (Plate 4F).

Neomorphism
The term Neomorphism was first identified by Folk (1965) which including all transformations between one mineral and itself or a polymorph.Within this term, Folk included both recrystallization and inversion.
The recrystallization processe has been used loosely for a number of processes that commonly cause a changes in crystal shape or grain size, predominantly an enlargement and occasionally a reduction in size, without causing a chemical alteration (Chilingar et al. 1979).Inversion is the process by which unstable minerals such as high-Mg calcite and aragonite change to a more stable from low-Mg calcite of the same chemical composition (Chilingar et al., 1979).
Neomorphism of marine sediments is attributed to meteoric phreatic conditions (Longman, 1980).The degree of neomorphism may change according to depositional environmental.The basianl sediments are least affected, where's the reef and lagoonal limestone are most extensively recrystallizated (Chilingar et al., 1979).Neomorphism effect is obvious on rocks of Sargelu sections particularly with shells of Posidonia (Plate 5A&B).The neomorphism process is merely an alteration of microcrystalline calcite (micrite) to microspar and pseudospar (Plate 5 A & B).

Silicification
Silicification, like the dolomitization process can take place during early or late atages of diagenesis.It takes the form of selective replacement of the fossils or the development of the chert layers and nodules (Tucker, 1981).Silicification in sargelu sections is common diagenetic process, which occurs particularly within the upper part of formations in all sections.The source of silica for this process is likely to be from solution of radiolarian skeletons commonly found in Sargelu sections, include sponge spicules, diatoms and radiolarians (Tuker, 1981) (Plate 5 C & D).

Compaction
Compaction process is refering to any process that cause decrease in the bulk volume of the rocks and affects carbonate sediments at the deep burial stage, where depth of overburden reaches up to 1000 m (Flugel, 2010).Compaction of sediments resulted in reduction of sediment thickness and intergranular porosity (Meyers, 1980;Shinn and Robbin, 1983).The degree of compaction is strongly influnenced by early cementation, which may inhibit or diminish the compaction (Flugel, 2010).Chilingar et al., (1979) had listed some factors that cause increase in compaction such as: • The ratio of fine to coarse sized particles.
• Amount of clay materials.
• Increasing in void ratio.
• Increasing in load of overburden.Finally, Tucker (1992) subdivided compaction in to two categories: • Mechanical compaction.

Stylolite
Stylolite are thinly zones of the discontinuity within carbonate rocks, in thin section they appear as undulated to zig-zig sutures (Flugel, 2010).Stylolite results from pressure solution, dissolution of the limestone along planes as a results of overburden or tectonic pressure (Wanless, 1979).Stylolite seems to be a common phenomenon in Sargelu section.The stylolite described by Wanless (1979), only two types of stylolite are noted.
Sutured-seam stylolite this type forms within unite having structural resistance and little or no platy insoluble minerals (Wanless, 1979).It is appeared in Rania Section at lower part and disappears in the Sargelu Section (Plate 5E).Non-sutured seam stylolite occurs in limestone that have significant amount of clay and platy silt (Wanless, 1979).It is showed in Sargelu Section at upper part (Plate 5F).
These microfacies are variably affected by several diagenetic processes including mechanical and chemical compaction, cementation, recrystallization and dolomitization.The chert shows a high degree of quartz crystallinity which indicates a diagenetically product from the amorphous silica of the many radiolarians, remnant of which is preserved in several units of the Sargelu Formation (Figs. 5 and 6).

Fig. 1 .
Fig.1.A geological map and setallite image showing the location of the two studied sections

Fig. 4 .
Fig.4.Main tectonic zones of Iraq(Fouad, 2015) for correlation between Sargelu and Sehkanayin Formations in Barsarin and Sehkaniyan Sections.BouDagher and Bosence (2007) studied the Early Jurassic benthic foraminifera diversification and biozones in shallow marine carbonate environment of western Tethys and recognized Haurania.The fossils of Haurania, Posidonia and Ammonite are authoritative in regional correlation according to (Krumbein and Sloss, 1963; Sharland et al., 2001) (Plate 3A, B, C, D, E and F).