Stratigraphic Sequence and Diagenetic Effects of the Upper Qamchuqa Formation in Khabaz and Kirkuk Oil Fields, Northern Iraq

Abstract


Introduction
The Upper Qamchuqa Formation in the current study is represents a part of the Wasia Group which deposited in the Zagros foreland basin during the Early Aptian-Late Albian age.This paper including petrographic, microfacies determination, digenetic features and stratigraphic sequence for two wells at Kirkuk and Khabaz oil fields.
The studied oil wells sited in northeastern part of Iraq within Kirkuk and neighboring areas (Fig. 1).These studied oil wells located in the within foreland basin in the Unstable Shelf (Low Folded Zone), bounded from the southeast by the Arabian Platform and the northeast by the Zagros Mountains, and consisting of linear and high folds with a trend from northwest to southeast in general.The folds in this area are affected the Mesozoic and Cenozoic eon (Henson, 1951 andPowers et al., 1966) Fig. 1.The location map of the studied oil wells with tectonic settings modified after (Al-Zaidy, 2013) Al-Zaidy et al. (2014) studied the Lower Aptian -Upper Albian sequence in the Zagros foreland basin, this study is included the petrographic and microfacies analysis which enabled to recognition the depositional environments and basin development.Abid et al. (2015) are studied the facies associations and development the diagenetic features during the Albian-Early Turonian period in Luhais Oil fields, Southern Iraq.While Al-Zaidy and Amer (2021) suggested that Albian sequence was deposited during two cycles of transgression stages, where showed deepening-upward in both cycles.The unconformable surface between these successions have been identified by the observation of glauconite detritus mineral and detected by well logs response in depth 2880m in well Balad-1.

Materials and Methods
The present study depends on data, that are got from two wells which belong to north and center Oil Company, the methodology includes: • Petrographic and microfacies analysis are based on examination of 100 thin sections of cuttings and core samples by transmitted microscope for the selected wells (K-229 and Kz-4).
• Analysis of gamma ray, density, neutron, and sonic well logs was used to study the volume of shale, and porosity types related to diagenetic development (Fig. 2).

Stratigraphy and Tectonic Settings
The studied oil fields lied in the northeast part of the Arabian plate within Unstable Shelf on the Foothill Zone (Fig. 1).The Qamchuqa succession in the study oil wells mainly represented by the Lower Qamchuqa Formation, Upper Sarmord Formation and Upper Qamchuqa Formation in middle and eastern parts of the area, while in the western and the southwestern parts the Jawan Formation is existing.Bellen et al. (1959) identified and described the Qamchuqa Formation from the Qamchuqa Gorge in the High Folded Zone where it comprised of thick dolomitized limestones which deposited during Hauterivian-Albian age.Chatton and Hart (1960) divided the Qamchuqa Formation into two members: lower member of Barremian-Aptian succession and an upper member of Albian succession.
In the typical locality, the Upper Qamchuqa Formation is consisting of dolomite, which removed organic-detrital limestone and of non-dolomitic limestone, locally argillaceous (Bellen et al., 1959).The lithological characterization of other sections of the succession is very similar.The differences are due to the different degree of dolomitization, sometimes marl occur.In some localities the occurrence of fresh water or brackish limestone interbeds was noted.(Bellen et al., 1959).
The paleo-environment of the Upper with the Jawan Formation partially evaporitic which suggests that it deposited in the lagoon environment to the southwest of the Qamchuqa High.Chatton and Hart (1960) considered the name of the succession by Nahr Umr Formation.
The Qamchuqa succession is an important Cretaceous succession in this area, and to understand the stratigraphic and depositional settings one must overview the vertical and lateral variation (Figs. 1.3,4,5,and 6).The lower boundary of the Lower Qamchuqa is conformable with the Ratawi, Yammama and Lower Sarmord formations, except in the area located to the northeastern part is unconformable with Garagu Formation during Late Valanginian period (Buday, 1980).To the Northeastern the Lower Qamchuqa Formation passes into the Balambo succession in the southwest parts of the Balambo-Tanjero Subzone.To the west and southwestern parts, the Shuaiba Formation is appeared everywhere except on the Stable Shelf west of Salman Zone (Jassim and Goff, 2006).
The upper contact of the succession is usually conformable in north and Northeast of Iraq with the upper Sarmord Formation.In the Southeastern part of the Low Folded Zone, in the Mesopotamian Zone, and along the margins of the Salman Zone the upper boundary is an unconformity surface (Buday, 1980).
The lower boundary of the Upper Qamchuqa succession is conformably and graded with the Lower Sarmord, Nahr Umr or Lower Balambo formations.The upper boundary representing a hiatus and is nonsequential; it is an unconformable contact in north and northeast of Iraq.The Upper Qamchuqa here is overlaying by the Dokan Formation, as it is found in the Makhul and the Mileh Tharthar areas (Templeton et al., 1956;Hay andAlgawi, 1958 in (Bellen et al., 1959).The upper contact is an unconformably surface in the Rutba Subzone (Jassim and Goff, 2006).

Restricted Shallow Marine Environment
This association facies is characterized in microfacies pellet wackstone to packstone or Peloidal wackstone-packstone where finite of diversity and not presence of effect of low energy sedimentary environment.The main facies content are s , pellets with micritized Milliolid and others skeletal (Plt.1.1).

Semi-Restricted Shallow Environment
The semiristricted shallow marine is representing with Orbitolinal wack -packstone and Miliolidal wackestone microfacies.which may consist of Orbitolina sp. with few small benthonic foraminifera referred to the semi-restricted shallow environment, where the microfacies with the miliolids component referred to shallower part for this environment (Plt.1.2).

Shallow Open Marine Environment
Orbitolina and Miliolids wackestone -packstone is the microfacies which represents the open marine settings.Where it is characterized by high diversity in components which included Orbitolina and Miliolids as main components addition to Echinoderms spines, Mollusca fragments, small benthonic foraminifera and lithothamnium red algae (Plt.1.3).

Deep Outer Ramp Environment
This association is consisting of Planktonic mudstone to wackestone with high occurrence of Globigerina sp. with small Benthonic Foraminifera with few occurrence"reflects a deep marine environment of the outer ramp" (Plt.1.5).

Diagenetic Framework
Main features of diagenesis are obscured, diagenesis leaves behind information of "postdepositional settings", water compositions, and temperatures.Diagenesis may decrease porosity and permeability, or the possibility to be high.commonly, however, The gradually absence of porosity and permeability could be trending with the increased time and depth of burial, this transformation is generally very large (Moore, 2004) (Fig. 5).Diagenetic process effect on the Upper Qamchuqa formation are: micritization, cementation, recrystallization, dolomitization, silicification, dissolution and compaction.The studied succession was affected by several genetic processes that affected upon the degree of porosity in this rock unit, as explained below:

Earlier and Latest Diagenesis
Diagenetic remains during the interval of sediments or organisms which found.Not setting clear limits within the diagenetic changes that replace sediments to the rocks, earlier and latest diagenetic stage are useful to characterize the time of diagenetic processes.An early diagenesis indicates that generating a diagenesis that occurs directly after deposition or after burial (Berner, 1980).The early diagenetic stage was written by Bathrust, 1975 andDickinson, 1984.After a long period of deposition, a late diagenesis occurs.

Micritization
It's the first diagenesis process advanced due to algal and bacterial activities through post depositional in a phreatic zone (Longman, 1980).
The basic size of any limestone is generally including mud or micrite.it's difficult to identify grains or crystals in a micrite due to their small size.geologist used different limits size between sand and mud for carbonate: for example, the size are putted at 0.02 mm depending on (Dunham, 1962) or put 0.004 mm depending on (Folk, 1962).
The envelope micrite are surrounding by skeletal bioclast or skeletal whole organism specially in the texture of wackestone and packstone (Ginsburg, 1957).the study of carbonate rocks showed most common type of diagenesis processes (Plt.2-1).

Cementation
It consists of filling the pores of a primary or secondary origin with substances that have been precipitated recently (Scholle and Schole, 2003).
Cementation is the diagenetic process by which different type of pores are filled by recently precipitated materials, through or after the deposition (Flugel, 2010).Calcite cementation blocks the porosity of Upper Qamchqa Formation reservoir.Main types of calcite cement are: Blocky (Granular) cement, calcite cement is made up of small, relatively equal-dimensional crystals that fill the pores between the particles and is a common occurrence.It's found in "meteoric and vadose, meteoric and phreatic and burial diagenetic environments".Granular cement is too originate from recrystallization process of previously existing cement (Flugel, 2004), granular cement in some microfacies filling the interparticle pores, and in the other filling the fragments (Plt.2-2).
Drusy cement, filling the pore space by cement liner is in the pores between the interparticle and intraparticle pores, fractures and molds, which characterizing by equant to elongated, anhedral to subhedral calcite crystals.Size usually increases to >10 µm and is towards the center of the void .Showing a distinct fabric.It appears in meteoric environments close to the surface thus in burial environments (Flugel, 2004).
The drusy cement was noticed basically in Shoal grainstone and other as quant crystals cement (Plt.2-3).Overgrowth syntaxial cement, the ground is controlled overgrowth round a base grain made by a single crystal.Overgrowth shows the differences in color between skeletal grains and cement.This type of cement is found in near marine surface, vadose to marine and meteoric to phreatic diagenetic environments, it is evident in deep burial environments (Flugel, 2004).It's recognize in Peloidal Packstone Microfacies and wackestone Microfacies (Plt.2-4).

Neomorphism
It's the process that includes changes in the mineralogic compositions and crystals and fabrics (Flugel, 2010).Neomorphism forms the metamorphosis relationship of minerals that are formed within the present water that including replacement process (Formation of another mineral through the dissolution of one mineral at a time, inversion process is change of a mineral by another mineral, and recrystallization process which represents the changes in crystal sized and shaped and no change in mineralogical composition (Flugel, 2004).
It turns out that the new transformation processes are of the type in which coarse crystals grow at the expense of fine crystals (Plt.2-5).

Dissolution
It is a dissolution process during the burial of carbonate deposits after fixing minerals.It is usually detected by a "selective dissolution non fabric, all the elements of the fabric will be cut off for example grains, cement, and matrix (Moore, 2004).
Depending on these types of pores, they usually form channels, vugs and caverns depending on the shapes and sizes (Choquette and Pray, 1970).Due to dissolution in Upper Qamchuqa Formation microfacies occur as moldic and vuggy pores (Plt.2-6).

Dolomitization
Many carbonates have been subjected to dolomitization, there probably an increase in porosity from net calcite to dolomite.There are many models used to classify the dolomite, in most of the models for dolomitization, most of the magnesium for the dolomite is derived from seawater though there may has been some modification in chemistry (Amthor and Friedman, 1991).
The texture of dolomite rocks was classified based on the crystal size and crystal distribution moreover to the shape of the crystal boundaries.Polymodel and unimodel are classified depending on the size distribution.The form of crystal limits is classified into two types planar or nonplanar.The classification of dolomite by Amthor and Friedman (1991) it's use to describe the types, mechanism, and origin of dolomite (Fig. 6).Sibley and Gregg (1987) The dolomite texture is classified according to the crystal limits distribution and crystal limits shape, and it is of seven types: According to Flugel (1982) we can describe 3 kinds of dolomite in concerned sections:-Scattered dolomite" is represent a coarse, euhedral dolomite rhombs with strong, often cloudy center clear rimmed"related to the stylolite and pressure solution compaction process (Plate.3-1).Pervasive dolomite" is characterized by coarse zones of crystalline dolomites with "cloudy center clear rim, that may exhibit"fabrics selectivity and well developed porosity, or may form dense interlocking mosaics"cement (Plate.3-2) and floaty dolomite"crystals (Plate.3-3).Saddle dolomite generally occurs as very late pore-fill cement and characterized by cloudy coarse crystals".(Plate.3-4).

Compaction
The compaction process refers to the reduction of the bulk volume of the rock.This type involves a mechanical process that reduces the bulk volume of single grains (grain distortion) or causes closer packing of the grains(reorientation) and the pressure solution is obtained as a result of dissolving and reducing the size of grains and cement minerals (Flugel, 1982).
This process includes mechanical and chemical compaction, where: Mechanical type is formed due to the burial sediments and caused reduction in the porosities of rocks.The overburden of thickness (load of sediments) necessary to product the structures of compaction is controversial.Overburden is resulting in the mechanical fracturing and fragmented of the grains.Mechanical compaction is mainly followed by pressure solution which detected by the stylolite and areas of solution (Flugel, 2010) (Plt. 3-5).
Chemical type of compaction is represented with the pressure solution which resulting in styloites and seams of solution which formed under the burial and overburden conditions.Solution seams are swarm like parting or isolated which distinguished by thin seams, often with insoluble residues accumulation (Flugel, 2010) (Plt.6).
Upper Qamchuqa Formation has been affected by the compaction process because of the overburden load of sediments with a result of pressure solution by grain connections causing stylolite and connected with fracture

Sequence Development
The present succession records deposition of three 4th order cycles during one 3rd order cycle, which are deposited during successive episodes"of the sea level stability.The lower most cycle (A) extends from the upper part of the underlying Upper Sarmord (TST) to the shallow open marine facies (HST) of the lower part of Lower Qamchuqa in K-229 which ended with shoal facies (Fig. 7).While in Kz-4 the facies deep open marine was started this cycle and then shallow open marine (Fig. 8).The mud dominated nature of this cycle indicates that carbonate buildup almost kept-pace with the relative rising of sea level (catch-up).Dissolution, cementation, compaction, floating dolomite rhombohedral and few authigenic iron oxides are the main diagenetic features in this microfacies.
Cycle (B) on the other hand, shows a grain dominated facies where the carbonates buildup kept pace with the still relative sea level (keep-up) during high stand which deposited under semi-restricted and restricted successively.Cycle (C) shows a marked variation in thickness within the sections (K-229 and Kz-4); it is greatest thickness was noticed at K-229 where its lower part is mostly mud dominated in Kz-4.Cycle (D) shows a distinct variation in facies change from grain-rich to mud-poor lithology of restricted to semi-restricted facies in the East (K-229) to predominantly restricted marine conditions at the central area (Kz-4), it is bounded at the top by a Type one sequence boundary (SB1) separating the studied succession from the overlying Dokan Formation.The Upper Qamchuqa sequence was controlled by 5 diagenetic "processes, these are: micritization, cementation, dissolution, dolomitization and compaction.The less affecting process was micritization, cementation and compaction, while the most effective was dolomitization and dissolution which have direct affect upon carbonates pores systems".
The present sequence is representing the deposition as four 3rd "order cycles, these cycles were deposited during successive episodes of relative" rising of sea level and stability.

Fig. 2 .
Fig. 2. Diagram showing steps and equations which used in the present study (Al-Zaidy et al., 2013)

Fig. 3 .
Fig.3.lithologic columnar section showing the distribution of microfacies and depositional environments of Upper Qamchuqa in K-229

Fig. 7 .Fig. 8 .
Fig.7.Columnar section showing diagenetic history and porosity types with shale values related to sequence stratigraphy in K-229