The origin, differential diagenesis and microporosity characteristics of carbonate mud across a late Paleogene ramp (Iraqi Kurdistan region)

Journal of Petroleum Science and Engineering 192 (2020) 107247

Contents lists available at ScienceDirect

Journal of Petroleum Science and Engineering journal homepage: http://www.elsevier.com/locate/petrol

The origin, differential diagenesis and microporosity characteristics of carbonate mud across a late Paleogene ramp (Iraqi Kurdistan region) Ala A. Ghafur Department of Natural Resources Engineering and Management, University of Kurdistan-Hewler, Erbil, Kurdistan Region, Iraq

A R T I C L E I N F O

A B S T R A C T

Keywords: Carbonate muds Microporosity Carbonate ramp Paleogene Kurdistan Iraq

Microporosity is common in many Middle Eastern carbonate reservoirs and various interpretations have been offered to explain its development, especially in Cretaceous reservoirs. In this study outcrops of late Paleogene carbonates of the north-eastern Iraq have been studied to assess the variation of porosity across a carbonate ramp and the factors that control its distribution. The depositional system of this late Paleogene succession was a homoclinal ramp dipping SW. Inner-mid ramp carbonate facies are massive to thick bedded units, mostly composed of packstone to grainstone, characterized by high diversity of benthic foraminifers, red algae, corals, gastropods and bivalves. Outer ramp carbonate facies are thin bedded wackestone to calcimudstone. In terms of matrix-dominated facies two distinct rock fabrics and pore systems have been identified: low porosity inner-mid ramp facies and higher porosity outer ramp facies. The inner-mid ramp carbonates show evidence of neo­ morphism and recrystallization of matrix mud which forms larger crystal size (average 3–4 μm) with smaller pore size (0.5 μm or less) and average porosity (8.7%). The outer ramp mud matrices exhibit the major component of planktic foraminifera and coccolith debris have smaller crystal size (average 1 μm) with larger pore size (1–2 μm), the average porosity (13.1%). For the inner-mid ramp matrices carbon and oxygen (C & O) stable isotopic values are negative indicating likely meteoric diagenesis. The relatively low strontium (Sr) and magnesium (Mg) values and high manganese (Mn) implies likely recrystallization and replacement of the marine precursor, probably high Mg-calcite and aragonite. In the outer ramp facies, the presence of coccolith and planktic fora­ miniferal debris indicates less diagenetic modification; the C & O stable isotopic values show less altered range with very low Mn and relatively high Sr and Mg also supporting less alteration. The textural and geochemical evidence supports an origin from a more stable, likely low Mg-calcite precursor.

1. Introduction Carbonate mud is the dominant component of most limestones, but its small size, polygenetic nature and its propensity for diagenetic modification has restricted its study. There are rare exceptions when it has been possible to “fingerprint” muds in ancient successions (Turpin et al., 2012). The diverse nature of carbonate muds has become increasingly clearer in recent years and our understanding of what might be termed primary mud, that is mud-grade carbonate that was formed syn-depositionally, has improved significantly (Perry et al., 2011; Salter et al., 2012; Gischler et al., 2013; Purkis et al., 2017). What has also become clearer is that many ancient mud-grade carbonates are the result of the recrystallization and replacement of both original muds and of other grain components such as the formation of mud grade carbonates in limestone-marl successions (Munnecke and Westphal, 2005; Arzani, 2006). (Fig. 1).

As the origin of some mud grade carbonates is strongly controlled by environment of deposition, especially the biogenic types, there should be changes in carbonate muds across depth gradients in both modern and ancient environmental settings. For example, shallow water muds are likely to be in large part derived from the breakdown of skeletal grains and calcareous algae (Gischler et al., 2013), and be dominated by high Mg calcite and aragonite. In pelagic settings, contributions from planktic foraminifera and coccolithophorids would result in higher proportions of low Mg calcite. The hypothesis being tested in this study is whether such differences can be identified in highly lithified Cenozoic limestones across a wide environmental (depth) gradient. 2. Materials and method The carbonate succession of late Paleogene has been studied in the S-

E-mail address: [email protected]. https://doi.org/10.1016/j.petrol.2020.107247 Received 11 April 2019; Received in revised form 18 February 2020; Accepted 27 March 2020 Available online 6 April 2020 0920-4105/© 2020 Elsevier B.V. All rights reserved.

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

SW of Sulaimani city of Iraq, bounded by latitude 34� 450 to 35� 200 and longitude 45� 100 to 45� 500 (Fig. 2 and Table 1). The details about field description, formation thickness, sample frequency, lithology, etc. are all summarized in the supplementary material as a sedimentary logs using sedlog software for each of the outcrops. 320 samples were collected and examined in Cardiff University laboratories in order determine textural and facies variations across mud-dominated carbonate ramp. For this purpose, thin sections were prepared for all samples and stained using the technique of Dickson (1965, 1966). Moreover, thirty mudstone-packstone samples were selected across different depositional environments for microporosity study using scanning electron microscopy (SEM) in Cardiff University laboratories. Samples were polished, cleaned and then etched in acetic acid. All the polished limestone samples were coated with gold, then examined with a scanning electron microscope (ESEM) and back scatter (BSEM). Furthermore, electron back-scattered diffraction (EBSD) was used for clay cages and some micrite crystals. Additionally, geochemical analyses were conducted in Cardiff Uni­ versity laboratories in order to measure trace elements. Trace elements were found using laser ablation Thermo X Series 2 inductively coupled plasma-mass spectrometry (ICP-MS) coupled to a New Wave Research

Fig. 1. Diagram showing different sources of micrite (Courtesy of Paul Wright).

Fig. 2. Geological map of the studied area. Number of the localities are described in Table 1. 2

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

4. Results

Table 1 Coordination for the studied sections. No

Sections

Latitude East

1

Section1 Section2 Section3 Section4 Section5 Section6 Section7 Section8

35� 2000 35� 3000 35� 0600 35� 3800 35� 5700 34� 5100 34� 1000 34� 3500

2 3 4 5 6 7 8

Longitude North

Exposed formations

180

45� 160 5500

Avanah, Anna, Fatha

160

45� 160 5000

Avanah, Bajawan, Anah, Jeribi

090

45� 170 0700

Avanah, Bajawan, Anah, Jeribi

080

45� 170 1500

Avanah, Bajawan, Anah, Jeribi

08

45 17 02

Avanah, Jaddala, Bajawan, Anah, Jeribi Avanah, Bajawan, Anah, Fatha

0

�

0

00

550

45� 440 0600

520

45� 460 2800

47

45 41 12

0

�

0

00

4.1. Facies distribution Interpretation of microfacies is based on texture especially the presence or absence of matrix and utilizes foraminifera as these are one of the main components, and in the diagenesis. Late Paleogene car­ bonate successions show a gradual deepening microfacies with no evi­ dence of slope break or effective barriers and is interpreted as having been deposited in a ramp setting Burchette and Wright (1992). The detailed sedimentary log for all sections is located in the supplementary material. In term of matrix-dominated facies two distinct rock fabrics have been identified: 1. Inner-mid ramp carbonate facies are structureless thickly to massively bedded units, mostly composed of packstone with presence of some grainstone, characterized by high diversity of benthic foramini­ fers, red algae, gastropods and bivalves (Fig. 3). The overall character­ istics of these facies suggest a very shallow marine setting above fairweather wave-base, in a relatively low energy environmental setting which extending to an open marine setting above the storm wave base, of a mid-ramp setting. The main diagenetic features associated with these microfacies include cementation in the form of non-ferroan calcite cement, including syntaxial overgrowths on echinoid fragments; micri­ tization; aragonite replaced by non-ferroan calcite cement; compaction and pressure solution, in addition to fractures and vugs partially filled with secondary calcite cement. Finally, limited amounts of dolomite are present in the form of scattered, very fine, idiotopic (euhedral) dolomite crystals. 2. Outer ramp carbonate facies are thin-medium bedded wackestone to calcimudstone. The outer ramp mud matrices exhibit the major component of planktic foraminifera and coccolith debris (Fig. 4). The overall characteristic of these facies represent an open deep marine setting on the outer ramp environment. The diagenetic features in the outer ramp microfacies are cementation in the form of non-ferroan calcite cement including cement filled fracture, as well as compaction including clay seams and low-amplitude stylolites are also present. Further information on microfacies study of the late Eocene to early Miocene of southern Kurdistan-Northern Iraq took place by Gfhafur (2012, 2015) and it is summarized in Table 2 and Fig. 5.

Avanah, Jaddala, Sheikh Alas, Fatha Azkand, Ibrahim

UP213 UV laser to analyse 100 μm diameter sections and helium gas was used for ablation and initial transport from the laser cell. Thermo Plas­ malab time-resolved analysis (TRA) data acquisition software was used for initial data reduction with post-processing in Excel. Finally, Oxygen/Carbon isotope analyses took place for twenty selected samples. Drilling was done on the polished surface limestone samples in order to differentiate the micrite matrix from grains. Drilling was used for coarse grained limestones which can easily see both matrix and grains, while micro-drilling, the Wave Research Micromill, was used to drill in finer grained limestone. The powder samples were weighed on a Sartorius microbalance and then sent to isotope laboratory in Cardiff University laboratories in order to determine oxygen and carbon iso­ topes for both shallow marine and deep marine micritic matrix on a gas bench III and analysed on a Thermo Electron Delta V advantage. Stan­ dards used were inhouse Carrara and international NBS19. Precision is 0.1 for both 13C and 18O. All values are reported to PDB scale. 3. Geological setting The late Eocene-Oligocene carbonate succession hosts major hy­ drocarbon reserves in Iraq besides Cretaceous carbonate sequence (Ghafur and Hasan, 2017; Ghafur et al., 2019). The term “Main Lime­ stone” is an informal term introduced to indicate the first main oil pay zone of the Kirkuk structure from middle-upper Eocene and Oligocene (Bellen, 1956). It consists of the Avanah and Jaddala Formations of Eocene age and nine Oligocene formations of the Kirkuk Group (Shurau, Sheikh Alas, Palani, Baba, Bajawan, Tarjil, Anah, Azkand and Ibrahim) in one stratigraphic package (Bellen, 1956; Bellen et al., 1959; Al-Naqib, 1960). The Early Miocene evaporite of the Fatha (formerly known as the Lower Fars) Formation (Burdigalian age ‘15.6–18.5 Ma’ according to Grabowski and Liu, 2009, 2010) acts as the cap rock for the reservoirs in the northern part of Iraq. Numbers of these formations are exposed in different outcrops across the study area (Table 1). Many earlier studies took place focusing on sedimentology, paleon­ tology, biostratigraphy and sequence stratigraphy of late Paleogene successions in the region although none of them focused on the source and origin of micrite in the mud rich facies. The following are examples of several previous studies: Bellen (1956); Ctyroky and Karim (1971a); Ditmar and Iraqi-Soviet team (1971); Youkhanna and Hradecky, 1978; Behnam (1979); Muhammed, 1983; Al-Hashimi and Amer, 1985; Al-Qayim, 2006; Jassim and Goff (2006); Kharajiany (2008); Al-Banna, 2008; Grabowski and Liu (2008) and Lawa and Ghafur (2015).

4.2. Scanning electron microscopic (SEM) study 4.2.1. SEM study of martices and bioclasts Samples from both shallow and deeper marine were investigated in polished slightly etched limestone blocks using SEM. Different miner­ alogically known grains such as miliolids, Nummulites, red algae, as well as peloids, along with both micritic matrices from both shallow and deep marine environments have been investigated in order to determine the original precursor mineralogy of micrite matrices. Micritic matrices from the shallow marine microfacies show straight curvilinear polyhedral crystal boundaries, in which many platy clays form partial or complete clay cages around the micrite crystals; where the micrite crystals adjoin clays they are straight or curvilinear sided. The crystal sizes of micrite matrix averages 3–4 μm. Very rare pitted micrite was observed with a square, elongated and irregular shape with 0.5–2 μm in diameter (Fig. 6). The deeper marine micrite also shows straight curvilinear polyhedral crystal boundaries in polished/etched and broken limestone blocks. Very few clay cages were found around the crystals. The crystal sizes for deeper micrite matrix ranged between 1 and 2 μm. No pitted crystals were found during the investigation of these deeper marine micrites (Fig. 7). Under SEM, primary aragonitic bioclasts, such as corals, show the replacement of original aragonite by microspar with no or very rare inclusions on the surface of the crystals (Fig. 8). Calcitic bioclasts, such 3

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

Fig. 3. (A&B) Field photos of thick to massive bedded limestone from shallow inner ramp, (C) Photomicrograph of peloidal, skeletal packstone partially grainstone microfacies from inner ramp, (D) Photomicrograph of skeletal packstone microfacies from mid ramp. Scale labels for C&D are 1 mm.

Fig. 4. (A&B) Field photos of fine to medium bedded limestone from outer ramp (field of view is 80 m & 100 m respectively), (C&D) Photomicrograph of skeletal wackestone and calicimudstone microfacies from outer ramp respectively. Scale labels are 1 mm.

as benthic foraminifera and red algae, with peloids, have a granular or elongated texture under high magnification. Miliolids have a granular texture with straight/curvilinear or polyhedral crystal shapes. Crystal sizes are 3–4 μm and neither inclusion on the surface of crystals nor clay cages between crystals was found (Fig. 9). Nummulites have an elongated crystal shape, with crystal sizes ranging between 15 and 20 μm; during

investigation, pitted crystals were observed with a four-sided type pit. No clay cages were found between crystals (Fig. 10). Red algae are another bioclast which was investigated under SEM, they have a gran­ ular texture with a straight/curvilinear crystal shape and are 3–4 μm in size. No inclusions were found on the surface of the crystals; clay cages were absent (Fig. 11). Finally, peloids may originally form from 4

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

4.2.2. Pits Pits, including different shapes and sizes, were observed in limited crystal types. Pitted crystals show distribution patterns that appear to be random. They are 2 μm or more long and 1 μm wide in elongated pits, and 0.5–2 μm diameter in square pits. Pits mostly occur inside crystals (i. e. intra-crystalline) with very few pits occurring at crystal boundaries (Fig. 14). In most cases, pits occur singularly, though they may also join to each other to form a cluster shape. Usually, pits are empty; rarely are they filled with clay debris (see Fig. 15).

Table 2 Distribution of the depositional environment zones of Eocene-Oligocene suc­ cessions with comparison to Buxton and Pedley’s (1989) ramp model (After Ghafur, 2015). Ramp zones

Microfacies

Sub-microfacies

Buxton and Pedley’s (1989) facies distribution

1

FT CG PS MP

Gypsum/Marl/Claystone Conglomerate Palaeosol MP-1: peloidal, bioclastic packstone/grainstone MP-2: peloidal wackestone/ calcimudstone Peloidal, skeletal grainstone PP: Peloidal packstone/ grainstone OG: Ooidal grainstone SP-1: Skeletal packstone with brecciation SP-2: Skeletal packstone with Austrotrillina howchini SP-3: Skeletal grainstone with Praerhapydionina delicate Nummulites-Alveolina packstone/grainstone Coral bioherm Rotalids-coralline red algae wackestone/packstone NR-1: Coralline red algaeNummulites wackestone NR-2: Coralline red algaeNummulites-Discocyclina packstone NR-3: Nummulites-Discocyclina packstone PK-1: peloidal packstone with planktonic foraminifera PK-2: Planktonic foraminifera calcimudstone PK-3: Planktonic-benthonic foraminifera wackestone

–

2 3

PG JB 4 5

SP

NA 6 7

CB RR NR

8

NR

9

PK

1

3 4

4.2.3. Clay cages Clay cages surround the micritic matrix as well as microspars in the form of incomplete and different thicknesses. They have straight slightly curvilinear boundaries with sizes ranging from 1 to 2 μm up to 7 μm (Fig. 16). They are largely found in-between shallow marine micritic crystals and are rarely seen in deeper marine micrite. Sometimes clay shards may be present in elongated pits which are slightly smaller than other pits. From X-ray diffraction, the composition of clay cages can be deter­ mined. The presence of potassium (K), aluminium (Al), silica (Si), magnesium (Mg) and iron (Fe) suggests that the clay cages are illite (see Table 4).

6 5

4.3. Oxygen–carbon stable isotope

2

Stable isotope analyses were carried out for both inner-mid ramp and outer ramp micritic matrices in order to investigate under which diagenetic environment they were formed and their relationship with micritic matrices texture from different depositional setting. The oxygen isotope composition of both outer ramp and inner-mid ramp marine matrices are relatively homogenous; the analysed data show ranges between 2.0 and 3.5 PDB (average 2.8 PDB) and 7.5 to 9.5 PDB (average 8.2 PDB), respectively. Meanwhile, carbon isotopes have a wider range from a deeper marine micrite positive value (between þ0.75 and þ2.0) to a shallow marine micrite negative value (between 1.5 and 7.5). The results of the carbon and oxygen isotopic analyses of different lithology are shown in Fig. 17. From the results we can conclude that in the inner-mid ramp micrite matrices, both oxygen and carbon isotopic values are very low, indi­ cating likely meteoric diagenesis. Both the oxygen and carbon isotopic values of inner/mid ramp carbonates mud are similar to the isotopic values of Late Miocene micritic carbonate from the Madrid Basin in Spain and lighter than the isotopic values of Holocene carbonate mud from the Everglades in Florida Bay seawater, which indicates that sta­ bilization occurred under meteoric water (Wright et al., 1997; Andrews,

7 8

micritized bioclasts and/or faecal pellets, they have a granular texture and crystal shapes are straight/curvilinear and 3 μm in size. Very rare pitted crystals were observed with a few clay cages around the crystals (Fig. 12) (see Table 3). In the outer ramp samples, a few bioclasts were recognized, such as planktonic foraminifera and coccoliths, in both polished/etched and broken limestone blocks (Fig. 13).

Fig. 5. Depositional model and microfacies change across the ramp, during Late Eocene and Oligocene-Early Miocene (After Ghafur, 2015). 5

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

Fig. 6. Scanning electron micrograph of the inner ramp carbonate matrix (A) General view, (B & C) Showing micrite with different thickness clay cages around the micrite crystals and different pit sizes, (D) Histogram showing the crystal size of shallow marine micrite (inner ramp), 346 crystal sizes were measured.

Fig. 7. Scanning electron micrograph of the outer ramp carbonate matrix showing porous matrix, (A) General view (B) Very thin clay cages developed around the crystals, (C) Broken surface of deeper marine carbonate micrites showing small sized, subhedral crystals, (D) Histogram showing the crystal size of deep marine micrite (outer ramp), 255 crystal sizes were measured.

1991). In contrast to outer ramp facies, the presence of coccolith and planktonic foraminifers debris indicates less diagenetic modification; and the carbon and oxygen isotopic values show a less altered range

which is likely be from marine diagenesis. The Carbone value of outer ramp facies is similar to. Oxygen isotopes can alter during stabilization and neomorphism in 6

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

Fig. 8. (A) Scanning electron micrograph of coral wall (B, C and D) Higher magnification of scanning electron micrograph of corals showing subhedral to anhedral crystal size with very few inclusions.

Fig. 9. (A) Scanning electron micrograph of micritized miliolid, (B) Higher magnification of the wall of miliolids showing very well sorted, subhedral crystals with no inclusions on the surface of the crystals, (C) Histogram showing the crystal size of miliolids, 399 crystal sizes were measured.

which small admixtures of diagenetic water can cause dramatic changes in oxygen isotopes (Martin et al., 1986), so this might be a good inter­ pretation for two different groups of oxygen isotopic values: (1) light oxygen isotopic values ( 2.0 to 3.5 PDB) in outer ramp micrite; and

(2) very light oxygen isotopic values ( 7.5 to 9.5 PDB) in inner/mid ramp micrite. Therefore, this depletion in oxygen isotopes is caused by mineralogical stabilization. From the oxygen Isotopic values of micritic matrices, temperature 7

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

Fig. 10. Scanning electron micrograph of Nummulite, (B) Higher magnification of the wall of Nummulite showing elongate crystals with inclusions on the surface of the crystals, (C) Histogram showing the crystal size of Nummulites, 108 crystal sizes were measured.

Fig. 11. (A) Scanning electron micrograph of red algae, (B) Higher magnification of red algae showing well sorted, subhedral crystals with no inclusions on the surface of the crystals, (C) Histogram showing the crystal size of red algae, 172 crystal sizes were measured. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

8

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

Fig. 12. (A) Scanning electron micrograph of peloids, (B) Higher magnification of peloids showing well sorted, subhedral crystals with no inclusions, (C) Histogram showing the crystal size of peloids, 489 crystal sizes were measured.

concentration to the influence of meteoric water (Wu and Wu, 1996) and suggested that the micritic matrix was formed from high-Mg calcite mud (Wright et al., 1997). In contrast, outer ramp micrite facies had an un­ expectedly high Sr value range of between 497 and 806 ppm and very low Mn of between 11 and 32 ppm, indicating that these limestones did not undergo significant meteoric diagenesis. The upper value for Sr is closer to the values for what are regarded as aragonite dominated pre­ cursors (ADPs), which were described by Lasemi and Sandberg (1984, 1993); but this is in contrast to the textural evidence that supports an origin from a more stable probably low-Mg calcite precursor (Fig. 20). This relatively high Sr value may be interpreted as there being some other Sr-bearing mineral present, such as celestite (strontium sulphate), that was leached during the dissolution procedure (Wheeley, 2006). Presence of celestite was reported by Al-Khateeb and Hassan, 2006 in central south part of Iraq. Celestite was formed from the combination of Sr from the deep ground water and the SO3, which was supplied from the surrounding environment due to high ground water level and contin­ uous evaporation. Both trace elements and oxygen-carbon isotopic values of Kirkuk Group in Kirkuk oil field ‘Kirkuk area’ shows similar result with the current study which the near shore facies composition of diagenetic solutions were subsurface meteoric type, in contrast to the basinal facies which either retained a considerable portion of the unaltered original low-Mg calcitic component or digenetic water were sourced somewhat from modified marine parentage (Majid and Veizer, 1986).

Table 3 Shows different crystal sizes and presence or absence of inclusion and clay cages in micrite and some selected bioclasts in addition to peloids. Miliolids Nummulites Red algae Peloids Shallow water micrite Deeper water micrite

Inclusions

Crystal size (μm)

Clay cage

No Yes No No or rare No or rare No

3–4 15–20 3–4 3 3–4 1–2

No No No Yes Yes Rare

can be estimated using the equilibrium relationship between oxygen of calcite (i.e. micritic matrices), temperature and oxygen of water (Fig. 18, Tucker and Wright, 1990). According to Zachos et al. (2001) the average deep sea oxygen isotope value for late Paleogene is around 2.3 latter value is plotted against average value of both outer ramp and inner-mid ramp micritic matrices 2.8 and 8.2 PDB respectively. It is revealed that estimated temperature of the sea was 40 and 75� respectively for the outer ramp and inner/mid ramp micritic matrices, the higher tempera­ ture value in the inner-mid ramp may resulted due to active diagenetic process in this zone. 4.4. Trace elements analysis The Sr value in inner-mid ramp facies ranges from 188 to 311 ppm, it falls below the value (400 ppm) of calcite dominated precursor (CDPs) which was described by Lasemi and Sandberg (1984, 1993), while the Mn value has a relatively high range of between 90 and 223 ppm, probably because the Sr value decreases with the uptake of Mn as a result of mineralogical stabilization and neomorphism during alteration from high-Mg calcite to low-Mg calcite (Mazzullo and Bischoff, 1992; Webb et al., 2009) (see Fig. 19). Several studies have attributed a low Sr

4.5. Microporosity Although the main focus of this study is on microporosity, it is worth mentioning that several types of porosity are observed in the study area which are formed during deposition as primary intergranular and intercrystalline porosity as well as during early (eogenetic) to burial (mesogenetic) stages of diagenesis in the form of secondary porosity; 9

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

Fig. 13. (A) Broken surface of deeper marine limestone showing cast of planktonic foraminifera chamber, (B) Broken surface of deeper marine limestone with presence of coccolith in the centre of the photo, (C) Scanning electron micrograph of polished-etched surface of deeper marine limestone with presence of coccolith.

such as intragranular, moldic, vugy and fracture which they are partially filled by secondary calcite mineral. The inner/mid ramp facies have both primary and secondary porosity, while the outer ramp facies mainly have primary porosity. Microporosity has been widely studied in many ancient micrite-rich limestones, including significant petroleum reservoirs. The most recent studies are: Lambert et al. (2006); Richard et al. (2007); Munnecke et al. (2008); Fournier and Borgomano (2009); daSilva et al. (2009); Volery et al. (2009); Volery et al. (2010a, b); Barnett et al. (2018); Brigaud et al. (2010); Volery et al. (2011); Deville de Periere et al. (2011); Norbisrath et al. (2012); Loucks et al. (2013); Lucia and Loucks (2013); Carpentier et al. (2015); Kaczmarek et al. (2015); Al Ibrahim et al. (2016); Hasiuk et al. (2016) and Stowakiewicz et al. (2016). Microporosity may occur as both primary intercrystalline and secondary microporosity during the early and late stages of diagenesis. Early cementation is a most impor­ tant mechanism for porosity reduction in carbonated micrite (Lasemi et al., 1990). During stabilization, metastable minerals can release cement and cause precipitation in adjacent micropores; the growth of cement in the micropore systems of fine-grained sediments leads to a decrease in microporosity. If cementation ceases shortly after stabili­ zation, microporosity can be retained to form a porous micro-rhombic micrite texture. In contrast, in the case of complete cementation, and occluding microporosity, a mosaic texture of anhedral micrite will be produced. According to Moshier (1989), numerous types of microporosity in micritic limestones are present, including both primary and solution-enhanced intererystalline micropores, in addition to micro­ molds, microvugs and microchannels. He also added that the polyhedral shaped intercrystalline microporosity with 5–10 μm diameter, inter­ connected by pore-throats with diameters of 0.5–2 μm is responsible for more than 20% of porosity with permeability exceeded 1md (Fig. 21).

Above porosity and permeability are measured by conventional core analysis using gas porosimeter (Moshier, 1989). Although, the amount of the microporosity is strongly controlled by crystal texture (Kaczmarek et al., 2015). The microporosity of micritic matrix from both inner-mid ramp and outer ramp settings have been examined using digital image analysis (Anselmetti et al., 1998) (Figs. 22 and 23) in order to determine the control of microporosity and its distribution and variation across ramp settings. In term of matrix-dominated facies, two distinct rock fabrics and pore systems have been classified: (1) low microporosity inner-mid ramp matrices and (2) higher microporosity outer ramp matrices. In the first pore system, pore sizes are up to 0.5 μm, with an average porosity of 8.7% (Fig. 22), while in the second pore system, pore sizes are between 1 and 2 μm with an average porosity 13.1% (Fig. 23). In the study area, porosity is not homogeneously distributed across the ramp setting, this is due to active processes of diagenesis such as cementation, stabilization of metastable minerals, neomorphism and recrystallization in addition to compaction during eogenetic and mes­ ogenetic stages of diagenesis. It is revealed that in the shallow (innermid ramp) setting of the basin, these diagenetic processes are active and caused primary porosity reduction. However, these processes are less active in the deeper (outer ramp) setting of the basin so that they retain some primary microporosity, in this respect they behave like chalk. The porosity of chalk decreases as a direct function of burial depth; under normal circumstances, a typical nano-fossil chalk oozes at the water-sediment interface and has 70% porosity; increasing the depth to 1 km, the porosity will reduce to 35%, at 2 km to about 15%, and at 3 km the porosity becomes almost zero (Scholle, 1977). Both mechanical and chemical compaction can cause porosity reduction in chalk; mechanical compaction is most effective in the early stages, but chemical compac­ tion in the late stages of diagenesis. 10

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

Fig. 14. (A–D) Scanning electron micrograph of elongate pits inside and at the boundary of the crystals mostly filled with clay shards.

Fig. 15. (A–D) Scanning electron micrograph of four-sided pits on the surface of the crystals as singular and aggregates.

11

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

Fig. 18. Equilibrium relationship between oxygen of calcite, temperature and oxygen of water. X-axis represents oxygen value of water; Y-axis represents temperature between 0 and 100� . The curved lines represent constant oxgen values (PDB) of calcite (After Tucker and Wright, 1990).

Fig. 16. Scanning electron micrograph shows the micrites and microspars from shallow marine is surrounded by clay cages in different thickness. Table 4 Element analysis of clay cage from shallow marine environment. Element

Weight %

Atomic%

Compd%

Formula

Mg Al Si K Fe O Totals

2.32 11.73 28.17 6.68 4.40 46.69 100.00

2.03 9.25 21.34 3.64 1.68 62.07

3.85 22.17 60.27 8.05 5.66

MgO Al2O3 SiO2 K2O FeO

Fig. 19. Inner-mid ramp facies minor elements (Mn and Sr versus Mg). Y-axis represents Mn and Sr; X-axis represents Mg in ppm.

Fig. 17. Cross plot of carbon-oxygen isotopic values. Inner-mid ramp micrite in blue have possible meteoric values while the outer ramp micrite in red have values closer to the marine value. X-axis represents oxygen values; Y-axis rep­ resents carbon values in PDB. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

5. Discussion Fig. 20. Outer ramp facies minor elements (Mn and Sr versus Mg). Y-axis represents Mn and Sr; X-axis represents Mg in ppm.

Selecting samples from known high-Mg calcites, such as miliolids, Nummulites and red algae, as well as peloids, micrite matrices from both shallow and deep marine environments were selected to determine the original precursor mineralogy of micrite matrices. During investigation of the selected samples, it was noticed that most of the micrite textures were lacking in relics with very few pits (no pits were found in deeper

marine micrite); this raises the possibility that micrite formed in an aragonitic sea is very low and/or diagenesis may overprint all primary differences in the mineralogy of precursors. In contrast to aragonite constituents, the process of stabilization of high-Mg calcite alters with 12

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

from different sources; on the one hand, microspars represent the pri­ mary cement formed in the early stages of diagenesis, as discussed in the case studies of Pliocene carbonates from the Bahamas and Silurian limestone from Gotland (Munnecke et al., 1997). On the other hand, microspar may fill secondary porosity when sand-sized grains may have dissolved, leaving an open space into which microspar is precipitated. Microspars that occur in patches may be interpreted as cement which precipitated into secondary pore spaces as cavities formed by solution (Steinen, 1978). Alternatively, microspars can form from a previously lithified micritic matrix by recrystallization and aggrading neo­ morphism, during mineralogical stabilization, through HMC replace­ ment by LMC leads to the release of Mg ions into the pore water. After removing these ions from the pores with fresh water, the micrite crystals start to grow until they reach microspar size (Folk, 1959, 1965, 1974). This study supports Folk’s (1959, 1965, 1974) opinion because of these facts: first of all, the non-uniform distribution thickness of clay cages around microspars, the partial remnants of clay within the crystals and displaying clay are evidence for the displacive growth of calcite cement as microspar crystals during the stabilization of metastable minerals. The pushing away of clay matrix by micritic crystals is thought to be because of the active force of crystal growth (Folk, 1965; Bathurst, 1975). This feature can easily be found in the shallow marine carbonate (inner-mid ramp) micrite. Moreover, the Sr value in inner-mid ramp facies falls below the value (400 ppm) of calcite dominated precursor (CDPs) which was described by Lasemi and Sandberg (1984, 1993). In contrast, clay cages are rarely found in outer ramp micrite, as they may originally have been formed from a low-Mg calcite source, from low-Mg calcite planktonic foram and coccolith, they indicate less diagenetic modification; and the carbon and oxygen isotopic values show a less altered range which is likely be from marine diagenesis. Microporosity is not uniformly distributed across matrix-rich ramp carbonates, reflecting different origins and precursor mineralogy; innermid ramp carbonates preserve lesser microporosity than outer ramp facies, this also supports neomorphism and recrystallization causing loss of primary microporosity, while in deeper marine micrite the evidence for recrystallization is less common and this retains some primary microporosity. We can conclude that the inner-mid ramp micrites are mostly sourced from high-Mg calcite benthic foraminifera and red algae, with little possibly of partial aragonite dominating. In contrast to outer ramp

Fig. 21. Scanning electron micrograph of fractured surface of porous crystalframework texture in the lime mud. Mircro-rhombic calcite forms matrix with a well-developed intercrystalline pore system. Measured porosity by conventional core analysis is 24%. Scale bar is 5 μm (After Moshier, 1989).

very small changes in the microfabric texture. A very good explanation of the process of stabilization of high-Mg calcite, from miliolids of the Eocene age to low-Mg calcite, is given by Towe and Hemleben (1976); miliolids had a magnesium content of about one-third of unaltered modern miliolids when tested, without any significant change in textural modification, even at the SEM level. A pitted texture with both elongated and square shapes was found in several micrite and microspar crystals, especially in shallow marine carbonates. The presence of clay shards within the crystals raises the possibility that square and elongated pits are not cross-sections or long sections of aragonite relics. Another possibility for presence of square pits is that they might be casts of pyrite, but traces of pyrite were not found inside the square pits, so they might have been plucked out of the surface of the crystals during the preparation (polishing and etching) of the sample. Based on petrographic observations, different crystal shapes, sizes and textures of microspars were recognized. Microspars may originate

Fig. 22. Scanning electron micrograph of the shallow marine carbonate matrix, the examined porosities using digital image analysis are A. 8.9%, B. 8.8%, C. 8.7% in three selected samples. The upper SEM images are indexed image and the lower darker are binary images generated from the original one to use in the porosity measurement. Scale bar is 5 μm. 13

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

Fig. 23. Scanning electron micrograph of the deeper marine carbonate matrix, the examined porosities using digital image analysis are A. 14.9%, B. 12.2%, C. 11% in three selected samples. The upper SEM images are indexed image and the lower darker are binary images generated from the original one to use in the porosity measurement. Scale bar is 5 μm.

micrites, “chalks”, which are largely composed of low-Mg calcite. Dur­ ing burial diagenesis chalks are formed by a combination of mechanical compaction and chemical recrystallization and cementation (Borre and Fabricius, 1998). The mechanism of chalk cementation is due to solution pressure and the reprecipitation of cement. Chalk diagenesis is related to two main factors: maximum depth of burial and pore-water chemistry (Scholle, 1977).

6. Conclusions The inner-mid ramp carbonates show evidence of neomorphism and recrystallization of micrite mud, causing loss of primary porosity, while the outer ramp facies shows the evidence of recrystallization is less than inner-mid ramp facies and retained some primary porosity; in this respect they have behaved like chalks. For the inner-mid ramp matrices C & O stable isotopic values are negative indicating likely meteoric diagenesis. In the outer ramp facies

Fig. 24. Ramp model showing two different rock fabrics and porosities. 14

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247

the presence of coccolith and planktic foraminifers debris indicates less diagenetic modification; the C & O stable isotopic values show less altered range which likely be marine diagenesis. Origin of micrite in inner-mid ramp shows relatively low Sr and Mg values and up-take of Mn implies likely recrystallization and replace­ ment of the marine precursor, probably high Mg-calcite and possibly aragonite during stabilization. While, outer ramp shows very low Mn and relatively high Sr and Mg supporting less alteration. The textural and geochemical evidence supports an origin from a more stable, likely low Mg-calcite precursor (Fig. 24). In conclusion, microporosity is strongly facies controlled within matrix-rich lithofacies with contrasting inner-mid and outer ramp types. So that microporosity is not uniformly distributed across the carbonate ramp reflecting different origins and precursor mineralogies and outer ramp carbonates preserve higher microporosities. This suggests that microporosity can have different origins reflecting depositional settings and sources.

Borre, M., Fabricius, I.L., 1998. Chemical and mechanical processes during burial diagenesis of chalk: an interpretation based on specific surface data of deep-sea sediments. Sedimentology 45, 755–769. Brigaud, B., Vincent, B., Durlet, C., Deconinck, J.F., Blanc, P., Trouiller, A., 2010. Acoustic properties of ancient shallow-marine carbonates: effects of depositional environments and diagenetic processes (middle Jurassic, Paris basin, France). J. Sed. Res. 80, 791–807. Burchette, T., Wright, V.P., 1992. Carbonate ramp depositional systems. Sediment. Geol. 79, 3–57. Buxton, M.W.N., Pedley, H.M., 1989. A standardized model for Tethyan Tertiary carbonate ramps. J. Geol. Soc. 146, 746–748. Carpentier, C., Ferry, S., Lecuyer, C., Strasser, A., Geraud, Y., Trouiller, A., 2015. Origin of micropores in late Jurassic (Oxfordian) micrites of the eastern Paris basin, France. J. Sediment. Res. 85, 660–682. Ctyroky, P., Karim, S.A., 1971. Stratigraphy and Paleontology of the Oligocene and Miocene Strata Near Anah, Euphrates Valley. Manuscript Report. Geosurv, Baghdad. daSilva, A.C., Loisy, C., Cerepi, A., Toullec, R., Kiefer, E., Humbert, L., Razin, P., 2009. Variations in stratigraphic and reservoir properties adjacent to the Mid-Paleocene sequence boundary, Campo section, Pyrenees, Spain. Sed. Geol. 219, 237–251. Deville de Periere, M., Durlet, C., Vennin, E., Lambert, L., Bourillot, R., Caline, B., Poli, E., 2011. Morphometry of micrite particles in cretaceous microporous limestones of the Middle East: influence on reservoir properties. Mar. Petrol. Geol. 28, 1727–1750. Dickson, J.A.D., 1965. A modified staining technique for carbonates in thin section. Geology 205, 587. Dickson, J.A.D., 1966. Carbonate identification and genesis as revealed by staining. J. Sediment. Petrol. 36, 491–505. Ditmar, V., Iraqi-Soviet team, 1971. Geological Conditions and Hydrocarbon Prospects of the Republic of Iraq-Northern and Central Parts. Iraq National Oil Company Library, Baghdad (unpublished). Folk, R.L., 1959. Practical petrographic classification of limestones. Am. Assoc. Petrol. Geol. Bull. 43, 1–38. Folk, R.L., 1965. Some aspects of recrystallization in ancient limestones. In: Pray, L.C., Murray, R.S. (Eds.), Dolomitization and Limestone Diagenesis: Tulsa, OK. Petrology of Sedimentary Rocks, vol. 13, pp. 14–48. Folk, R.L., 1974. The natural history of crystalline calcium carbonate: effect of magnesium content and salinity. J. Sediment. Petrol. 44, 40–53. Fournier, F., Borgomano, J., 2009. Critical porosity and elastic properties of microporous mixed carbonate-siliciclastic rocks. Soc. Explor. Geophys. 74, E93–E109. Ghafur, A.A., 2012. Sedimentology and Reservoir Characteristics of the Oligocene-Early Miocene Carbonates (Kirkuk Group) of Southern Kurdistan (Ph.D Thesis). Earth and Ocean Sciences, Cardiff University, Cardiff, UK, p. 312. Ghafur, A.A., 2015. Integrated depositional model of the carbonate Kirkuk group of southern Kurdistan-Iraq. J. Nat. Sci. Res. 5, 79–106. Ghafur, A.A., Hasan, D.A., 2017. Petrophysical properties of the upper Qamchuqa carbonate reservoir through well log evaluation in the Khabbaz Oilfield. UKH J. Sci. Eng. 1, 72–88. Ghafur, A.A., Salad Hersi, O., Sissakian, V.K., Omer, H.O., Abdulhaq, H.A., 2019. Faciescontrolled Dolomitization of the Lower Cretaceous Qamchuqa Formation, Kurdistan Region, Northern Iraq. Geoconvention conference, Calgary-Canada. Gischler, E., Dietrich, S., Harris, D., Webster, J.M., Ginsburg, R.N., 2013. A comparative study of modern carbonate mud in reefs and carbonate platforms: mostly biogenic, some precipitated. Sediment. Geol. 292, 36–55. Grabowski, G., Liu, C., 2008. Sequence stratigraphy and depositional history of the Eocene-Miocene carbonates and evaporites in the subsurface of the northern Mesopotamian Basin, northeast Iraq. In: 8th Middle East Geosciences Conference and Exhibition, Bahrain, Manama. Grabowski, G., Liu, C., 2009. In: Ages and Correlation of Cenozoic Strata of Iraq. International Petroleum Technology Conference. Grabowski, G., Liu, C., 2010. Strontium-isotope age dating and correlation of phanerozoic anhydrites and unfossiliferous limestones of Arabia. In: American Association of Petroleum Geologist, 9th Middle East Geosciences Conference, GEO 2010. GeoArabia, Abstract, vol. 16, p. 173. Hasiuk, F.J., Kaczmarek, S.E., Fullmer, S.M., 2016. Diagenetic origins of the calcite microcrystals that host microporosity in limestone reservoirs. J. Sediment. Res. 86, 1163–1178. Ibrahim, M.A.A., Sarg, R., Hurley, N., Cantrell, D., Humphrey, J.D., 2016. Micrite, Microporosity and Total Organic Carbon Content: Depositional and Diagenetic Controls Linking Small- and Large-Scale Observations in the Tuwaiq Mountain and Hanifa Formations. AAPG Annual Convention and Exhibition, Denver, Colorado. Article # 51229. Jassim, S.Z., Goff, J.C., 2006. Geology of Iraq, p. 341. Dolin. Kaczmarek, S.E., Fullmer, S.M., Hasiuk, F.J., 2015. A universal classification scheme for the microcrystals that host limestone microporosity. J. Sediment. Res. 85, 1197–1212. Kharajiany, S.O.A., 2008. Sedimentary Facies of Oligocene Rock Units in Ashdagh Mountain-Sangaw District-Kurdistan Region. MSc. Thesis (un-published). University of Sulaimani, Sulaimani, p. 106. Lambert, L., Durlet, C., Loreau, J.P., Marnier, G., 2006. Burial dissolution of micrite in Middle East carbonate reservoirs (Jurassic-Cretaceous): keys for recognition and timing. Mar. Petrol. Geol. 23, 79–92. Lasemi, Z., Sandberg, P.A., 1984. Transformation of aragonite-dominated lime muds to microcrystalline limestones. Geology 12, 420–423. Lasemi, Z., Sandberg, P.A., 1993. Microfabric and Compositional Clues to Dominant Mud Mineralogy of Micrite Precursors. Springer Verlag, pp. 173–185.

Declaration of competing interest Author confirm that there is no conflict of interest to be reported. CRediT authorship contribution statement Ala A. Ghafur: Conceptualization, Data curation, Visualization, Investigation, Validation, Writing - original draft, Writing - review & editing. Acknowledgement The author thanks professor V. Paul Wright for reviewing the manuscript and giving valuable feedback. The acknowledgement also goes to all lab technicians from Cardiff University in UK and those whom helped and supported this research. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.petrol.2020.107247. References Al-Banna, N.Y., 2008. Stratigraphic Note: Oligocene/Miocene Boundary in Northern Iraq. GeoArabia-Manama, vol. 13, p. 187. Al-Hashimi, H.A.J., Amer, R.M., 1985. Tertiary Microfacies of Iraq. Directorate General for Geological Survey and Mineral Investigation. Al-Khateeb, A.A., Hassan, K.M., 2006. Distribution of celestite in Karbala-najaf area, central southern part of Iraq. Iraqi Bull. Geol. Min. 2, 45–56. Al-Naqib, K.M., 1960. Geology of the southern area of Kirkuk Liwa, Iraq. In: 2nd Arab Petroleum Congress, Beirut, vol. 2, pp. 45–81. Al-Qayim, B., 2006. Sag-Interior Oligocene Basin of North-central Iraq: Sequence stratigraphy and basin overview (Abstract). In: Middle East Conference and Exhibition; Manama, Bahrain. Andrews, J.E., 1991. Geochemical indicators of depositional and early diagenetic facies in Holocene carbonate muds, and their preservation potential during stabilisation. Chem. Geol. 93, 267–289. Anselmetti, F.S., Luthi, S., Eberli, G.P., 1998. Quantitative characterization of carbonate pore systems by digital image analysis. Am. Assoc. Petrol. Geol. Bull. 82, 1815–1836. Arzani, N., 2006. Primary versus diagenetic bedding in the limestone-marl/shale alternations of the epeiric seas, an example from the lower lias (early Jurassic) of SW Britain. Carbonates Evaporites 21, 94–109. Barnett, A.J., Wright, V.P., Chandra, V.S., Jain, V., 2018. Distinguishing between eogenetic, unconformity-related and mesogenetic dissolution: a case study from the Panna and Mukta fields, offshore Mumbai, India, 435. The Geological Society of London. Special Publications, pp. 67–84. Bathurst, R.G.C., 1975. Carbonate Sediments and Their Diagenesis: Development in Sedimentology. Second Enlarged Edithion. Elsevier Science Ltd, Amsterdam, p. 658. Behnam, H.A., 1979. Stratigraphy and Paleontology of Khanaqin Area NE Iraq. D. G. of Geological Survey and Mineral Investigation, Iraq. Bellen, R.C.V., 1956. The stratigraphy of the main limestone of the Kirkuk, Bai Hassan and Qara Chauq Dagh structures in northern Iraq. J. Inst. Petrol. 42, 233–263. Bellen, R.C.V., Dunnington, H., Wetzel, R., Morton, D., 1959. Lexique Stratigraphique. Interntional. Asie, Iraq, p. 333.

15

A.A. Ghafur

Journal of Petroleum Science and Engineering 192 (2020) 107247 Salter, M.A., Perry, C.T., Wilson, R.W., 2012. Production of mud-grade carbonates by marine fish: crystalline products and their sedimentary significance. Sedimentology 59, 2172–2196. Scholle, P.A., 1977. Chalk diagenesis and its relation to petroleum exploration: oil from chalks, a modern miracle. AAPG (Am. Assoc. Pet. Geol.) Bull. 61, 982–1009. Steinen, R.P., 1978. On the diagenesis of lime mud: scanning electron microscopic observations of subsurface material from Barbados, W. I. J. Sediment. Petrol. 48, 1139–1148. Stowakiewicz, M., Perri, E., Tucker, M.E., 2016. Micro-and nanopores in tight zechstein 2 carbonate facies from the southern Permian basin, NW Europe. J. Petrol. Geol. 39, 149–168. Towe, K.M., Hemleben, C., 1976. Diagenesis of magnesian calcite: evidence from miliolacean foraminifera. Geology 4, 337–339. Tucker, M.E., Wright, V.P., 1990. Carbonate Sedimentology. Wiley-Blackwell. Turpin, M., Emmanuel, L., Immenhauser, A., Renard, M., 2012. Geochemical and petrographical characterization of fine-grained carbonate particles along proximal to distal transects. Sediment. Geol. 281, 1–20. Volery, C., Davaud, E., Foubert, A., Caline, B., 2009. Shallow-marine microporous carbonate reservoir rocks in the Middle East: relationship with seawater Mg/Ca ratio and eustatic sea level. J. Petrol. Geol. 32, 313–325. Volery, C., Davaud, E., Foubert, A., Caline, B., 2010a. Lacustrine microporous micrites of the Madrid Basin (late Miocene, Spain) as analogues for shallow-marine carbonates of the Mishrif reservoir formation (Cenomanian to early Turonian, Middle East). Facies 56, 385–397. Volery, C., Davaud, E., Durlet, C., Clavel, B., Charollais, J., Caline, B., 2010b. Microporous and tight limestones in the urgonian formation (late Hauterivian to early Aptian) of the French Jura Mountains: focus on the factors controlling the formation of microporous facies. Sediment. Geol. 230, 21–34. Volery, C., Suvorova, E., Buffat, P., Davaud, E., Caline, B., 2011. TEM study of Mg distribution in micrite crystals from the Mishrif reservoir Formation (Middle East, Cenomanian to Early Turonian). Facies 57, 605–612. Webb, G.E., Nothdurft, L.D., Kamber, B.S., Kloprogge, J.T., Zhao, J.X., 2009. Rare earth element geochemistry of scleractinian coral skeleton during meteoric diagenesis: a sequence through neomorphism of aragonite to calcite. Sedimentology 56, 1433–1463. Wheeley, J.R., 2006. Taphonomy, Sedimentology and Palaeoenvironmental Interpretation of Middle Ordovician Limestones, Jamtland, Sweden. Unpublished PhD thesis, Cardiff University, Earth-Sciences Department, p. 198. Wright, V.P., Alonso Zarza, A.M., Sanz, M.E., Calvo, J.P., 1997. Diagenesis of late Miocene micritic lacustrine carbonates, Madrid Basin, Spain. Sediment. Geol. 114, 81–95. Wu, Y., Wu, Z., 1996. Two-phase diagenetic alteration of carbonate matrix: middle Ordovician limestones, Taiyuan City area, China. Sediment. Geol. 106, 177–191. Youkhanna, R.Y., Hradecky, P., 1978. Report on Regional Geological Mapping of Khanaqin - Maidan Area (Part I). Geological Survey Department, State Organization of Geological Survey and Mineral Investigation. Zachos, J., Pagani, M., Sloan, L., Thomas, E., Billups, K., 2001. Trends, Rhythms, and Aberrations in global Climate 65 Ma to present. Science 292, 686–693.

Lasemi, Z., Sandberg, P.A., Boardman, M.R., 1990. New microtextural criterion for differentiation of compaction and early cementation in fine-grained limestones. Geology 18, 370–373. Lawa, F.A.A., Ghafur, A.A., 2015. Sequence stratigraphy and biostratigraphy of the prolific late Eocene, Oligocene and early Miocene carbonates from Zagros fold-thrust belt in Kurdistan region. Arab. J. Geosci. 8, 8143–8174. Loucks, R.G., Lucia, F.J., Waite, L.E., 2013. Origin and Description of the Micropore Network within the Lower Cretaceous Stuart City Trend Tight-Gas Limestone Reservoir in Pawnee Field in South Texas, vol. 2. A Publication of the Gulf Coast Association of Geological Societies, pp. 29–41. Lucia, F.J., Loucks, R.G., 2013. Microporosity in Carbonate Mud: Early Development and Petrophysics, vol. 2. A Publication of the Gulf Coast Association of Geological Societies, pp. 1–10. Majid, A.H., Veizer, J.A.N., 1986. Deposition and chemical diagenesis of tertiary carbonates, Kirkuk oil field, Iraq. AAPG (Am. Assoc. Pet. Geol.) Bull. 70, 898–913. Martin, G.D., Wilkinson, B.H., Lohmann, K.C., 1986. The role of skeletal porosity in aragonite neomorphism-Strombus and Montastrea from the Pleistocene Key Largo limestone, Florida. J. Sediment. Res. 56, 194–203. Mazzullo, S.J., Bischoff, W.D., 1992. Meteoric calcitization and incipient lithification of recent high-magnesium calcite muds, Belize. J. Sediment. Petrol. 62, 196–207. Moshier, S.O., 1989. Microporosity in micritic limestones: a review. Sediment. Geol. 63, 191–213. Muhammed, Q.A., 1983. Biostratigraphy of Kirkuk Group in Kirkuk and Bai-Hassan Areas. MSc. Thesis. University of Baghdad, Baghdad-Iraq. Munnecke, A., Westphal, H., Reijmer, J.J.G., Samtleben, C., 1997. Microspar development during early marine burial diagenesis: a comparison of Pliocene carbonates from the Bahamas with Silurian limestones from Gotland. Int. Assoc. Sedimentol. 44, 977–990. Munnecke, A., Westphal, H., 2005. Variations in primary aragonite, calcite, and clay in fine-grained calcareous rhythmites of Cambrian to Jurassic age-an environmental archive? Facies 51, 611–626. Munnecke, A., Westphal, H., K€ olbl-Ebert, M., 2008. Diagenesis of plattenkalk: examples from the Solnhofen area (Upper Jurassic, southern Germany). Sedimentology 55, 1931–1946. Norbisrath, J.H., Eberli, G.P., Weger, R.J., Knackstedt, M., Verwer, K., 2012. Modeling Electrical Resistivity in Carbonates Using Micro-CT Scans and Assessing the Influence of Microporosity with MICP. AAPG Annual Convention and Exhibition, Long Beach, California. Article # 40953. Perry, C.T., Salter, M.A., Harborne, A.R., Crowley, S.F., Jelks, H.L., Wilson, R.W., 2011. Fish as major carbonate mud producers and missing components of the tropical carbonate factory. Proc. Natl. Acad. Sci. Unit. States Am. 108, 3865–3869. Purkis, S., Cavalcante, G., Rohtla, L., Oehlert, A.M., Harris, P.M., Swart, P.K., 2017. Hydrodynamic control of whitings on great Bahama bank. Geology 45, 939–942. Richard, J., Sizun, J.P., Machhour, L., 2007. Development and compartmentalization of chalky carbonate reservoirs: the Urgonian Jura-Bas Dauphine platform model (Genissiat, southeastern France). Sediment. Geol. 198, 195–207.

16