Depositional environment and diagenesis of Asmari Formation in the well-A (oileld at western Lorestan basin) and Syah-koh Anticline (Zagros fold–thrust belt, west Iran): microfacies, sequence stratigraphy and geochemistry

The Asmari Formation (Oligocene-Miocene), as one of the main carbonate petroleum reservoirs of the Middle East, represents strongly variable reservoir quality on regional and local scale. One outcrop analogue (Syah-koh Anticline) and a well-A (oileld at western Lorestan basin), both situated in western Iran, were investigated to address the reservoir characteristics. The 12 microfacies identied reect deposition on a broad carbonate ramp reecting an outer, mid and inner ramp setting. Three third-order depositional sequences are deduced based on interpreted Gamma-ray, sonic logs and stratal stacking patterns. The identied sequences and existence of a thicker evaporite succession in well-A points to more restricted depositional conditions with reduced connection to the open sea from the Syah-koh Anticline towards well-A. The diagenetic modications revealed micritization, dissolution, cementation, fracturing, dolomitization and compaction. Petrographic and δ 13 C and δ 18 O analysis of cements points towards marine, meteoric and burial diagenetic alteration, whereby the cements with δ 13 C and δ 18 O values similar to those of enclosing micrite (0 to -2 ‰ V-PDB) reect a marine diagenetic environment. The micrite phases enriched in aforementioned isotopes resulted from evaporation which is supported by widespread associated dolomitization. The J trend present in the δ 13 C and δ 18 O cross plot reects the meteoric diagenetic overprint. The cements with highly depleted δ 18 O signatures (<-10 ‰ V-PDB) are linked to burial diagenetic processes. The isotopic values are in agreement with the luminescence characteristics of the cements including dull for marine, alternating bright and dull zones for meteoric and bright for burial cements.


Introduction
The Asmari Formation (Late Oligocene -Early Miocene) is one of the major hydrocarbon reservoirs in the Zagros region of Iran. This formation at its type section in Tang-e Gel-e Tursh (southern Iran) (Motiei, 1993) consists of limestones, dolomitic limestones and argillaceous limestones. Geological investigations of this formation started already since the early 20th century by Busk and Mayo (1918) and continue till today. Several studies have been carried out on microfacies, especially addressing the depositional environments and sequence stratigraphy of the Asmari Formation (e.g. Motiei, 1993; . However, the extensive regional distribution of this formation in the Zagros Basin, its paleo-climate and tectonic setting causes a large variability in microfacies that subsequently were affected by diagenetic processes. Due to this variability, no speci c study can fully address all reservoir aspects of this formation in the entire Zagros. For instance, the most obvious variability, reported by Motiei (1993), is that the evaporitic Kalhor Member transitionally changes to limestone of the middle Asmari Formation in the northwestern part of the Zagros Basin, whereas in the southeast it changes to the sandy Ahwaz Member. Consequently, the study of reservoir analogues nearby different oil-elds of Asmari in Zagros can provide complementary information.
In this study two sections of the Asmari Formation in the Lorestan Basin have been assessed, i.e. a surface section in Syah-koh Anticline and a subsurface section in well-A (oil eld in the western part of Lorestan Basin). The aim is to reconstruct the depositional environment and to provide a relevant sequence stratigraphy model, which both are essential to understand and improve the predictive aspect of economic exploration and production (as shown in Catuneanu, 2006) of the Asmari Formation. Moreover, the diagenetic modi cations of the studied intervals and related processes have been investigated. This study is based on eld work and systematic sampling, petrography, and stable carbon as well as oxygen isotope analyses.

Geological Setting
The Zagros Basin formed as a foreland basin in response to the development of Zagros Mountain Belt, which is part of the Alpine-Himalayan system of Iran, extending from the NW border of Iranian through to SW of Iran. It is divided into two main structural units from south to north: the Zagros Fold-Thrust Belt and High Zagros Belt. The Zagros Fold-Thrust Belt corresponds to the tectonically deformed part of the Zagros sedimentary basin. This occurred during Early Cretaceous to Recent Zagros orogeny (Alavi, 2007) and extends for about 2000 kilometers from SE Turkey through northern Syria and Iraq to W and S Iran.
The study area is located in the SW-part of Lorestan Basin (Fig. 2). An excellent section of Asmari Formation with a thickness of about 283 m is exposed at the upper part of Syah-koh Anticline (Fig. 3).
This section displays a sharp conformable contact with the underlying Pabdeh and overlaying Gachsaran Formations (Fig. 3). In this area, the Asmari Formation can be subdivided into a lower Amari (53 m), middle Asmari (98 m) which is equivalent to the Kalhor Member and upper Asmari (132 m). The section in the well-A drilled in an oil eld at the W-part of Lorestan Basin penetrates the lower Amari (52 m), Kalhor Member (50 m) and upper Asmari (50 m).

Methods
In this study 63 thin sections from Syah-koh and 300 samples (cuttings) from well-A were stained with potassium ferricyanide and alizarin-red S (Dickson, 1965). Detailed petrographical and sedimentological analyses based on the macroscopic description (bed geometry, texture, sedimentary features, and faunal content) and characteristics of accessory grains and matrix composition served as evidence for deducing microfacies, sequence stratigraphy, paleoenvironmental reconstruction and identi cation of diagenetic products. Microscope analysis has been done by an Olympus BX60, Leica DM LP Parallel and Crossed Polar Optical and Cathodoluminescence (CL: using a modi ed Technosyn 8200MK2 instrument at 10 kV and 300-400 µA gun current). Moreover, a scanning electron microscope (SEM, model Hitachi TM-1000) with magni cations up to 11000x was used to study the micro-fabrics of fresh-cut, un-polished surfaces.
The classi cation of studied carbonate rocks follows the nomenclature of Dunham (1962), while microfacies and their depositional setting are compared to standard ramp microfacies (RMF) descriptions by Wilson (1975), Pedley (1996) and Flügel (2010). Sequences are de ned as system tracts based on two main surfaces (i.e. maximum ooding surfaces (MFS) and sequence boundaries (SB)). Arabian Plate sequences reported by Haq and Al-Qahtani (2005) together with universal sequences (Ogg, et al., 2016) are compared with the results obtained in this study. Stable carbon and oxygen isotopes were analysed at KU Leuven on a Thermo Delta V Advantage isotope ratio mass spectrometer coupled to a GasBench II. The analytical procedure is the same as reported in Mohammadi et al. (2019). For TOC 19 samples were analysed at KU Leuven. The carbonates were removed by repeated acidi cation with diluted (2%) HCl. For the determination of %OC (organic carbon and the stable carbon isotope composition of the OC fraction (δ 13 C OC ), samples were combusted in an elemental analyser -isotope ratio mass spectrometer (EA-IRMS, ThermoFinnigan Flash HT, and ThermoFinnigan DeltaV Advantage), and data were calibrated using an in-house Leucine and IAEA-C600 standard (caffeine).

Sedimentary Microfacies
The Asmari Formation in the studied sections consists of both skeletal and non-skeletal carbonates. Skeletal grains mainly correspond to benthic and some pelagic foraminifera, red algae, bivalves, crinoids, bryozoa and some coral fragments. Benthic foraminifera are the main skeletal component and display a broad diversity range (e.g. Operculina sp., Miliolids, Elphidum sp. Archaias sp, Dendritina rangi ( Fig. 4A to F). Planktonic foraminifera consist mainly of Globigerinids (Fig. 4G). Red algae is the second most important skeletal component representing variable types like Peyssoneliacean, Subterraniphyllum and Lithophyllum ( Fig. 4H to K). Non-skeletal grains consist mainly of ooids, intraclasts and peloids (Fig. 4L).
Based on lithology, texture and sedimentary characteristics, 12 marine microfacies could be differentiated in the studied Asmari Formation. These microfacies re ect a succession that was deposited upon a vast extended carbonate ramp consisting of an outer, middle and inner carbonate ramp. The latter can be further subdivided into different sub-environments including shoal, open lagoon, restricted lagoon and intertidal setting. In the following discussion, the different microfacies types are described and interpreted in terms of paleoenvironment.

I) Outer ramp
Planktonic foraminifera wackestone (MF1) This microfacies is characterized by grains oating in a micrite matrix with loose packing (Fig. 5A). Clastic carbonate grains are uncommon. The constituents of these wackestones consist dominantly of planktonic foraminifera (globigerinids and globorotalids), and rarely echinoid fragments, without benthic foraminifera and any features characteristic of shallow-water and/or high-energy sedimentation. Sand-sized planktonic foraminifera are the main components, which mixed with minor thin-shelled bivalves, echinoderm and ostracod shell fragments in the context. They show intergranular and moldic porosity lled with a cement composed of calcite and iron oxide. Furthermore, replacement of rhombohedral dolomite is common.
The depositional environment is interpreted to be situated just below wave base within the photic zone of an outer ramp based on sedimentological characteristics and faunal content (cf. Buxton and Pedley 1989;Pedley 1998). Abundant planktonic foraminifera within a micritic matrix indicate calm and aphotic sedimentation conditions, below storm wave base(SWB). This interpretation is based on co-occurrence of planktonic foraminifera which can be used as index for open marine conditions (Geel, 2000) and by the ne-grained matrix. Moreover, according to Ćosović et al. (2004) the lack of benthic foraminifera and red algae indicates the lower limit of the photic zone. This microfacies occurs at the bottom of the lower Asmari.

Bioclastic wackestone/mudstone (MF2)
This microfacies is made up by bioclasts such as echinoids, bryozoans, ostracods, echinoderms and pelagic foraminifera that oat in a micritic to calci-siltitic matrix (Figs. 4, 5B). In this light-yellow clayey wackestone/mudstone main planktonic fauna consists of dispersed Globigerina. Furthermore, rarely bentic large foraminifera such as Amphistegina and Lenticulina are recognized in this facies. Bioturbation and burrowing are widespread and locally the matrix is neomorphosed to microsparite. The decrease in the proportion of planktonic foraminifera and common bioturbation differentiates this microfacies from MF1.
The depositional environment of MF2 is placed below wave base in the photic zone. This interpretation is based on accumulation of planktonic foraminifera with bioclasts such as bryozoans, as well as bioturbation and muddy matrix (Wilson, 1975;Geel, 2000;Flügel, 2010).

II) Mid ramp
Operculina algal packstone/rudstone (MF3) This microfacies is dominated by large planktonic foraminifera, especially Operculina, Miogypsina and coralline and Peyssoneliacean red algae (Fig. 5C). Additional components are fragments of corals, echinoderms and bryozoans. The packstone is composed of a variety of allochems in terms of type and size, including mainly sand to silt-sized fragments of planktonic foraminifera. The fabric is grainsupported with slightly tight packing. Foraminifera and algae display co-existence and in some cases are encrusted (see Fig. 4K). Coralline algae rudstone to packstone is a common Miocene ramp microfacies (Buxton and Pedley, 1989) comprising Echinoderm, rotalia and red algae. Both moldic and framework porosity are well developed and partially lled with cements.
The depositional environment of this microfacies corresponds to the deepest part of a mid ramp just above the lower limit of the photic zone. This interpretation is supported by occurrence of large foraminifera such as vast range of Operculina which is often encountered in the latter environmental setting (Geel, 2000;Romero et al., 2002). Moreover, the rare occurrence of planktonic foraminifera and absence of lagoonal foraminifera like miliolids are additional arguments in support of a mid ramp setting.

Bioclastic bindstone (MF4)
This microfacies is characterized by a bindstone fabric with Peyssoneliacean red algae which trap and bind allochems and ne carbonate particles (Fig. 5D). Principal constituents which were trapped are fragments of corals, gastropods, bivalves, echinoids and benthic foraminifera. The fabric displays a tight packing with rare intergranular porosity, which is locally lled with sparitic equant cement.
The depositional environment of this microfacies corresponds to the upper part of a mid ramp situated between an euphotic and mesophotic zone below the fair-weather wave base. Although the environment of this microfacies is interpreted as low energy lagoonal environment by Rasser (2000Rasser ( , 2003, the limited presence of skeletal grains representative of low energy environment, like millolids, point toward sedimentation under relatively lower energy conditions. This suggests sedimentation at a lower energy level and deeper setting compared to the former microfacies MF3. Oncoidal peloidal packstone to grainstone (MF5) This microfacies is characterized mainly by oncoids (Fig. 5E) which formed by encrusting and binding of sedimentary grains. Well-sorted peloids are the second important component. The other grains consist of some bioclasts such as echinoids, benthic foraminifera, ostracods and also ooids. The fabric is grain supported with intergranular pores with sparitic in ll which locally changes to a micritic matrix. The pores are partially lled by blocky calcite.
The depositional environment of this microfacies corresponds to the border of mid and distal inner ramp in the euphotic zone around fair-weather wave base. This interpretation is supported by the presence of well sorted peloids and rare ooids re ecting a hydrodynamic shoal environment of with oncoids and rare large bentic foraminifera support a moderately high-energy setting as encountered at the transition of inner to mid ramp.

III) Inner ramp Bioclastic and ooid grainstone (MF6)
This microfacies is made up by the presence of ooids displaying an oval, circular or elongate outline and concentric internal structure (Fig. 5F) showing several cortical layers around a core which is usually dissolved. They are well-sorted and densely packed. In some cases, they are micritized. Other components consist of a broad range of peloids, benthic foraminifera and fragments of bivalves and gastropods. Intergranular and/or moldic are common pore types of this microfacies (Gharechelou et al. 2015).
The depositional environment of this microfacies corresponds to a distal inner ramp near-shoal to lagoon with deposition above fair-weather wave base. This interpretation is supported by the high frequency and well sorting and rounding of ooids and peloids making-up a grainstone texture indicating that this facies was deposited in a high energy shoal/barrier environment (Tucker and Wright, 1990;Tucker et al., 2009). However, co-occurrence of ooids with porcelaneous foraminifera (miliolids and dendritina rangi) within a ne-grained matrix point to a relatively low energetic environment, indicating a textural inversion and redisposition. This means that after forming ooids in an energetic shoal environment, components were transferred to a low lagoonal setting (Warren, 2006;Flügel, 2010).

Reefal rudstone to boundstone (MF7)
This microfacies is characterized by coral of scleractinia or hexacorals together with red algae and large foraminifera especially acervulina sp. (Fig. 5G). Other components consist of very small skeletal debris and intraclasts occurring in a micritic matrix. They show a tightly packed fabric with poor sorting, as well as some framework porosity. These pores are usually lled by both calcite and evaporitic cement.
The depositional environment of this microfacies corresponds to the distal inner ramp that is situated above fair-weather wave base (FWWB). This interpretation is supported by the occurrence of coral rudstone (see Wilson, 1975 andFlügel, 2010). They are restricted to two samples which point to limited and a discontinuous distribution, likely forming patch reefs.

Bioclastic miliolid packstone (MF8)
This microfacies consists of a vast range of small benthic foraminifera especially miliolids (Fig. 5H), making up a packstone. Calcareous algae like halimeda and dasycladacea as well as dendrita rangi foraminifera are poorly represented. Miliolid, dendritina packstone is recognized with beige to gray color and grainy texture. Skeletal allochems, porcelaneous foraminifers, specially miliolids and denderitina, are the main component of MF8. Additionally, echinoderms, bivalves and gastropods can be found. The fabric is grain-supported with relatively tight packing.
The depositional environment of this microfacies corresponds to the mid part of an inner ramp in the euphotic zone above fair-weather wave base. The occurrence of small to medium-sized porcelaneous foraminifera (miliolids) suggests an inner shelf environment (Geel 2000;Brandano and Corda 2003). Abundant allochems re ect a restricted lagoonal environment while gastropods indicate an open lagoon.
The diversity and grain-supported fabric suggest higher hydrodynamic conditions than present in a restricted lagoon pinpointing to an open lagoonal environment.

Bioclastic echinoid wackestone (MF9)
This microfacies is characterized by abundant echinoid fragments with intraclasts and rarely benthic foraminifera (Fig. 5I). It shows a poor sorting and loose packing. It is micrite matrix supported. In this microfacies vuggy pores are common and partially lled by cement.
The depositional environment of this microfacies corresponds to a lagoon within an inner ramp setting that is situated above fair-weather wave base. This interpretation is supported by the co-occurrence of echinoids evidencing open marine conditions with the small benthic foraminifera pointing to a more restricted environment (Flügel 2010). This microfacies differs from the former by its low diversity in skeletal grains.

Evaporites (MF10)
Evaporite phases are made up by laminated gypsum (Fig. 5K) displaying two main textures, namely alabastrine consisting of ne crystalline anhedral to subhedral crystals and porphyroblasts displaying coarse crystalline subhedral to euhedral crystals. Another texture that is infrequently observed, associated with the former, consists of daisywheel gypsum displaying rosette structures with needle gypsum crystals at their margins. There are some lath-shaped phases that usually correspond to pseudomorphs of anhydrite, especially occurring in the alabastrine textures.
According to Warren, (2006) the main evaporite settings are 1) mud at evaporites with sabkha, saline pans and salinas, 2) salteran evaporites and 3) deep water evaporites. Absence of textures like nodular and chicken-wire evaporites that relate to mud at evaporites within sabkha, saline pans and salinas suggest that such a setting did not existed in the study area. The rst study thin layer of the evaporite called basal evaporite possibly precipitated in a deep water setting (Kavoosi and Sherkati, 2012; Ra ei and Rahmani, 2017). This explanation is based on the continues position of this evaporitic layer between deep water sediment of Pabdeh and Lower Asmari Formations. However, the main evaporite unit followed directly upon mid ramp deposits, intercalated with carbonate wackestones and mudstones that re ect a restricted environment. Furthermore, the presence of elongated nodular anhydrite (Fig. 8I) together with the lack of microfossils indicate a saltern environment within a shallow and restricted lagoon, an interpretation which is in line with Daraei et al. (2015).

Dolomitized mollusk wackestone with evaporites (MF11)
This microfacies is characterized by abundant micritized bivalves and high degree of matrix dolomitization (Fig. 5J). Other components are represented by micritic agglutinated foraminifera, ostracods, brachiopods and calcareous algae. It corresponds to a matrix-supported microfacies with tight packing. Vugs, intergranular and microfracture pore types are common. They are lled with dolomite and evaporite cements. Replacement of micrite by rhombohedral dolomite is the most common diagenetic feature in this microfacies.
The depositional environment of this microfacies corresponds to a restricted lagoon above fair weather wave base with poor connection to the open basin. Evidence for this interpretation relates to the low diversity in skeletal grains (Gadzicki, 1983;Wilson and Evans, 2002). In addition, the micritic matrix, the poorly sorting of intraclasts and good taphonomic preservation of brachiopods point to an environment with low-energy conditions.

Mudstone (MF12)
This microfacies is characterized by a mud-supported fabric containing irregular and amalgamated discontinuous fenestral pores and solution vugs (Fig. 5L). Partially to entirely pore-lling by blocky to granular evaporitic and dolomitic cement has been observed. Dolomitization is common forming rhombohedral crystals in the matrix.
The depositional environment of this microfacies corresponds to an intertidal at and/or restricted lagoon. This interpretation is supported by well-preserved and widespread fenestral pore types which according to Hardie and Shinn (1986) originated from desiccation-shrinkage in an intertidal at. Absence of skeletal grains also supports this interpretation.

Sequence Stratigraphy Interpretation
Based on eld observations and petrography, both re ecting temporal and spatial changes in the microfacies that can be translated into sedimentary environments, together with petrophysical log data (gamma ray (GR) and sonic logs), the Asmari Formation in the two studied sections was subdivided into three depositional sequences (Fig. 6). These sequences re ect 1 to 10 million year corresponding to third order cycles. Each depositional sequence is characterized by a TST (transgressive systems tracts), HST (highstand systems tracts), with MFS (maximum ooding surface) and SB (sequence boundary).

Depositional sequence A (DS A)
Depositional sequence A, which is not well exposed, consists of thin layers of evaporite and thick and thin bedded limestones in the Syah-koh section and thin layers of evaporite as well as marl and thick bedded limestone in well A. The thickness of this sequence is respectively about 50 and 53 m. The only system tract recognized in this depositional sequence is a HST. The TST more likely is preserved in the underlying Pabdeh Formation since a depositional deepening for this formation is assumed when compared to Asmari (Vaziri-Moghaddam et al., 2010).
The maximum GR log response along with the low response of the sonic log at the lower part of "DS A" indicates a 2 m thick evaporite that continues in a 4 m thick shale layer which marks the boundary between the Asmari and Pabdeh Formation in well A. This can be correlated with the sharp transition of the thin bedded evaporite of Asmari Formation to olive marls in the lower Asmari Formation interval in the Syah-koh section (Fig. 6).
The HST of this DS mainly includes mid ramp deposits in the Syah-koh section with low API in the GR log and also the transition of shale to anhydrite layers of the Kalhor Member at the end of this sequence re ecting a shallowing upward trend in well-A (Fig. 6). The upper boundary of this DS is located at the base of an anhydrite succession in the Syah-koh section and well-A which re ects a restricted lagoonal environment that developed in an inner shelf setting.

Depositional sequence B (DS B)
Depositional sequence B, comprising thin anhydrite layers with occasional dolomites and thick bedded limestones in both study locations correspond to the Kalhor Member (Fig. 6). The thickness of this sequence in the Syah-koh section and in the well-A was about 100 and 50 m, respectively. It should be noted that the distinct difference in thickness of the Kalhor Member is due to presence of halite and anhydrite in well-A. The lower sequence boundary, marked by anhydrite sediments was already mentioned and the upper sequence boundary corresponds to restricted lagoonal deposits in both sections (Fig. 6).

Depositional sequence C (DS C)
Depositional Sequence C primarily comprises thick bedded limestones in the Syah-koh section and in well-A. Its thickness varies around 120 and 100 m, respectively. The lower and upper sequence boundary is marked by anhydrite sediments in both sections (Fig. 6). The TST of this DS consists of sediments deposited in an open lagoon in Syah-koh section and in a mid-ramp setting in well-A. MFS of "DS C" in well-A is characterized by mid-ramp microfacies along with high API values in the GR log re ecting an increase in clay content, and it is represented by mid ramp deposits in the Syah-koh section (Fig. 6). The shallowing upward trend of the HST in this sequence is characterized by medium to thick bedded limestone re ecting mid and inner ramp depositional environment in the Syah-koh section and in the well-A.

Diagenesis And Paragenesis
The main diagenetic features observed in the studied Asmari intervals include micritization, cementation, dissolution (porosity development), dolomitization, physical and chemical compaction, and fracturing.
Micritization is evident as micritic envelopes around skeletal fragments. The most pervasive calcite cement types are isopachous brous, drusy mosaic, dogtooth, granular, veins-lled and blocky cements. All the cements, except for the blocky ones which show bright CL colours, are dull when viewed under cathodoluminescence (Fig. 7A to G). Evaporitic cements (gypsum) were also observed in the study area (Fig. 7H, 8G and H). The thickest dolomitization interval occurs as an interlayer within the Kalhor Member (re ecting restricted lagoon settings). These dolomites include ne to medium euhedral crystals (100-200 µm) with lilac CL colour ( Fig. 7L and M) and saddle dolomite. In the carbonate section of the Upper Asmari, dolomitization occur as replacement phases as well as cement (Fig. 8A to C). Physical compaction and stylolitisation are other diagenetic features that can be recognized in the studied area ( Fig. 8D to F). Moreover, microfractures lled by veins (both calcite and evaporite) also occur widespread mainly in matrix support intervals like mudstone (Fig. 7D to H and 8E). The dissolution observed in the study area consist of moldic, vuggy and enlarged inter-and intragranular porosity. Moldic porosity is most probably caused by the aragonitic nature of skeletal grains and the vuggy porosity usually is often produced by enlargement of moldic porosity during different stages of the diagenesis (Fig. 7I to K). Enlarged intragranular caused by dolomitization which is common in the study area is very apparent in carbonate interlayers of the Kalhor Member. Based on these features the paragenesis (Fig. 9) can be split up into three different diagenetic settings: i.e. marine, meteoric, and burial realms.

Stable Carbon And Oxygen Isotopes
Stable isotope analyses were performed on micrite and cements. The obtained values are shown in Table 1 and plotted in Fig. 10.

Discussion
Depositional environment The 12 microfacies are interpreted to have been deposited in a broad ramp-type setting following the criteria presented by Buxton and Pedley (1989), Wilson (2002) and Flügel (2010). The reconstructed paleo-depositional environment is depicted in Fig. 11. This depositional system started with an outer ramp (deep water) where pelagic microfacies accumulated and gradually evolved into an extended mid ramp where large benthic foraminifera, coral and red algae (e.g. Peyssoneliacean) accumulated to form grain-and rudstones. The subsequent changes re ect an evolution towards open lagoon to mid ramp depositional settings as re ected by the occurrence of large and small benthic foraminifera and red algae. These depositional settings evolved into a restricted lagoon with noticeable decrease in fauna diversity on the one hand and deposition of evaporites on the other hand. The restricted lagoon eventually evolved into an intertidal setting marked by fenestral porosity and vadose cements. Among the samples studied from the Siah-koh Anticline and well-A, it appears that coral boundstone only occur in the subsurface well, whereas, large foraminifera especially operculina is more common in the outcrop section (i.e. Siah-koh Anticline). Most importantly, the deposits studied in the aforementioned two sections show obvious differences in thickness of evaporites due to presence of thick halite packages in well-A. This indicates that the basin system became more enclosed away from the more open marine Siah-koh Anticline in direction to well A. Moreover, according to Jahani et al. (2007) salt mobility occurs due to salt tectonics causing the movement of salt from syncline to anticline. Accordingly, as well-A was drilled in an anticlinal structure this explains to some degree the thickness differences caused by salt movements.

Diagenetic evolution
Early marine diagenesis started in the studied intervals with micritization of bioclasts (e. precipitation under marine conditions. This interpretation is in agreement with the non-luminescent characteristics of the brous cements (see Fig. 7F). The observed J trend (negative δ 13 C and δ 18 O) in the isotope cross plot of the cements (Fig. 11A) most likely point to the near-surface in uence of meteoric waters (Allan and Matthews, 1982;Lohmann, 1988). This can be related to organic matter resulting from soil CO 2 caused by karst related phenomena or soil formation which is typical for eogenetic conditions.
Besides, it also can originate from oil and methane oxidation, however, then normally the δ 13 C values are much more negative. Further evidences testifying to such eogenetic realm is the presence of concentric non-bright-dull CL zonation of the cement (see Fig. 7C). The most depleted δ 18 O V−PDB values of cements as low as -10‰ may re ect precipitation or recrystallization from uids with elevated temperatures under burial conditions (Choquette and James, 1987). This is in line with petrographic data showing uniform bright luminescence in blocky calcite (see Fig. 7G). Notably, the samples with almost the same ranges of stable isotope in both cement and micrite showing also a relatively high concentration of TOC suggest extensive recrystallization. Three third-order depositional sequences were deduced based on lithology, Gamma-ray and sonic log re ecting deepening and shallowing trends in the depositional environment of the studied area. The depositional patterns observed in these sequences suggest the deepening of the basin from well-A (oil eld located at the western part of the Lorestan Basin) towards the Syah-koh Anticline.

Conclusions
The diagenetic modi cations include early (marine and meteoric) and late (burial) diagenesis constrained by petrography and δ 13 C and δ 18  Declarations