The late Oligocene-early Miocene Qom Formation in the Central Iran Basin contains oil and gas in the Alborz and Sarajeh fields. Organic geochemical analyses in previous studies indicated that the hydrocarbons migrated from deeper source rocks, likely of Jurassic age. In the Central Iran Basin, the Qom Formation is 1,200 m thick and is bounded by the Oligocene Lower Red Formation and the middle Miocene Upper Red Formation. In previous studies, the Qom Formation was divided into nine members designated from oldest to youngest: a, b, c to c4, d, e and f, of which “e” is 300 m thick and constitutes the main reservoir. Our study focused on a Qom section located in the Gooreh Berenji region of central Iran which is 294 m thick. The lower part of the formation was not deposited, and only the following four members of early Miocene age (Aquitanian and Burdigalian) were identified between the Lower and Upper Red formations: “c2”? (mainly greyish to greenish gypsiferous marls); “d” (thin- to thick-bedded anhydrite with intercalation of thin-bedded sandstone); “e” (argillaceous or sandy limestone); and “f” (fine-grained coral and bryozoan boundstone). In contrast to the Central Iran Basin, the “e” member in Gooreh Berenji is only 15 m thick and does not have a good reservoir potential.

A detailed petrographic analysis of the Gooreh Berenji section resulted in the identification of 13 microfacies (MF) that were interpreted in terms of their depositional environments according to the following categories: MF1 (sabkha), MF2 (intertidal river channel), MF3 (lower intertidal), MF4 (peritidal), MF5 (supratidal), MF6 and MF7 (shallow restricted lagoon), MF8 and MF10 (proximal open-marine), MF9 (leeward lagoon), MF11 (shoal), MF12 (reef and patch reef formed within lagoon), and MF13 (open-marine). The Qom Formation constitutes a regional transgressive-regressive sequence that is bounded by two continental units (Lower and Upper Red formations). The transgression started from the south in the late Oligocene and by the early Miocene the sea covered all of central Iran. In the Gooreh Berenji area, carbonate deposition occurred on a shallow-marine ramp. The presence of a wide range of lagoonal facies indicates that reefal facies (“f”) developed in a narrow elongated strip away from the shoreline.

The Central Iran Basin is located in the western part of the Lut microplate, and is bounded to the north by the Alborz Mountains and to the southwest by the Sanandaj-Sirjan Zone (Stöcklin and Setudehnia, 1971) (Figure 1). The basin lies within the Turkish-Iranian Plateau that reaches elevations of 1.5 to 2.0 km and forms part of the Alpine-Himalayan collisional system (Sengör and Kidd, 1979; Dewey et al., 1986; Allen et al., 2004).

Figure 1:

The distribution of Oligocene-Miocene Qom Formation in the central Iran sedimentary basin. Also shown are exploration blocks 1 to 3 and the transverse section (blue line, Figure 3) (modified from Darvishzadeh, 1992).

Figure 1:

The distribution of Oligocene-Miocene Qom Formation in the central Iran sedimentary basin. Also shown are exploration blocks 1 to 3 and the transverse section (blue line, Figure 3) (modified from Darvishzadeh, 1992).

Hydrocarbons were discovered in this basin in the early 1950s in the Alborz and Sarajeh fields by the National Iranian Oil Company (NIOC, 1963) in the Oligocene-Miocene argillaceous limestones of the Qom Formation. This formation is in part equivalent to the petroliferous Asmari Formation in the southern Iranian fields. Whereas the Alborz field has now been nearly depleted and abandoned, the Sarajeh field continues to produce large volumes of gas and condensates.

In order to further understand the lithofacies, and the general environment and depositional characteristics of the Qom reservoir, we sampled a 294-m-thick section of the Qom Formation located at Gooreh Berenji in the central part of Dasht-e Kavir (the Great Desert of central Iran), 40 km northwest of Jandaq city (Figures 1 and 2). This paper describes the results of a sedimentological and petrographic study of 172 samples from the studied section. Thin sections were stained by Alizarine red solution and detailed microscopic analyses were carried out to identify the different microfacies (Figure 6) so as to interpret their depositional settings. We also correlated the Gooreh Berenji section to a more complete section at Kuh-e Nardaghi, located 300 km away in the Central Iran Basin (Figure 3).

Figure 2:

Satellite image of the study area showing the location of the Gooreh Berenji section, the Jandaq area (modified from Google Earth, 2006). The Qom Formation is separated from the Oligocene Lower Red Formation by a NE-trending fault. In this section, the Qom Formation is of Miocene age and is overlain by the mid-Miocene Upper Red Formation.

Figure 2:

Satellite image of the study area showing the location of the Gooreh Berenji section, the Jandaq area (modified from Google Earth, 2006). The Qom Formation is separated from the Oligocene Lower Red Formation by a NE-trending fault. In this section, the Qom Formation is of Miocene age and is overlain by the mid-Miocene Upper Red Formation.

Figure 3:

Correlation of the Qom Formation between the Gooreh Berenji and Kuh-e Nardaghi locations.

Figure 3:

Correlation of the Qom Formation between the Gooreh Berenji and Kuh-e Nardaghi locations.

The Alborz oil and Sarajeh gas fields are located in the northwestern part of the Central Iran Basin, about 140 km southwest of Tehran, between the cities of Qom and Kashan (Figures 1 and 4). Both fields occur in northwest-trending anticlines having a surface expression that is complicated by thrust, and possibly strike-slip faults.

Figure 4:

Isopach map of late Oligocene-early Miocene Qom Formation in the middle part of the Central Iran Sedimentary Basin (Qom area) contoured at 100 m intervals (modified from Baghbani et al., 1995). The thicknesses of the surface sections and the wells are shown in meters. The Alborz oil field and Sarajeh gas field are located adjacent to the Qom depocentre where formation reaches over 1,300 m.

Figure 4:

Isopach map of late Oligocene-early Miocene Qom Formation in the middle part of the Central Iran Sedimentary Basin (Qom area) contoured at 100 m intervals (modified from Baghbani et al., 1995). The thicknesses of the surface sections and the wells are shown in meters. The Alborz oil field and Sarajeh gas field are located adjacent to the Qom depocentre where formation reaches over 1,300 m.

Alborz Oil Field

The surface expression of the Alborz field is a large asymmetrical anticline with a height of 900 m. The steeply dipping northwestern plunge is partly covered by gravel plains. At the surface, the Qom salt plug occurs at the northern end of the anticline (Figure 4). Despite this complexity, reflection seismic maps have proven quite accurate. The field is located 17 km southwest of the Mil oil seep. The main reservoir reaches depths between 2,677 and 2,977 m above sea level. About 15 wells have been drilled in the field. They encountered extremely high pressures, which caused difficult drilling problems, including lost circulation at many levels and low formation strength in the upper section.

The main Qom reservoir zone, “e”, is 97 m and 79 m thick in the Alborz-6 and Alborz-8 wells, respectively, and the porosity varies from 0.5% to 15.0% (NIOC, 1959; Tabibian et al., 1995). The oil in the “e” reservoir is light and paraffinic with a gravity of 39° API (NIOC, 1964; Baghbani et al., 1995). The “c1” member (130 m thick) constitutes a separate reservoir zone that contains 24° API oil (NIOC, 1964; Tabibian et al., 1995).

The sulfur and asphaltene contents are about 0.14% and 0.20% by weight (Tabibian et al., 1995). The gas-oil ratio ranges between 1,034 and 1,172 standard cubic feet per stock tank barrel (SCF/STB) l, and at 60°F is about 1,100 SCF/STB (NIOC, 1959). The water saturation ranges between 42.0% and 91.0% in the “e” and “c” members.

Sarajeh Gas Field

The Sarajeh field has a complex stratigraphic and structural architecture due to heterogeneous lithofacies and faulting. The depth of the reservoir ranges between 2,409 and 2,794 m above sea level. Nine wells have been drilled in the field, of which three still produce gas; the others are abandoned.

The main Qom reservoir, section “e”, is 375 m and 270 m thick in the Sarajeh-3 and Sarajeh-4 wells, respectively. The porosity ranges between 4.3% and 11.8%, and the permeability between 0.19–1.85 mD. The condensates are sweet (nil sulfur) and have an API gravity of 51.5° at 60°F. The gas-to-oil ratio at 60°F is about 30 MSCF/STB (NIOC, 2004).

Source Rock

Geochemical studies by Tabibian et al. (1995) indicate that the Qom Formation source rocks are rich in types II and III kerogen. The source rocks are early-mature, ranging between late diagenesis and early catagenesis (the oil-generation window). The maturity index of clay minerals and the presence of smectite, illite and chlorite indicate that the formation falls in the early catagenesis stage. Moreover, the central Iran burial history shows that only the lower parts of the Qom Formation fall in the catagenesis stage. The Qom source rocks are poor in organic matter (total organic content (TOC) values of less than 1.0%) and the kerogen type is amorphous.

Since the Alborz and Sarajeh traps are not filled to the spill point and contain light oil and gas/condensates, it seems unlikely that the Qom Formation sourced the reservoirs. Instead it seems more probable that hydrocarbons migrated vertically through faults and fractures from much deeper source rocks (Tabibian et al., 1995).

To identify alternative source rocks for the Qom-reservoired hydrocarbons samples of Jurassic and Cretaceous sediments were collected from central Iran, analyzed and compared with those of the Qom Formation (Tabibian et al., 1995). The coal content of the Jurassic sediments in central Iran is mostly of type III kerogen (maceral vitrinite type) and, to a lesser degree, of type I kerogen. They are rich in organic matter (TOC > 4.0 %) and are highly mature, falling in the metagenesis phase (the gas generation window), and are poor in oil generation.

The saturate hydrocarbons distributions (gas chromatograms) of the Mil oil seep are similar to those of the extracted organic matter from Jurassic sediments in the Qom region. The biomarker distribution of the Mil oil seep matches that of the organic matter of the Jurassic sediments (Tabibian et al., 1995). The frequency of the Gammacerane biomarker (an indicator of evaporative lake environment) and the concentration of rearranged steranes (C29> C27> C28) of the oil seep are similar to the organic matter of Jurassic sediments. This similarity is not observed in the Cenozoic sediments.

Based on the geochemical similarity and maturity of the oils from the Mil seep and those extracted from the Jurassic sediments, we conclude that the Qom-reservoired hydrocarbons were sourced from Jurassic sediments.

Lithology

Due to the variation in the age and thickness, no type section has been defined for the Qom Formation. The Qom Formation is mainly composed of carbonates, volcaniclastics, siliciclastics and evaporites deposited in a northwest-trending belt in central Iran (Baghbani et al., 1995). Distinctive key markers are gypsum beds in the uppermost part of the “c2” member, the entire “d” member, and in the upper part of the “f” member just below the Upper Red Formation, which is a cap rock for the petroleum reservoir.

Soder (1955) divided the formation into the following six members, from oldest to youngest: “a” (basal limestone); “b” (sandstone and sandy marl); “c” (alternation of limestone and marl); “d” (gypsum and anhydrite); “e” (gray and green marl and argillaceous limestone); and “f” (reefal limestone). Abaie et al. (1964) further subdivided the “c” member into four submembers: “c1” (limestone and marl alternation); “c2” (marl, sandstone and gypsum); “c3” (bryozoan limestone); and “c4” (green marl). In this paper, all nine rock units are considered as members, and their correlation in the Qom area is shown in Figures 5a and 5b.

Figure 5:

Correlation of the Qom “a” to “f” members showing facies changes in the Qom area (Shakeri, 1995).

(a) Whereas all the members can be correlated in the Alborz-7, Rostam-1 and Sarajeh-3 wells, only the “a”, “b” and some parts of “c1” members are identified in the Neyzar surface section.

(b) facing page: Surface sections for each member, except for Sarajeh-3.

Figure 5:

Correlation of the Qom “a” to “f” members showing facies changes in the Qom area (Shakeri, 1995).

(a) Whereas all the members can be correlated in the Alborz-7, Rostam-1 and Sarajeh-3 wells, only the “a”, “b” and some parts of “c1” members are identified in the Neyzar surface section.

(b) facing page: Surface sections for each member, except for Sarajeh-3.

Due to the presence of faults in the Gooreh Berenji area, the lower part of the Qom Formation is not exposed (Figure 2). Studies carried out by Feiznia and Mosaffa (1998) in the Semnan area (the nearest section to Gooreh Berenji), show that the lower part of the formation was not deposited. Therefore the formation in the Gooreh Berenji area only consists of the “c2”, “d”, “e” and “f” members (Figures 3). The “c2” member is composed of gypsiferous marl containing oysters, gastropods, and pecten with intercalations of thin-bedded gypsum, sandstone and fossilifereous limestone. The “d” member is mostly composed of medium- to thick-bedded gypsum with intercalation of sandy and gypsiferous limestone. The “e” member is composed of thin-bedded fossilifereous limestone with intercalation of gypsiferous marl. The “f” member is composed of massive thick-bedded bryozoa and coral boundstone.

Lower Boundary and Underlying Formation

The Qom Formation unconformably overlies the Lower Red Formation (Rahimzadeh, 1994). The latter formation is composed of conglomerates, sandstones, gypsum, salt, silt, clay and rare pyroclastic sediments (Table 1). In the Gooreh Berenji section, the formation consists of reddish thin-bedded sandstones, siltstones and claystones. The occurrence of Nummulites intermedius and Neoalveolina cf. pygmaea led Soder (1955) to assign an Oligocene age to the formation. Bozorgnia (1965) identified Eulipidina cf. dilatata at the boundary of the Lower Red and Qom formations, and assigned the former an Oligocene age (Rupelian Stage).

Table 1

Stratigraphy column of studied area (Modified from the Geological Quadrangle Map of Iran no. H6, Jandaq sheet).

Upper Boundary and Overlying Formation

The Upper Red Formation unconformably overlies the Qom Formation (Rahimzadeh, 1994). The first study of the Upper Red Formation in the Central Iran Basin was carried out by National Iranian Oil Company geologists (Aghanabati, 2004), who divided it into three units:

  • (1) “M1” (red color claystone, siltstone and gypsiferous sandstone, thick anhydrite layers in the basal part);

  • (2) “M2” (sandstone containing cavities); and

  • (3) “M3” (gypsiferous siltstone, light yellow marl along with intercalation of calcareous sandstone).

In the Gooreh Berenji section, the Upper Red Formation lies paraconformably on the Qom Formation. Only the “M1” member was observed in this section.

Regional Distribution and Thickness

The Qom Formation is widely distributed in central Iran (Figure 1). It crops out in the south and southeast of the Kavir Plain on the following regions: Naien (north of Akhoreh River), Anarak, Millsahlab Kuh, Rizab Mishouri and Soheil Pakoo (Davoudzadeh, 1972; Sharkovski et al., 1984; Rahimzadeh, 1994) and, in the northwest, Zanjan, Hamadan and west Azerbaijan.

The thickness of the formation varies considerably in central Iran (Figures 4 and 7). In the middle part of the Central Iran Basin (the Qom area), its thickness reaches about 1,200 m (Baghbani et al., 1995), whereas in the Jandaq area it is 294 m thick. Towards the southeastern part of the basin (Kashan area), its thickness is approximately 300–600 m. An exceptional case occurs in the northeastern part of the Kashan area (Aran), where the thickness reaches 883 m (Dashtban et al., 2004).

The northwestern part of the Central Iran Basin is divided into three exploration blocks (Figure 1): Block 1 (the Razan area and the northeast part of Hamadan Province), Block 2 (northeastern Hamadan Province and the northeast of the Kurdistan Province) and Block 3 (the northeast of the Kurdistan Province and the west of Azerbaijan in the Takab area) (Shakeri, 2006). In Blocks 1 and 3, the thickness varies from 100–300 m, whereas in Block 2 it reaches 1,343 m (Shakeri, 2006). In the Tafresh area (the Koriyan-Amjak area), the thickness of the Qom Formation is up to approximately 2,300 m (Hadjian, 1970).

Paleontology and Age

In the middle part of the Central Iran Basin (Qom area), the Qom Formation is of early Miocene age (Aquitanian to Burdigalian). In the southeastern part of the basin, the occurrence of microfauna such as Nummulites intermedius indicates an Oligocene to early Miocene age (Rupelian to Burdigalian) (Rahaghi, 1973). Towards the northwestern part of the basin (in the Razan and Takab area), the age of the formation is interpreted as Aquitanian to Burdigalian. In the Qoshejeh anticline (located in Block 2, the Hamedan area), Nayebi (1989) identified different species of Lepidocyclina from the Qom Formation. In this area, only some units (“c1” and undifferentiated members) are known (Shakeri, 2006).

Biostratigraphic studies in the Central Iran Basin indicate that the Aquitanian “Miogypsinoids complanatus” and “Nephrolepidina tourneuri” occur in the “c1” member (Nayebi, 1995 and Karimi Mosadaq, 1999). These fauna do not occur in Gooreh Berenji. This is the main evidence for the absence of the lower part of the Qom Formation in this section.

The microfacies (Figure 6) and sedimentary environments (Figure 11) of the Qom Formation in the Gooreh Berenji section were studied and interpreted in detail using the classifications of Dunham (1962) and Carozzi (1989). The interpretation of microfacies was carried out based on Wilson (1975) and Flugel (2004).

Figure 6:

The stratigraphic column of Gooreh Berenji surface section indicating the distribution of the different facies.

Figure 6:

The stratigraphic column of Gooreh Berenji surface section indicating the distribution of the different facies.

Anhydrite and Gypsum (MF1)

This facies was scattered with sections of different thicknesses and horizons which varied in an interval of 200 m. In thin sections, the anhydrite layers appeared to be in the form of chicken wire (Figure 8a). The depositional environment was interpreted to be sabkha.

Figure 7:

Isopach map of Qom Formation in North Central Kavir (modified from Haghshenow, 1963).

Figure 7:

Isopach map of Qom Formation in North Central Kavir (modified from Haghshenow, 1963).

Figure 8:

(a) Anhydrite with chicken wire fabric attributed to primary deposition in a sabkha environment (MF1) (micrograph in cross-polarized light).

(b) Fine- to medium-grained lithic arenite with secondary gypsum cement filling the spaces between the grains, deposited in a tidal channel in a supratidal environment (MF2) (micrograph in plane-polarized light).

(c) Calcareous siltstone with small skeletal grains deposited in lower intertidal environment (MF3) (micrograph in plane-polarized light).

(d) Gypsiferous marl deposited in a supratidal environment (MF4) (micrograph in plane-polarized light).

Figure 8:

(a) Anhydrite with chicken wire fabric attributed to primary deposition in a sabkha environment (MF1) (micrograph in cross-polarized light).

(b) Fine- to medium-grained lithic arenite with secondary gypsum cement filling the spaces between the grains, deposited in a tidal channel in a supratidal environment (MF2) (micrograph in plane-polarized light).

(c) Calcareous siltstone with small skeletal grains deposited in lower intertidal environment (MF3) (micrograph in plane-polarized light).

(d) Gypsiferous marl deposited in a supratidal environment (MF4) (micrograph in plane-polarized light).

Sandstone (MF2)

Fine- to medium-grained sandstones (grain sizes less than 0.5 mm) occured mainly as thin layers at different levels of this section. They were mainly lithic wacke and lithic arenite. The main constituents of the lithic wacke were mainly detrital quartz, polycrystalline quartz (cherts), volcanic and metamorphosed grains, which were mainly of angular to sub-angular forms bounded in a micritic groundmass. The lithic arenite types were well sorted and were more texturally mature (Figure 8b). In some samples, gypsum cement was observed. This type of sandstone was generally deposited in shallow channels formed in tidal flat environments.

Siltstone (MF3)

In thin sections, the detrital grains forming the calcareous siltstones ranged from being sub-rounded to rounded. In some samples they contained fragments of benthic forms, ostracods and detrital carbonate grains (Figure 8c). This facies was generally deposited in lower intertidal environments.

Fossiliferous Gypsiferous Marl (MF4)

This gypsiferous facies was mainly observed in outcrop with gentle topography, and in numerous horizons occurring between siltstone and thin-bedded limestone. It was soft in hand specimens and was difficult to prepare for thin-section analysis. It contains ostracods, oysters, bivalves and gastropods (Figure 8d). This facies was mainly deposited in a peritidal environment.

MF5 - Mudstone

This facies was found in thin layers throughout the section. They were mainly gypsiferous, containing macrofossils such as oysters, gastropods and bioclasts. In some thin sections, several casts of different forms occur, mainly due to the dissolution of evaporite minerals such as salt and anhydrite. This evidence indicates deposition in a supratidal environment (Figure 9a).

Figure 9:

(a) Mudstone exhibiting the evaporitic mineral casts deposited in supratidal environment (MF5) (micrograph in plane-polarized light).

(b) Scattered bioclast in a matrix background along with silt and fine-grained sand deposited in a shallow restricted lagoonal environment (MF6) (micrograph in plane-polarized light).

(c) Lagoonal faunas, mainly bivalve and miliolid, with compaction phenomena deposited in high-energy lagoonal environment.

(d) Combination of allochems, such as bryozoa and red algae, deposited in a proximal open-marine environment (MF8) (micrograph in plane-polarized light).

(e) Large allochems with well-developed calcite cementation deposited in leeward-lagoonal environments (MF9) (micrograph in plane-polarized light).

(f) Echinoderm-red algal grainstone deposited in a proximal open-marine environment (MF10) (micrograph in plane-polarized light).

Figure 9:

(a) Mudstone exhibiting the evaporitic mineral casts deposited in supratidal environment (MF5) (micrograph in plane-polarized light).

(b) Scattered bioclast in a matrix background along with silt and fine-grained sand deposited in a shallow restricted lagoonal environment (MF6) (micrograph in plane-polarized light).

(c) Lagoonal faunas, mainly bivalve and miliolid, with compaction phenomena deposited in high-energy lagoonal environment.

(d) Combination of allochems, such as bryozoa and red algae, deposited in a proximal open-marine environment (MF8) (micrograph in plane-polarized light).

(e) Large allochems with well-developed calcite cementation deposited in leeward-lagoonal environments (MF9) (micrograph in plane-polarized light).

(f) Echinoderm-red algal grainstone deposited in a proximal open-marine environment (MF10) (micrograph in plane-polarized light).

Silty-Sandy Skeletal Wackestone (MF6)

The main allochems in this facies were fine-grained broken fragments of bivalves, such as oysters. Sparry calcite cements filled the open spaces in some of the chambers in oysters. Intra-granular spaces were filled by gypsum cement. Gastropods, miliolids, ostracods and secondary dolomitization were observed (Figure 9b). This facies was apparently deposited in a shallow, restricted lagoonal environment.

Gastropod/Bivalve Packstone (MF7)

The main allochems in this facies consisted of gastropods (30–40%) and bivalves (20–25%). In addition, miliolids and ostracods occured in a scattered pattern (Figure 9c). Most of the open spaces in the bivalves and gastropods were filled by a sparry calcite cement. Compaction, dolomitization, bioturbation and neomorphism were observed in some thin sections. The presence and abundance of the major allochems in this facies indicates sedimentation in a vast lagoonal environment, rich in bioclasts.

Red Algal Bryozoan Packstone (MF8)

The main allochems in this facies were red algae (40–50%), bryozoans (7–10%) and coarse-grained broken echinoderms (3–5%). Compactional features and syntaxial cements at the margins of echinoderms were observed in some samples. Porosity was not developed in this facies. The identified allochems played important roles in developing reefs (Figure 9d), implying an environment that was proximal to open-marine (reefs towards the open sea).

Gastropod/Bivalve Grainstone (MF9)

The main allochems in this facies were gastropods (60–70%) and, less commonly, coarse-grained bivalves (Figure 9e). The open spaces inside the gastropod chambers were filled by a sparry calcite cement. Compactional features, secondary dolomitization, and dark-colored grain minerals such as iron oxides and moldic porosity were readily identified. This facies was mainly formed in a high-energy or bioclastic bank.

Echinoderm/Red Algal Grainstone (MF10)

The main allochems in this facies consisted of fine- and coarse-grained echinoderms (50–60%), bryozoans and red algae (10–20%) (Figure 9f). Cementation, compaction, dolomitization and fracturing were observed. Sparry calcite cements filled the spaces between the grains. Syntaxial cements were observed at the fringes of echinoderms. The opaque minerals and pyrites were also observed. The major organisms were reef forming, suggesting a fore-reef and proximal open-marine environment.

Ooid Skeletal Grainstone (MF11)

This facies mainly consisted of ooid grains (60–70%) and minor bioclasts (5–8%). Cementation, compaction, moldic porosity and fracturing were observed. The intra-grain spaces were filled by a sparry calcite (Figure 10a). A shoal environment was interpreted for this facies.

Figure 10:

(a) Fine-grained ooids with well-developed oomoldic porosity deposited in a shoal environment (MF11) (micrograph in plane-polarized light).

(b and c) Coral boundstone with growth framework porosity (arrow) forming a reef body and scattered patch reefs in lagoon environment. Some of the growth framework porosity is filled by secondary gypsum cement (MF12) (micrograph in plane-polarized light).

(d) Bryozoan boundstone with recrystallized zoacium, partially filled by mud, deposited in an open-marine environment (MF13) (micrograph in cross-polarized light).

Figure 10:

(a) Fine-grained ooids with well-developed oomoldic porosity deposited in a shoal environment (MF11) (micrograph in plane-polarized light).

(b and c) Coral boundstone with growth framework porosity (arrow) forming a reef body and scattered patch reefs in lagoon environment. Some of the growth framework porosity is filled by secondary gypsum cement (MF12) (micrograph in plane-polarized light).

(d) Bryozoan boundstone with recrystallized zoacium, partially filled by mud, deposited in an open-marine environment (MF13) (micrograph in cross-polarized light).

Coral Boundstone (Framestone) (MF12)

The main components of this facies were corals, mostly hexacorals (Figures 10b and 10c). In a few samples, borings into the corals were filled by mud and silt. In some of the hexacorals, growth framework porosity was observed (Figure 10b). The reefs separated the lagoon from the open-marine environment. Patch reefs may also have developed within the lagoonal environment.

Bryozoan Boundstone (MF13)

The main allochems in this facies were bryozoans (Figure 10d). Zoecia of bryozoans was filled by mud. Neomorphism was the main agent for the conversion of mud to sparry calcite cement. The bryozoans formed build-up structures similar to those formed by coral reefs; some were in the form of isolated branching bryozoans scattered widely along the entire section. The evidence indicates deposition in an open-marine environment.

Several geologists have studied the depositional environment of the Qom Formation in central Iran. Dadfar (2002) used geochemical analyses to divide the Qom “f” member into six microfacies that represent the basin margin, fore-reef and back-reef environments. Azim and Lasemi (1997) and Noori and Lasemi (1997) divided the formation into carbonate facies related to open-marine, barrier, lagoonal and tidal-flat environments. Shakeri and Lasemi (1994) identified nine microfacies related to lagoonal, barrier, reef and open-marine environments that were deposited on a carbonate ramp. Okhravi (1998) studied the uppermost part of the Qom Formation (“f” member) of the Dobaradar, Kamar Kuh and Khur Abad regions in the middle part of the Central Iran Basin. They identified various microfacies, sedimentological cycles and diagenetic processes, and used them to interpret a depositional model. Ashrafzadeh and Lasemi (1997) identified four groups of microfacies related to tidal flat, lagoonal, barrier and open-marine environments.

In our study, five depositional environments were identified in the Qom Formation of the Gooreh Berenji region (Figure 11).

Figure 11:

Cross-section and model of the sedimentary environment in the Jandaq area during the Miocene period.

Figure 11:

Cross-section and model of the sedimentary environment in the Jandaq area during the Miocene period.

Supratidal-Sabkha Environment

The first group of microfacies consisted of gypsum and anhydrites found in variable thicknesses in numerous horizons. This facies was associated with sea-level lowstands that caused supratidal and sabkha environments, as consistent with facies belt number 9 of Wilson (1975).

Peritidal Environment

The second group of microfacies consists of silty limestones containing large shell fragments (oysters, gastropods and ostracods) or gypsiferous mudstones. The main environment of this facies is peritidal, as consistent with facies belt number 8 of Wilson (1975). The channels of siltstone and sandstones (lithic arenite and lithic wacke with gypsum cements) are also attributed to this environment.

Back Reef-Lagoonal Environment

This facies consisted of bivalve wackestone-packstone, gastropod wackestone, bivalve-gastropods packstone, gastropod and red algal packstones. The fossil content varied from 10–60%. In this facies, the main components belonged to a back reef-lagoonal environment. In some cases, the secondary dolomites and gypsiferous cements were widespread. In the Gooreh Berenji section, this facies alternated with sandy limestone and thin anhydrite layers showing a shallow restricted lagoonal environment. This facies was consistent with facies belt number 7 of Wilson (1975).

Peri-Reefal Environment

The facies encompassed different corals and was referred to as boundstone. The corals were not continuous and mostly appear as patchy lenticular beddings, suggesting a barrier between lagoonal and open-marine environments. This facies was consistent with facies belt number 5 of Wilson (1975).

Fore-Reef and Open-Marine Environments

The most diagnostic facies in this environment was bryozoan boundstone. Several types of bryozoa floated in the matrix, and they were mostly isolated branching forms. Under the microscope, the major zoecia in the bryozoans were filled by mud and sometimes appeared in the packstone facies. The evidence indicates an open-marine low-energy shallow basin as consistent with facies belt number 2 of Wilson (1975).

Diagenesis in the Qom Formation was characterized by calcite, anhydrite or dolomite. Five types of calcite cements were observed:

  • (1) isopachous rim cement that fringed around the allochems in the form of blades (MF6, 8, 11 and 12);

  • (2) coarse sparry calcite cement that filled the pore spaces between the grains as equant crystals (MF6, 11, 12 and 13);

  • (3) overgrowth cement that was found in skeletal packstone and grainstone, particularly surrounding the echinoderms stems and spines (MF8, 9, 11 and 12);

  • (4) poikilotopic cement consisting of large calcite crystal enclose smaller minerals of different sizes (MF11); and

  • (5) equant cement that occupied the original spaces between the grain, or chambers of skeletal grains, in a limestone (MF7, 8 and 11).

The anhydrite cements were mostly found in supratidal and intertidal environments (MF2). In some samples, the evaporite cements were observed in reef environments (MF12).

Where observed, dolomitization was microcrystalline with poor subhedral dolomite-types filling the poor spaces. This submarine cementation occurred in deeper basins as well as on the leeward and seaward sides of the reef. Okhravi and Amini (1998) made similar interpretations in the lower Miocene carbonate of the Central Iran Basin (Qom “f” member). These cements had a similar fabric to modern submarine cements.

The samples also showed micritization (micritic rim surrounding the skeletal grains, MF6, 8, 11 and 12), neomorphism (transformation of aragonite to calcite, MF6, 8, 9, 10, 11 and 12), dissolution (formation of porosities such as vuggy, interparticle, intraparticle and moldic porosity), compaction (packing of the grains) and bioturbation (in situ modification of texture, packing, sorting and clay content).

In the late Cretaceous and Cenozoic periods, several micro-continents, interspersed with zones of ophiolites and mélanges, amalgamated together in the collisional zone between the Arabian and Eurasian plates (Sengör, 1990). These micro-continents included the Sanandaj-Sirjan Zone, lying parallel to and northeast of the Zagros Suture, and the Lut Block in eastern Iran. Volcanic rocks of late Cretaceous to early Miocene age occur in the Sanandaj-Sirjan Zone and central Iran and represent Andean-type magmatism associated with the subduction of the Neo-Tethyan oceanic crust beneath Iran (Berberian et al., 1982).

North of the Neo-Tethyan subduction zone, alkaline volcanic and turbidite successions, up to 5 km thick, represent Eocene back-arc extension in central Iran, the Alborz Mountains, the Lesser Caucasus and the eastern Black Sea regions (Brunet et al., 2003). In central Iran, the successions are commonly overlain by terrestrial clastics, evaporites and volcanics of the Oligocene Lower Red Formation (Stöcklin and Setudehnia, 1971). The late Oligocene-early Miocene transgression deposited limy and marly sediments, including the Qom Formation, which covered vast areas of central Iran reaching Azerbaijan (Bozorgnia, 1965).

In the Jandaq (Gooreh Berenji) study area, the early transgression above the Lower Red Formation was represented by a sabkha environment (gypsum and anhydrites). As the transgression progressed, water depths in the basins gradually increased, resulting in restricted shallow, lagoonal basins and carbonate sedimentation. Variations in accommodation space, due to eustatic and tectonic influences, caused the deposition of various facies, such as gastropod packstone or bivalve gastropod wackestone to packstone. During maximal flooding, the sea covered all of central Iran and its margins where coral reefs evolved (Qom “f” member). In the Jandaq area, deposition occurred over a gentle shallow carbonate ramp and the maximum flooding was marked by bryozoan boundstone of the “f” member (Figure 11).

The transgression did not flood central Iran simultaneously (Aghanabati, 2004). In some places (e.g. Ardestan and Kashan region) it started in the early Oligocene (Rupelian), while in other places (Idaghchi Mountains) it occurred during the late Oligocene (Chattian) or as late as the early Miocene (Abedini, 1963–1964, inRahaghi, 1980). The transgression appears to have arrived from the south and progressed to the north and northwest of central Iran (Emami, 1991). After the early Miocene (after c. 16 Ma), orogenic movements and/or a sea-level lowstand again reset central Iran in a continental environment resulting in the deposition of the Upper Red Formation (Aghanabati, 2004).

The sedimentological study of the Qom Formation in the Gooreh Berenji section led to the identification of 13 microfacies deposited in several environments, including supratidal-sabkha, peritidal, back reef-lagoonal, peri-reefal and fore-reef, and open-marine. Whereas the Oligocene-early Miocene Qom Formation in the Central Iran Basin consists of the “a” to “f” members and is 800–1,200 m thick, in the Gooreh Berenji section, the lower part of the Qom section was not deposited and the formation is only 295 m. Only early Miocene members “c2” to “f” were identified above the Lower Red Formation, indicating that the Oligocene-Miocene sea, which flooded the Central Iran Basin, reached the Gooreh Berenji area much later in the early Miocene.

In the Central Iran Basin, the Qom Formation contained two distinctive carbonate microfacies, benthic and pelagic, whereas in Gooreh Berenji the pelagic sediments were not observed. In the Central Iran Basin, the pelagic and benthic sediments were deposited on a carbonate ramp that was deeper than in the Gooreh Berenji area. The reduced thickness and facies changes in the “e” member (15 m) in Gooreh Berenji area indicate that the hydrocarbon potential was not as high as in the Central Iran Basin, where it is 300 m thick.

The authors thank the Exploration Department of the National Iranian Oil Company (NIOC) and the Research Institute of the Petroleum Industry (RIPI) for permission to publish this paper. They also thank G.H.A. Fakouri, M. Moeini and other NIOC geologists and managers for their support. A.R. Chegini is thanked for drawing the figures and logs. The authors thank M. Memariani, Head of the Geochemical Department, and M. Moinpour for providing geochemical reports on the Qom Formation. Thanks are due to two anonymous reviewers for their suggestions that were useful in improving the manuscript. Finally, the authors thank GeoArabia’s Editor-in-Chief Moujahed Al-Husseini for his useful comments and suggestions, the Production Manager Nestor Buhay, and Arnold Egdane for designing the final paper.

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Alireza Shakeri is a Carbonate and Clastic Sedimentologist working with the Research Institute of Petroleum Industry (RIPI) since 2002. He gained a BSc (Honors) in Geology from Tehran University in 1990 and an MSc in Sedimentology from Azad University of Tehran in 1995. He has worked as a Field Geologist at the National Iran Oil Company (NIOC) for 11 years where his work focused on the Zagros, Central Iran, Makran and Caspian Sea sedimentary basin on Tertiary, Cretaceous and Paleozoic sediments. Since joining RIPI, Alireza has worked on joint projects with Statoil Co. (Norway), Shell (Netherlands), Total (France) and Hol (Austria). He is presently working on the reservoir characterization of the Kangan and Dalan Formation (Khuff equivalent) onshore, as well as offshore.

shakeriar@ripi.ir

Jalal Douraghinejad is a Sedimentologist working in the Research Institute of Petroleum Industry (RIPI) since 1999. Jalal has a BSc (Honors, 1985) in Geology from the University of Poona in India, and an MSc Tech of Applied Geology from the Indian School of Mines in Dhanbad. He obtained his PhD in Sedimentology in 1996 from the University of Poona. Jalal worked as a Sedimentologist in the Marine Geology Department of the Geological Survey of Iran (GSI) between 1996 and 1998 where his focus was on the marine sediments of the northeast Caspian Sea. Since joining RIPI he has worked on various projects such as reservoir characterization and sedimentologoical studies of Bibihakimeh field jointly with Statoil Co. (Norway) and on the sedimentology and reservoir characterization of South Pars field and Tabnak wells.

douraghij@ripi.ir

Mehran Moradpour is a Reservoir Geologist at the Research Institute of Petroleum Industry (RIPI) of Iran since November 2001. He received an MSc in Sedimentology and Sedimentary Petrology from the Department of Geology at Shahid Beheshti University, Tehran, Iran in 2000. His thesis was the study of sedimentary environment and diagenesis of Lower Cretaceous deposits around Esfahan city. He then joined the Oil and Energy Instruction Company as a junior Geologist. Meheran is presently working on carbonate reservoir characterization. His main interest of research is on sedimentary environments and diagenesis.

moardpourm@ripi.ir