Microfacies analysis, depositional environment, and diagenesis of the Asmari–Jahrum reservoir in Gulkhari oil field, Zagros basin, SW Iran

Abstract

The Asmari and Jahrum formations are studied in terms of microfacies, depositional environment, and diagenesis in the Gulkhari oil field based on data provided from well nos. 2, 10, and 11. Petrography, fauna content, biofacies, and depositional textures lead to recognition of 21 carbonate microfacies and 3 non-carbonate lithofacies which were deposited in four depositional environments including tidal flat, lagoon, bar, and open marine on a homoclinal ramp. Dissolution, bioturbation, cementation, neomorphism, silisification, micritization, physical compaction, and dolomitization affected on the Asmari and Jahrum formations, of which cementation, dolomitization, and dissolution are most prominent. Evidently, cementation reduced reservoir quality whereas dolomitization, dissolution, and burrowing, promoted porosity development of the reservoir. Diagenetic processes points toward uplift and a set of syngenetic processes on seafloor and epigenetic processes during shallow to deep burial. Consequently, the microfacies with improper reservoir quality were subjected to subsequent porosity development through diagenetic processes.

Introduction

Gulkhari oil field is located in the southern part of the Dezful embayment in the Zagros basin, wherein majority of entrapped hydrocarbons of the Zagros basin have been discovered since 1908. Carbonates are reservoirs (including the Asmari and Jahrum formations (Fms.)) hosts most of the hydrocarbons entrapped. Hence, in this research, microfacies and diagenetic processes affecting the Asmari and Jahrum formations in the Gulkhari oil field were studied. Due to insufficient primary porosity in the Asmari and Jahrum formations, it is supposed that the reservoir potential of these formations mostly depend on the development of secondary porosity, originated from different diagenetic processes, mainly dolomitization, fracturing, and dissolution. Gulkhari oil field is located 70 km NW of Bushehr port in Zagros basin (Fig. 1). Some 12 wells bored in this reservoir, among them well nos. 1 and 2, are abandoned. The present study focuses on data (including thin sections) provided for wells 2, 10, and 11 drilled in the Gulkhari oil field through the Asmari–Jahrum reservoir. James and Wynd (1965) published lithostratigraphic characteristics and biostratigraphy of Triassic to Pliocene of the Zagros basin. They introduced these formations with detailed lithofacies as well as biostratigraphy. Lithofacies of the Jahrum Fm. in Dezful embayment (as the major subdivision of the Zagros basin) was studied by Safaei et al. and Zakerzadeh in 1998 and Aghanabati (2004). Seyrafian (1998) studied sedimentary facies and depositional environment of the Jahrum Fm. in the Borojen area. Najafi et al. (2004) studied sequence stratigraphy and depositional history of tertiary carbonates of the Asmari–Jahrum formations exposed in Shiraz area. Khatibi Mehr and Moallemi (2009) compared depositional history of the Jahrum Fm. (in Zagros) and the Ziarat Fm. (in Alborz) based on benthic foraminifera. Mirzaei Mahmoudabadi and Afghah (2009) studied depositional environment and sequence stratigraphy of the Jahrum and Sachun formations in two stratigraphic sections (Sarvestan and Tang-e-khiareh) in the Shiraz area. Biostratigraphy, microfacies analysis, depositional environment, diagenesis, and sequence stratigraphy, of the Asmari Fm. were recently the focus of enormous investigation, as this formation is well appreciated due to huge amounts of hydrocarbons trapped in it. Vaziri-Moghaddam (2002). Vaziri-Moghaddam et al. (2006). Amirshahkarami, et al. (2007). Ehrenberg et al. (2007). Ranjbaran, et al. (2007). Hakimzadeh and Seyrafian (2008). Mohseni et al. (2008). Mossadegh, et al. (2009). Sadeghi, et al. (2010). Allahkarampour, et al. (2010). Vaziri-Moghaddam, et al. (2010). Sooltanian et al. (2011) and Laursen et al. (2009) examined the geochemistry and petrography of the Asmari Fm. The main goal of this research is to unravel the major factors that may control the reservoir quality of these reservoirs in the Gulkhari oil field. Most of the previously published works focus on the Asmari formation in the Dezful Embayment rather than the Asmari–Jahrum reservoir beyond this prolific subdivision of the Zagros basin, particularly in the coastal Fars and adjacent area. Attempts were made to construct a reasonable depositional model for the study area which may assist for further possible development of this oil field.

Fig. 1

Location map of Gulkhari oil field in Zagros basin (above) and structure contour map on top of the Asmari-Jahrum reservoir along with location of studied wells (below), after (Yazdani and Raki 2001)

Geologic setting

The Gulkhari oil field is located in the coastal Fars as a subdivision of the Zagros basin. In this part of the basin, these formations are not differentiable in subsurface, so they are described as an integrated reservoir as we do in this research. The Jahrum Fm. gradually overlays the Pabdeh Fm. and disconformably underlined by the Asmari Fm. (James and Wynd 1965). the Jahrum Fm. is no younger than middle Eocene in age and disconformably overlain by the Oligo–Miocene Asmari Fm. Evidence of post Middle–Eocene break is not seen in the coastal Fars where the complete Eocene is presented, although in inner Fars, the Razak Fm. unconformably overlay the Jahrum Fm. (James and Wynd 1965). See Fig. 2 for more information.

Fig. 2

Stratigraphic chart of tertiary formations in southwest of Iran (after Ala 1982)

Methods and Materials

Thickness of the Asmari and Jahrum formations in well nos. 2, 10, and 11 are 188, 204, and 290 m and 272, 262, and 290 m, respectively. A total of 750 thin sections from cores and cutting chips were examined under petrographic microscope. Some 420 samples are from the Jahrum Fm. and 330 samples are from the Asmari Fm. Thin sections were stained by Alizarin Red S. after Dickson (1965). All constituents of the samples including skeletal and non-skeletal grains, matrix, textural characteristics, and diagenetic features were examined under the microscope. Classification was after Dunham (1962) and Embry and Klovan (1971). Microfacies analysis and description was after Wilson (1975) and Flugel (2010) as well. Description of clastic lithofacies was after Pettijohn (1975). Dolomites were described based on a combination of Friedman (1965). Sibley and Gregg (1987). and Mazzullo (1992) classifications.

Results and discussion

Biostratigraphy

According to thin section examinations, 41 genus and 44 species of foraminifera were determined as being classified in five assemblage zones of which the first two assemblage zones belong to the Asmari Fm. and the latter three ones belong to the Jahrum Fm. (Plate 1) (see also Figs. 3, 4, and 5 for more details).

Plate 1

a Archaias sp., b Red algae - Quinqueloculina sp., c Bryozoa d Orbitolites complanatus–Amphistegina sp., e Miogypsina–Rotalia sp.–Miliolid, f Rhapydionina sp.–Lituonella roberti–Saudia sp., g Prearhagydionina sp., h Elphidium sp., i Dictyoconus sp., j Dendritina, k Operculina, l Orbitolites complanatus, m Discucyclina sp.–Heterostegina sp., n Nummulites sp. (Scale bar 0.2 mm)

Fig. 3

Microfacies analysis and depositional environment of the Asmari and Jahrum formations in well no. 2 in Gulkhari oil field

Fig. 4

Microfacies analysis and depositional environment of the Asmari and Jahrum formations in well no. 10 in Gulkhari oil field (see Fig. 4 for symbols)

Fig. 5

Microfacies analysis and depositional environment of the Asmari and Jahrum formations in well no. 11, in Gulkhari oil field (see Fig. 4 for symbols)

Assemblage zone no. 1

This biozone corresponds to Miogypsinoides, Archaias, and Valvulinid, and comprises the middle part of the Asmari Fm. early Miocene (Aquitanian) in age (Plate 1). This biozone extends in well no. 2 from depth 2397 to 2535 m, in well no. 10 from depth 2392 to 2530 m, and in well no. 11 from depth 2402 to 2525 m intervals. It is characterized by genus and species including the following: Archaias asmaricus, Archaias sp., Archaias hensoni, Miogypsinoides sp., Dendritina sp., Peneroplis thomasi, Peneroplis evolutus, Valvulinid sp., Quinqueloculina sp., Textularia sp., Spirolina sp., Discorbis sp., Triloculina trigonula, Rotalia viennoti, Pyrgo sp., Ammonia beccarri, Reusella spinulosa, Dendritina rangi, Neorotalia sp., gastropods, and miliolid.

Assemblage zone no. 2

This biozone is characterized by Nummulites–Nephrolepidina–Eulepidina biozone and comprises the lower part of the Asmari Fm. suggesting an early Oligocene age (Plate 1). This biozone extends in well no. 2 from depth 2536 to 2575 m, in well no. 10 from depth 2531 to 2579 m, and in well no. 11 from depth 2526 to 2590 m intervals. It was determined by genus and species as follows: Nummulites cascus, Nummulites fichteli, Nummulites fabianii, Amphistegina sp., Rotalia viennotti, Heterostegina sp., Operculina complanata, Triloculina trigonata, Triloculina tricarinata. Textularia sp., Peneroplis thomasi, Miliola sp., Nephrolepidina sp., Austerotrillina asmariensis, Rotalia sp., Quinqueloculina sp., and Lithothaminium sp.

Assemblage zone no. 3

This biozone belongs to late Eocene and is characterized by Orbitolites complanatus, Rhapidionina urensis, Coskinolina sp., Dictyoconus aegyptiensis, which extends in well no. 2 from depth 2576 to 2640 m, in well no. 10 from depth 2580 to 2635 m, and in well no. 11 from depth 2591 to 2650 m intervals and belongs to the upper part of the Jahrum Fm. The biozone no. 3 also contains: Biloculina sp., Valvulinid sp., Elphidium sp., Spiroculina sp., Rotalia sp., Nummulites fabiani prever, Silvestriella tetraedra gumbel, Orbitolites sp., echinoid, red algae, and bryozoa (Plate 1).

Assemblage zone no. 4

This biozone is known as Nummulites globulus–Heterostegina sp. Discocyclina sell–Lituonella roberti. It extends in well no. 2 from depth 2641 to 2720 m, in well no. 10 from depth 2636 to 2714 m, and in well no. 11 from depth 2651 to 2735 m intervals of the Jahrum Fm. (Plate 1). This fossil assemblage determines the middle Eocene (Lutetian) age. Accompanied fossils are: Miliola sp., Rotalia sp., Actinocyclina sp., Quinqueloculina sp., Alveolina cf., Rhapidionina sp., Valvulina sp., Biloculina sp., red algae, echinoid, and bivalve.

Assemblage zone no. 5

This biozone is known as: Globigerina sp.–Nummulites galensis–Nummulites striatus. It extends in well no. 2 from depth 2721 to 2850 m, in well no. 10 from depth 2715 to 2848 m, and in well no. 11 from depth 2736 to 2882 m intervals in the Jahrum Fm (Plate 1). This fossil assemblage suggests early to middle Eocene age. Associated fossils include: Rotalia sp., echinoid.

Microfacies analysis and depositional environment

Based on microfacies analysis, 21 carbonate microfacies were recognized, deposited in five facies belt. Furthermore, three non-carbonate lithofacies were also determined. These facies assemblages are labeled as A through E which is classified according to subenvironments they were deposited, from offshore to shoreline, as will be discussed in the following section.

Open marine facies assemblage

Microfacies A₁ (mudstone with few planktonic fauna)

This microfacies contains Globigerina scattered in mud matrix Along with sparse skeletal fragments. Some pyrite crystals were found in matrix (Fig. 6a). Presence of planktonic foraminifera and lack of large species suggest that this microfacies was deposited beneath photic zone in open marine (Cosovic and Drobne 2004). Stagnant deep water with normal salinity beneath storm wave base (SWB) is expected for this environment (Wilson 1975; Lasemi 1995; Bernaous et al. 2002). Abundant calcareous mud indicates low-energy hydrodynamic regime (Brigaud et al. 2009). This microfacies is comparable to ramp microfacies (RMF)-2, Flugel (2010) which probably deposited in facies belt 7 of Buxton and Pedley (1989) in outer ramp.

Fig. 6

Photomicrographs of microfacies recognized in the Asmari-Ahrum formations in the Gulkhari oil field. a Mudstone with a few pelagic fauna, well no. 10, depth 2870 m. b Argillaceous lime mudstone, well no. 11, depth 2849 m. c Bioclast Nummulites wackestone-packstone, well no. 11, depth 2588 m. d Bioclast Nummulites Discocyclina wackestone-packstone, well no. 10, depth 2682 m. e Dolomitized bioclast Nummulites Amphistegina Heterostegina wackestone-packstone, well no. 10, depth 2607 m

Microfacies A₂ (argillaceous lime mudstone)

This microfacies contains less than 5 % planktonic fauna and up to 65 % clay content (Fig. 6b). Depositional environment of this microfacies is similar to A₁ indicating proximal outer ramp.

Microfacies A₃ (bioclastic nummulites wackestone-packstone)

This microfacies consists of large foraminifera such as Nummulites striatus, Nummulites galensis, Nummulites intermedius, Nummulites fabiani, and rotalia and echinoderm fragments (Fig. 6c) in dolomitized matrix. Obviously, orientation of elongate grains by currents action is evident. Stenohaline fauna such as perforate foraminifera and echinoid, suggest shallow open marine environment (Romero et al. 2002). Racey (1994) reported several types of Nummulites, Assilina, Discocyclina, and Alveolina from Sib Fm. in Oman region and attributed them to middle ramp. This microfacies is similar to RMF-7 Flugel (2010) that was deposited above storm wave base (SWB) and below fair weather wave base (FWWB). Hence, we interpreted this microfacies as indicative of middle ramp.

Microfacies A₄ (bioclast nummulites discocyclina wackestone–packstone)

This microfacies is more frequent in the lower to middle parts of the Jahrum Fm. Bioclasts consist of echinoid comminuted benthic foraminifera fragments (Fig. 6d) rest on dolomitized matrix. In some cases, grain size varies up to gravel. Large planar Discocyclina and Nummulites suggest this microfacies was probably deposited in relatively deep water (Geel 2000). Existence of calcareous mud and abundant open marine fauna such as bryozoa and foraminifera fragments, confirm low to medium energy, between storm wave base and fair wave base (Wilson 1975; Flugel 2010) in photic zone (Geel 2000; Pomar 2001; Romero et al. 2002; Nebelsick et al. 2005; Renema 2006; Barattolo et al. 2007 and Bassi et al. 2007). This microfacies was also deposited in middle ramp setting.

Microfacies A₅ (dolomitized bioclast nummulites amphistegina heterostegina wackestone–packstone)

This microfacies comprises considerable amounts of large benthic foraminifera (such as Nummulites, Amphistegina, and Heterostegina). A few echinoderm fragments and red alga are also observed (Fig. 6e). Up to 16 % of this microfacies is dolomitized. Evidences such as diversity of large perforate foraminifera, existence of echinoderm, and slightly comminuted bioclastic fragments, suggest deposition between storm wave base and fair weather wave base. Perforate foraminifera like planar and wide Nummulitides, Operculina, and Heterostegina could exist down to the lower boundary of photic zone in the deep parts of the basin (Romero et al. 2002). High taxonomic diversity includes large benthic perforate foraminifera; echinoderm, corallinaceae red alga, and calcareous mud matrix were attributed to deep sea setting (Amirshahkarami et al. 2007). This microfacies is equivalent of RMF-7 Flugel (2010) which was probably deposited in facies belt 5 of Buxton and Pedley (1989) in the middle ramp.

Bar facies assemblage

Microfacies B₁ (peloidal bioclast miliolid nummulites miogypsina grainstone)

The important characteristic of this microfacies is mixed perforate and imperforate foraminifera. Grain supported, sorted and rounded grains (Fig. 7a), lack of mud, and fairly rounded bioclastic fragments, indicate medium to high energy condition. Subordinate amount of Rotalia, echinoderm, red alga, bryozoa, and Dendritina suggest high energy bar/ barrier in platform margin. This microfacies is equal to RMF-26 Flugel (2010) which was deposited in facies belt 3 of Buxton and Pedley (1989) in the lower boundary of mid-ramp.

Fig. 7

Photomicrographs of microfacies recognized in the Asmari-Jahrum formations in Gulkhari oil field. a Amphistegina peloid grainstone, well no. 10, depth 2581 m. b Grainstone with intergranular porosity and cement, well no. 10, depth 2585 m. c Grainstone with Orbitolites and Miogypsina, well no. 10 depth 2502 m. d Ooid grainstone bioclast with Miliolid, well no. 11, depth 2547 m. e Intraclast bioclast packstone–grainstone, well no. 10, depth 2453 m. f Peloid Orbitolites Nummulites wackestone–packstone, well no. 11, depth 2649 m

Microfacies B₂ (bioclast nummulites Orbitolites sp. grainstone)

Both perforate and imperforate benthic foraminifera such as Nummulites and Orbitolites (0.2–1.8 mm size) are predominant components of this microfacies. Some Rotalia, echinoderm, bryozoa, Miogypsina, Elphidium, and red alga fragments are also observed (Fig. 7b). Winnowed rounded grains point to high energy conditions. Similar interpretation inferred for microfacies B₁ is inferred for this microfacies which is abundant in the middle to the upper part of the Jahrum Fm.

Microfacies B₃ (bioclast Quinqueloculina sp. and Orbitolites sp. grainstone)

Bioclasts are the main constituent of this microfacies of which 45 % consists of imperforate benthic foraminifera, (Orbitolites, Quinqueloculina, Dendritina, Miogypsina, Elphidium, Peneroplis and Textularia); other bioclasts are Rotalia, echinoid, red alga, and bivalve fragments (Fig. 7c). Furthermore, this microfacies contains about 2 % peloid. The most important diagenetic processes are cementation and dolomitization. Both intergranular and intraparticle porosities are occluded by cement. Negligible mud content or lack of interparticle mud, suggests high energy condition. This microfacies is equivalent of RMF-26 Flugel (2010). It is correlatable to facies belt 3 of Buxton and Pedley (1989) deposited in bar/barrier environment of inner ramp and comprises the upper part of the Jahrum Fm.

Microfacies B₄ (bioclast ooid packstone–grainstone)

Predominant grains consist of 65 % of porcelanous foraminifera and ooids. Accessory components are ostracod, echinoderm, and gastropod (Fig. 7d). Anhydrite cement occluded the porosities in some samples. The same interpretation given for B₄ is inferred for this microfacies.

Lagoon facies assemblage

Microfacies C₁ (bioclast–intraclast packstone–grainstone)

Major components of this microfacies are porcelanous benthic foraminifera (Miliolid, Peneroplis, Dendritina rangi, Elphidium, Quinqueloculina, and Archaias) and subrounded to rounded intraclasts (Fig. 7e). Rare echinoderm and green alga fragments are also observed. Porosities filled by anhydrite cement. Micritized skeletal grains, imperforate foraminifera, and abundant intraclasts mixed by lagoon fauna suggest open lagoon close to leeward of bar in platform margin (Hallock and Glenn 1986). This microfacies is equivalent of RMF-14 Flugel (2010) and facies belt 2 Buxton and Pedley (1989) and comprises the upper part of the Asmari Fm.

Microfacies C₂ (peloid bioclast nummulites, Orbitolites sp. wackestone to packstone)

Nummulitides and Orbitolites are the predominant skeletal grain (up to 35 %), their size vary between 0.3–1.8 mm. Accessory grains include echinoderm, Rotalia, red alga, bryozoa, Elphidium, Miogypsina, and Dendritina rangi (Fig. 4f). A few fecal pellets are also observed. This microfacies comprises middle to upper parts of the Jahrum Fm. Large and rigid Nummulites, peloid, Orbitolites, and mud-supported texture suggest lagoon environment. Mixed porcelanous fauna with oval/lenticular hyaline test points toward photic zone, in an open lagoon (Pomar 2001; Romero et al. 2002 and Renema 2006). This microfacies is equivalent of RMF-13 Flugel (2010) and facies belt 2 of Buxton and Pedley (1989). Although some researchers believe that Orbitolites could exists in deeper parts of the ramp (Hohenegger et al. 1999; Hohenegger 2000; Langer and Hottinger 2000).

Microfacies C₃ (peloid imperforate foraminifera wackestone–packstone)

This microfacies is characterized by the maximum diversity of imperforate foraminifera (40–45 % abundance). A variety of imperforate foraminifera (Miliolid, Dendritina rangi, Peneroplid, Quinqueloculina, Archaias, and Elphidium) were recognized in this microfacies. Minor echinoid fragments, green alga, bivalve, and intraclast are also present (Fig. 8a). Sediments are poorly to moderately sorted, subangular to subrounded, and fine to medium sand size. Lack of normal marine fauna, diverse imperforate porcelanous foraminifera, and peloid, imply a saline lagoon. Numerous imperforate porcelanous foraminifera imply hypersaline environments (Flugel 2010; Vaziri-Moghaddam et al. 2006; Brandano et al. 2009 and Allahkarampour et al. 2010). Depositional environment interpretation is similar to C₂.

Fig. 8

Photomicrographs of microfacies recognized in the Asmari-Jahrum formations in Gulkhari oil field. a Imperforate foraminifera wackestone, well no. 10, depth 2462 m. b Miliolid wackestone, well no. 10, depth 2453 m. c Framework porosity in coral boundstone, note to the anhydrite and gypsum which occluded the porosity, well no. 2, depth 2484 m. d Asservolina encrusting on a bivalve shell fragment, pore filling anhydrite cement is visible, well no. 10, depth 2576 m. e Wackestone with scattered detrital quartz grains, well no. 10, depth 2430 m. f Lime mudstone with detrital quartz grains, well no. 10, depth 2390 m

Microfacies C₄ (bioclast miliolid wackestone)

Miliolid is the most abundant skeletal grain in this microfacies (40 %) with up to 4 % peloids and intraclasts (Fig. 8b). Mud-supported texture with abundant Miliolid and low fauna diversity imply shallow lagoon with low to medium energy level. It is similar to RMF-16 Flugel (2010) probably deposited in facies belt 2 of Buxton and Pedley (1989).

Microfacies C₅ (coral boundstone)

This microfacies is mainly consists of frame-builder corals. Septa were dissolved; subsequently the porosity was occluded by anhydrite/calcite cement (Fig. 8c). This microfacies were formed in leeward of bar within the lagoons (Flugel 2010) and in platform margin above fair wave base (Wilson 1975). The most important diagenetic processes in this microfacies are dissolution and precipitation of calcite and anhydrite cements. This microfacies is equivalent of RMF-15 Flugel (2010) and belongs to facies belt 2 of Buxton and Pedley (1989).

Microfacies C₆ (algal boundstone)

The major components of this microfacies are Lithophyllum and Subtraniphyllum thomasi. Although benthic foraminifera (Miliolid, Rotalia, Peneroplis evolutus, Quinqueloculina, Amphistegina, and Heterostegina), bivalve fragments, gastropod, and echinoderm (Fig. 8d) are also present. Encrusting of Asservolina on red alga is obvious. Coexisting red alga and porcelanous benthic foraminifera confirm their deposition in lagoon environment. Interpretation on depositional environment is similar to C₅.

Microfacies C₇ (peloid wackestone to packstone)

Peloid is the main component of this microfacies (15–20 %). Less than 2 % quartz and intraclast were observed (Fig. 8e). Abundant peloids suggests relatively calm water (e.g., lagoon) depositional condition for this microfacies, which is equivalent of RMF-21 Flugel (2010). and belongs to facies belt 2 of Buxton and Pedley (1989).

Microfacies C₈ (dolomitized mudstone with bioclast)

This microfacies consist of lime mudstone. Skeletal and non-skeletal components are less than 10 %. Despite dolomitization, which left vague of bioclasts, some fossils including Miliolid, Peneropli, and gastropod are visible (Fig. 8f). Sand size angular and subangular detrital quartz grains scattered throughout the mud matrix. Pellet and gypsum crystal laths were also recognized in some samples. This microfacies is equivalent to RMF-19 Flugel (2010). Scars fauna with low diversity imply unfavorable conditions for much of benthic organisms (Sadeghi et al. 2010). These features indicate restricted lagoon environment.

Peritidal facies assemblage

Microfacies D₁ (dolostones)

This microfacies consists of xenotopic to idiotopic, subhedral to euhedral dolomite crystals, varying in 20–200 μm crystal size. Coarse dolomite with straight to curved crystal boundaries, undulate extinction, and cloudy appearance filled most of the fractures (Fig. 9a). Dolomitization of peritidal sediments was promoted by sea level fall after evaporation, excluding the fracture filling dolomites. This microfacies is equivalent of RMF-22 Flugel (2010) of peritidal environment, deposited in facies belt 1 of Buxton and Pedley (1989).

Fig. 9

Photomicrographs of microfacies recognized in the Asmari-Jahrum formations in Gulkhari oil field. a Dolostones, well no. 11, depth 2626 m. b Peloid intraclast packstone, well no. 10, depth 2455 m. c Dolomudstone with anhydrite cement, well no. 10, depth 2634 m. d Burrow and cavern porosity in mudstone, well no. 10 depth 2442 m. e Quartz wacke, well no. 2, depth 2420 m. f Anhydrite, well no. 11, depth 2544 m. g Dolomitized shale, well no. 11, depth 2738 m

Microfacies D₂ (peloid, intraclast packstone)

Major constituents of this microfacies are subrounded to well-rounded intraclast and peloid indicate intermittent transport due to wave action (Fig. 9b). Anhydrite is the predominant cement that filled both burrow and intergranular porosities. This microfacies is equivalent to RMF-24 Flugel (2010) deposited in intertidal environments in facies belt 1 Buxton and Pedley (1989).

Microfacies E₁ (dolomudstone)

Finely crystalline (4–14 μm), anhedral, nonplanar dolostone with xenotopic texture are characteristics of this microfacies (Fig. 9c). Vague lamination was observed in some samples. Burrow porosity are filled by anhydrite cement. Very finely crystalline texture and lamination indicate near surface conditions during deposition of this microfacies (Gregg and Shelton 1990; Warren 2000; Haeri Ardakani et al. 2013). Dolomudstone mainly forms either simultaneously or during the early stage of diagenesis in supratidal to upper intertidal setting (Adabi 2002). This microfacies is equivalent of RMF-22 Flugel (2010) deposited in facies belt 1 Buxton and Pedley (1989).

Microfacies E₂ (mudstone)

This microfacies is laminated barren–lime mudstone with scattered detrital quartz grains (Fig. 9d). Lacks of bioclasts probably reflect restricted water circulation brought unfavorable condition for stenohaline biota (Alsharhan and Kendall 2003). It is totally accepted that lime mudstones and dolomudstones formed in proximal tidal flat (Warren 2000). This microfacies is equivalent of RMF-22 Flugel (2010) and facies belt 1 Buxton and Pedley (1989).

Lithofacies E₃ (quartz wacke)

Fine to medium sand size, poorly sorted, angular to subangular monocrystal quartz is the major component of this lithofacies (Fig. 9e) which rest on lime mud matrix. The detrital quartz grains were probably transported into the depositional environment by sporadic flood events (Warren 1989) in supratidal environment.

Lithofacies E₄ (anhydrite)

This lithofacies is barren, probably deposited in a restricted basin which intermittently over flooded by marine water (Fig. 9f). Much of gypsum and anhydrite could be deposited during dolomitization of carbonates after evaporation (Qing et al. 2001; Melim and Scholle 1995). This lithofacies was deposited in supratidal setting in an inner ramp.

Lithofacies E₅ (shale)

Two-meter-thick shale layer was recognized in depth intervals of 2738 to 2740 m in borehole #11. This shale bed is sandwiched between peloid wackestone and lime mudstone at base and top, respectively. This barren shale bed represents crude lamination. About 8 % dolomite crystals are scattered throughout the matrix (Fig. 9g). Based on facies association, we interpret the depositional environment of this shale bed as supratidal pond. Apparently, shift from carbonate to clastic sedimentation occurs in shoreline of the inner ramp, where inlets brought detritus into the basin.

Facies association and depositional model

Microfacies analysis of the Jahrum and Asmari formations in the Gulkhari oil field and comparison with Buxton and Pedley (1989) and Flugel (2010) indicate that they were deposited in a carbonate ramp. According to gradual changes between facies, lack of barrier reef, lack of oncoids, pisoids, and grain aggregates which normally are distinctive of carbonate shelf or rarely could be found in carbonate ramp (Flugel 2010). lack of resedimented facies indicative of steep slope depositional environment, we purposed a homoclinal carbonate ramp (Fig. 10) for the study area. During the early to middle Eocene, a ramp was developed wherein the Pabdeh Fm. was accumulated (Mohseni and Al-Aasm 2004), followed by the deposition of the Jahrum Fm. During middle Eocene (Lutetian), the basin became shallower and wackestone–packstone contains benthic foraminifera such as Nummulites, Discocyclina, and Heterostegina were deposited, indicating sedimentation in low to medium photic zone. Afterward, during late Eocene, Orbitolites, Coskinolina, and Dictyoconus were the dominant biota, which preferentially live in lower parts of the mid-ramp. Environmental condition in such parts of the ramp is characterized by mild turbation, low bed stability, and medium to low photic zone and high diversity of imperforate foraminifera that terminates the Jahrum Fm. According to fauna content and diagenetic features, apparently, no hiatus was recognized between the Asmari and Jahrum formations in this oil field (Figs. 4 and 5). The Asmari Fm. deposited during Oligocene, characterized by perforate foraminifera such as Nummulites, Amphistegina, and Heterostegina as predominant biota, indicating mid-ramp with medium to low photic zone (Hottinger 1997; Pomar 2001). Large benthic foraminifera such as Heterostegina and Amphistegina (A₅) particularly abundant in tropical to subtropical environments (Brandano and Corda 2002) in a wide range of hydrodynamic conditions vary between 40 and 70 m in depth (Hottinger 1983, 1997). However, coexisting red alga and foraminifer imply mid-ramp (Brandano and Corda 2002; Corda and Brandano 2003; Brandano et al. 2009) with low to medium photic zone (Hottinger 1997 and Pomar 2001). Basin uplift during early Miocene (Aquitanian) evolved the mid-ramp into an inner ramp where the majority of the Asmari Fm. was accumulated. Abundant imperforate foraminifera such as Peneroplis, Archaias, Miliolid, Dendritina, and Quinqueloculina imply shallow water depth, medium to high photic zone, and low turbation of the basin. During late Aquitanian, the basin exhumed, hence upper part of the Asmari Fm. missed. Consequently, the deposition of evaporates of the Gachsaran Fm. commenced (Fig. 11). Although in the coastal Fars, the Razak Fm. was deposited in part of the shallow ramp with predominate evaporitic interval (Motiei 1995). In general, the Jahrum and Asmari formations were deposited in outer ramp, mid-ramp and inner ramp, in Gulkhari oil field.

Fig. 10

Purposed depositional environment for the Asmari and Jahrum formations in the Gulkhari oil field (without scale), the most prominent microfacies types are inserted in different parts of the carbonate ramp

Fig. 11

Cartoon illustrating the evolution of the depositional environment of the Asmari and Jahrum formations in Gulkhari oil field through Eocene to Oligocene. (Data for the Razak and Pabdeh Fm. after Motiei 1995 and Mohseni and Al-Aasm 2004 respectively)

Diagenetic processes

Petrographic examinations on thin sections revealed various diagenetic processes in the Asmari and Jahrum formations which will be discussed in the following section.

Dissolution

Dissolution may takes place in various environments including near surface and meteoric environments, mixing zone and burial diagenetic environments (Moore 1989; Choquette and James 1990). Dissolution of unstable grains left moldic porosity in the Asmari and Jahrum formations. Probably this process occurred in burial setting, left moldic porosity associated with stylolite (Fig. 12a). Although further investigation, particularly geochemical approach could unravels uncertainly of the origin of these porosities.

Fig. 12

Photomicrographs of diagenetic features recognized in the Asmari-Jahrum formations in Gulkhari oil field. a Anhydrite after partial dissolution of Nummulites, well no. 11, depth 2628 m. b Boring on red alga, well no. 10, depth 2531 m. c Blocky cement-filled biomoldic porosity, well no. 11, depth 2695 m. d Poikilotopic anhydrite cement, well no. 10, depth 2576 m. e Rim cement around echinoid, well no. 11, depth 2701 m. f Centripetal increase in crystal size, well no. 10, depth 2582 m. g Chertification, well no. 11, depth 2750 m. h Micritic envelope around carbonate grain, well no. 2, depth 2772 m

Bioturbation

Bioturbation as burrow and boring, including micro and macro boring observed in mudstones of lagoon and peritidal environments (Fig. 12b).

Cementation

Several fabrics of cements were recognized in the Asmari and Jahrum formations which are briefly discussed in the following section.

Blocky calcite cement

Xenotopic to hypidiotopic, medium to coarse crystals (crystal sizes varies between several μm to several mm) with planar boundary, characterize this type of cement (Fig. 12c). It could generate from both fresh water and burial environments as second and third generations (Tucker 2001; Flugel 2010). Blocky cement implies low Mg⁺²/Ca⁺² ratio in pore fluids (Ahmad et al. 2006). This cement is also observed as open space filling cement and could indicate a fresh water diagenetic environment after uplift (Seeling et al. 2005).

Poikilotopic cement

Large crystals (normally anhydrite) embedded several dolomitized grains, as well as dolomite crystals (Fig. 12d). Normally, this type of cement forms in burial environment due to low nucleation rate and crystallization from supersaturated fluids with respect to CaCO₃ (Tucker 1993).

Syntaxial rim cement

This type of cement overgrowths as clear rim around echinoderm fragment (Fig. 12e). Rim cements are often considered as fresh water cements; however, they are reported from marine and burial diagenetic environments (Ahmad et al. 2006).

Neomorphism

Normally aggrading neomorphism was observed in samples of the Asmari and Jahrum formations in the study area (Fig. 12f).

Silicification

Silicification is observed as selective silicification of bioclasts, and chert also was observed either as nodules or discrete layers in some samples (Fig. 12g).

Micritization

A process in which carbonate particles substitute by aphanitic or micritic crystals during early stages of marine diagenesis in sediment/water interface (Samankassou et al. 2005). normally in calm water. It is observed on mollusks fragments in peritidal facies, particularity in the middle parts of the Asmari Fm. (Fig. 12h).

Compaction

Physical compaction in mud-supported sediments caused realignment, preferred orientation of bioclasts (mainly foraminifera), and plastic deformation of grains. Chemical compaction is observed as dissolution seams, stylolites, and fitted fabrics. Stylolites are discrete and irregular (after Lagan and Semeniuk’s 1967 classification) with low amplitude irregular anastomosing appearance. Fitted fabrics also developed (Fig. 13a, b), which decreased the porosity and permeability of the reservoir.

Fig. 13

Photomicrographs of diagenetic features recognized in the Asmari-Jahrum formations in Gulkhari oil field. a Mechanical compaction caused deformation of bioclasts, well no. 10, depth 2605 m. b Concentration of insoluble residue along stylolites surface, well no. 11, depth 2691 m. c Dolomicrite with intercrystalline and burrow porosity, well no. 10, depth 2623 m. d Dolomicrosparite with intercrystalline porosity, well no. 10, depth 2634 m. e Dolosparite, well no. 10, depth 2574 m. f Pore filling dolomite, well no. 11, depth 2523 m

Dolomitization

Both the Asmari and Jahrum formations are subjected to dolomitization. Based on petrography, four types of dolomites were recognized within these formations, which are described as follows.

Very fine crystalline dolomites or dolomicrite

This is first generation of dolomite as anhedral crystals comprise about 25 % of the Asmari and Jahrum formations (Fig. 13c) associated by evaporate pseudomorphs. Crystal sizes vary between 5 and 16 μm. This type is equivalent to xenotopic texture (Friedman 1965). planar-A (Mazzullo 1992) and xenotopic-A (Sibley and Gregg 1987). The dolomicrite usually obscured skeletal grains and peloids. According to these evidences, it seems that first generation of dolomites formed under low temperature near surface conditions above tidal flat.

Fine crystal dolomites (dolomicrosparite)

The second type of dolomites is dense unimodal, subhedral to anhedral crystals with planar (planar-S) intercrystalline boundary (Fig. 13d). Crystal size varies between 16 and 62 um which appear dark brown (due to excess inclusion). These dolomites are associated with dissolution seams and stylolites. The second type dolomite is equivalent of hypidiotopic (Friedman 1965). idiotopic (Sibley and Gregg 1987). and planar (Mazzullo 1992) textures. They may probably form after recrystallization of type 1 precursor.

Medium crystal dolomites (dolosparite)

Crystal sizes of the type three dolomites are between 62 and 250 μm. Crystals are often anhedral with curve and tongue-shaped intercrystalline boundaries, nevertheless, in some cases appear as subhedral crystals with compromise boundaries. Distribution of crystal sizes is polymodal (Fig. 13e). Crystals appear cream to brown color under ordinary light. We described them as hypidiotopic (Friedman 1965) and idiotopic-S (Sibley and Gregg 1987) textures.

Pore filling dolomite cement

Zoned coarse transparent crystals are often euhedral with planar boundaries filled fractures (Fig. 13f) are main features of this type. Their variable size (between 200 and 500 μm) often controlled by the size of the pore spaces. They could be formed in temperatures less than 50–60 °C (Mazzullo 1992). in shallow burial conditions in late diagenetic stages (Mutti 1990).

Paragenetic sequence of diagenetic processes observed in the Gulkhari oil field

Based on petrographic observations and cross- cutting relationship between diagenetic features, attempt was made to constrain the paragenetic sequence of diagenetic processes affected on the Asmari–Jahrum reservoir (Fig. 14).

Fig. 14

Suggested paragenetic sequence of observed diagenetic processes in the Asmari-Jahrum reservoir in Gulkhari oil field

Conclusion

Asmari and Jahrum formations were deposited on a shallow carbonate ramp in Gulkhari oil field. The Jahrum Fm. was deposited in distal part of mid to outer ramp (as revealed by microfacies). In late Eocene, the basin became shallower. However, the Asmari Fm. was deposited in mid-ramp during Oligocene. Although during Aquitanian, the depositional environment considerably shifted into shallower ramp. Gradual change in fauna and mixed open marine and lagoon fauna suggest no prominent bar/ barrier on this ramp. Comparison of the studied wells revealed no distinct facies changes them. However, negligible changes in microfacies were recognized, which are probably due to normal lateral facies changes and/or due to sample missing (gaps)? For example, microfacies B2 and C3 were not detected in well no. 2, C5 and D2 in well no. 11, as well. Although the thickness of outer ramp facies (unit 7) increases from well no. 2 toward well no. 10, while those of mid-ram decreases toward well no. 10. This could reflect the position of boreholes drilled into the Jahrum Fm. Other changes are negligible (Fig. 15). Diagenetic processes affected on the Asmari and Jahrum formations in Gulkhari oil field are, bioturbation, neomorphism, silicification, micritization, cementation, dissolution, compaction, and dolomitization which successfully influenced the these formations in marine, meteoric, and burial diagenetic environments. Dolomitization and cementation are the most important diagenetic processes. The dolomites have different origins, so the first type (dolomicrite) is considered as syngenetic. The second type (dolomicrosparite) was formed during shallow burial stages. The third type (dolosparite) is probably a diagenetic product of pre-existing types after recrystallization. The fourth type (pore filling) was formed during shallow burial diagenetic stage. These diagenetic processes had main impact on peritidal, lagoon, bar (mid-ramp) microfacies and little influence on open marine (outer ramp) microfacies.

Fig. 15

Correlation between Asmari-Jahrum reservoir in well nos. 2, 10, and 11 in the Gul-Khari oil field. Thickness of outer ramp facies (unit 7) increases from well no. 2 toward well 10 and 11, while those of mid-ramp (unit 6) decreases