Abstract
The aim of this paper is investigation of microbiostratigraphy and sedimentary environment of the Sarvak and Kazhdumi formations in Bahregansar oil field. To investigate these units, microbiostratigraphy and microfacies analyses were carried out on nearly 600 m of cuttings, including cutting plug samples and thin sections prepared from Bahregansar oil field in the Persian Gulf, SW Iran. 22 Species and genera of foraminifera were recognized and four biozones were identified. Also the microfacies analysis of the Sarvak and Kazhdumi formations in Bahregansar oil field is led to recognition fourteen microfacies of three facies belts (depositional environment) including open marine, bar and lagoon environments.
Keywords: Microbiostratigraphy, Depositional environment, Microfacies, Sarvak, Kazhdumi, Albian, Cenomanian.
Received: 6 September 2016 / Revised: 14 July 2017 / Accepted: 31 July 2017 / Published: 9 August 2017
Contribution/ Originality
This study is one of very few studies which have investigated the Upper Cretaceous of oil fields. The results of this study will help to make Upper Cretaceous biozones, interpretation of microfacies and depositional environments..
1. INTRODUCTION
Many geologists have focused on the carbonate petroleum reservoirs of the Persian Gulf because of the well-known oil productivity of this region (Afghah and Farhoudi, 2012). The Sarvak Formation is one of the most significant carbonate reservoirs known to be an oil producer in the Bahregansar oil field (Tabiyejou et al., 2014). Bahregansar field consists of several separate hydrocarbon accumulations within following formations and from top to bottom respectively includes Ghahr, Asmari, Sarvak, Kazhdumi and Fahliyan formations.
Although the Bangestan carbonates are known as significant reservoirs in the Middle East, their detailed biostratigraphy is still ambiguous. The stratigraphic position of the Sarvak Formation is also another reason for the importance of the biostratigraphic study. According to James and Wynd (1965) Zagros is divided into four main zones: Khuzestan, Lurestan, Coastal Fars and Interior Fars (James and Wynd, 1965). They suggested that Cenomanian benthic foraminifers are well expanded in the Khuzastan, Coastal and Interior Fars areas whereas pelagic facies are well dominant in the Lurestan region in order to determine the lower and upper lithostratigraphic boundary of the Sarvak Formation (Wynd, 1965).
Sarvak Formation consists of limestone and dolomitic limestone. The depositional environment shows a carbonate shelf near to the reef grading downward to open and deep marine facies. The top of this formation is characterized by an erosional surface in this field which truncates the reservoir creating an unconformity trap (Madadi, 1986). This formation has been drilled within 7 wells of this field and shows an average thickness of about 128m. The period of erosion and non-sedimentation in this field towards the northern areas (e.g. Hendijan, Tangu fields) is remarkable and. cover the western part of this field. The Variety of thickness in the Sarvak Formation is presumably due to middle cretaceous uplifting (Touronian). The age of the formation has been defined as Albian-Cenomanian
Kazhdumi Formation is consisted of marl, limestone and dolomitic limestone. Nahr-Umr Formation is its equivalent in the south of Persian Gulf.
In the Bahregansar oil field, 3 wells were drilled in the Kazhdumi Formation. The average thickness in this field is about 194m. Toward the south and south-eastern part of the field the thickness of the basal sandy unit increased. This unit has been distinguished as hydrocarbon bearing zone.
The age of Kazhdumi Formation is Lower Cretaceous (Albian to Cenomanian) and the depositional environments include Neritic to Pelagic environments.
The Ilam Formation overlies the Sarvak Formation disconformable and lower boundary of Sarvak Formation is conformable with Kazhdumi Formation.
The main objectives of this paper are to describe and interpret the microbiostratigraphy, facies and depositional environments of Sarvak and Kazhdumi formations in the Bahregansar oil field, Persian Gulf in order to understand the geological evolution of the area during that time interval.
2. MATERIAL AND METHODS
This study is based on data from one well drilled in the Bahregansar oil field in the Persian Gulf, SW Iran. 200 collected samples from the well (600m) were examined and microbiostratigraphy of the Sarvak and Kazhdumi formations used to characterize biozones which are equivalent of Wynd`s biozones (Wynd, 1965). For classification purposes the concepts of Dunham (1962) and Flugel (2010) have been used for carbonate facies.
3. STUDY AREA
This oilfield is located 56 kilometers, west of Bahregan District. It includes a main platform and 7 satellite platforms, with 12 wells. The oil produced from Bahregansar platform is transferred to onshore installations of Bahregan District for treatment and storage by a 16inch pipeline.
Figure-1. Location map of the study area which represent Bahregansar oil field in Northwestern part of the Persian Gulf.
Source: Kalantary (1969)
4. MICROBIOSTRATIGRAPHY OF THE SARVAK FORMATION IN THE STUDIED WELL
In this study, 13 genera and 9 species of benthic foraminifera and 5 genera and 8 species of planktonic foraminifera were detected, based on which the following 3 biozones were recognized:
1- Nezazata- Alveolina assemblage zone
This biozone is located on the upper side of Sarvak Formation and has a thickness of 130 meters (2805m-2678m). The biozone can be specified with the concentration of the following fossils:
Ovalveolina ovum, Prealveolina sp.
Microfossils of this zone are as follow:
Alveolina sp.; Nezzazzata sp.; Dicyclina sp.; Heterohelix globulosa; Heterohelix reussi; Ticinella primula; Favusella washitensis; Rotalipora reichli; Trocholina sp., Miliolids
This biozone is relevant to biozone (21) by Wynd. Also Khalili (1974) divided this zone into two subzones. He marked the lower zone with Orbitolina-Nezzazata and believes that the upper zone (which contains Orbitolina) can also be divided based on the different species of Orbitolina, Bolz 1977 claims that this zone base on selected samples only represents the upper part of Sarvak Formation. Bourgeois (1969) believe that the upper part of this biozone shows the Late Cenomanian (Husinec et al., 2000) that all Alveolinids of this zone were diminished during the Late Cenomanian due to a word wide Oceanic Anoxic Event known as OAE2. It should be noted that this is a major disconformity which is common among the various parts of the SW Iran and the surrounding areas in the Middle East (Husinec et al., 2000).
2-Trocholina- Orbitolina Assemblage zone
This biozone which is located in the middle of the Sarvak Formation include the sediment of Albian- middle to Late Cenomanian. The thickness of this biozone is 50m (2840m-2805m).
The biozone can be specified with the concentration of the following fossils:
Lenticulina sp., Trocholina sp., Orbitolina concava
Microfossils of this zone are as follows:
Textularia sp., Prealveolina cretasea, Trocholina arabica; Trocholina intermedia, Trocholina sp., Orbitolina concave, Dictytoconus sp., Nezzazzata sp., Ovalveolina ovum, Pesudolitunella reicheli, Hemicyclammina sp., Cuneolina pavonia, Cylindroporella sp.,Miliolids
This biozone is equivalent to Wynd biozone (25).This zone is well-developed in the costal Fars (Motiee, 1992).
3- Favusellawashitensis Rang zone
This biozone located in the lower Sarvak Formation include the sediment of Albian -Cenomanian in the studied stratigraphic section of the Sarvak Formation. The thickness of this biozone is 70m (2898m-2840m). The biozone can be specified with the concentration of the following fossils:
Favusella washitensis, Oligostegina sp., Prealveolina sp., Algae. This biozone is equivalent of Wynd biozone (23), and its microfossils include:
Calsipherula sp., Lenticulina sp., Rotalipora sp., Ticinella sp., Rotalipora gandolfii, Rotalipora subticinensis.
In this zone, due to the fact that Prealveolina and Oligostegina are dominated, it can be proved that it was formed at the beginning of Albian to Cenomanian. This zone is viewed in Coastal Fars with Prealveolina–Algae Assemblage zone together (Motiei, 1992).
5. MICROBIOSTRATIGRAPHY OF THE KAZHDUMI FORMATION IN THE STUDIED WELL
In the study of Kazhdumi Formation, 9 genera and 3 species of benthic foraminifera and 3 genera and 2 species of planktonic foraminifera were identified and 2 biozones were recognized as follow:
1- Oligostegina facies
This biozone includes Albian-Cenomanian sediments and is equivalent to Wynd biozone (26). This facies is widespread in the Zagros area. The biozone can be specified with the concentration of the following fossils:
Calcisphaerula sp., Stomiosphaera conoida. This biozone has a thickness of 150m (3040m-2900m). Its microfossils include:
Favusella washitensis, Oligostegina sp.; Calsipherula sp., Textularia sp., Trocholina sp., Orbitolina concave, Dictytoconus sp., Nezzazzata sp., Cylindroporella sp., Miliolids.
By seen Stomiosphaeraconoida in begging of this zone, it concluded that it belongs to the beginning of the Albian period.
2- Favusella Washitensis Rang Zone
This biozone located in the lower Sarvak Formation include the sediment of Albian -Cenomanian in the studied stratigraphic section of the Sarvak Formation. The thickness of this biozone is 150m (3040m-3100m).
This biozone is equivalent to Wynd biozone (23), and its microfossils include: Orbitolina concava; Textularia sp., Orbitolina sp., Heterohelix sp., Rotalipora gandolfii, Ticinella sp.
Figure-2. Biostratigraphy chart of the studied section. In the biozones column.1-Nezzazata- Alveolina assemblage Rang zone, 2 -Trocholina- Orbitolina Assemblage Zone, 3- Favusella washitensis Rang Zone
Source: Ahmadi (2016)
Figure-3. Biostratigraphy chart of the studied section. In the biozones column.1-Oligosteginid facies, 2- Favusella washitensis Rang Zone.
Source: Ahmadi (2016)
6. MICROFACIES ANALYSIS AND DEPOSITIONAL ENVIRONMENTS OF THE SARVAK FORMATION IN THE STUDIED WELL;
The microfacies analysis of the samples taken from the Sarvak Formation in Bahregansar oil field led to the recognition of 7 microfacies of 3 facies belts (deposition environment) based on Flugel (2010) including open marine (A), bar (B), lagoon (C) environment. Open marine facies (A) consist of oligostegina wackestone (A1), pelagic foraminiferal packestone (A2) pelagic foraminiferal wackestone (A3). This facies is characterized by pelagic foraminiferal such as Favusella washitensis, Oligosteginid and context of lime mud and represent low energy environment that demonstrate open marine
Bar facies (B) consist of bioclastic grainstone (B1) and dolomitized peloid grainstone (B2). It consists of coral debris, bioclast, dolomitized peloid grinstone. Context of grainstone and very high energy condition indicates in barrier environment.
Lagoon facies (C) consist of benthic foraminiferal wackestone (C1) and bioclast peloid wackestone (C2). This facies zone is mainly consisted of benthic and some pelagic forams. The skeletal allochems are abundant and diversity. With benthic foraminiferal such as Nezzazata and Miliolids in context of lime mud and represent low energy environment that demonstrate lagoon.
7. MICROFACIES ANALYSIS AND DEPOSITIONAL ENVIRONMENTS OF THE KAZHDUMI FORMATION IN THE STUDIED WELL
Microfacies analysis of the samples taken from the Kazhdumi Formation indicate 7 microfacies of 3 facies belts (deposition environment) based on Flugel (2010) including open marine (O), barrier (BA), lagoon (L) environment. Open marine facies (O) consist of bioclast packestone (O1), Bioturbated limy mudstone (O2), Oligostegina pelagic wackestone (O3) Oligostegina wackestone (O4), Spicule sponge oligostegina wackestone (O5). This facies zone is characterized with pelagic forams such as Favusella sp., Oligosteginids, spicule sponge, and to Echinoid fragments and bioturbated limy mudstone which indicates deep open marine environment. This facies is characterized by pelagic foraminifera such as Favusella washitensis, Oligosteginid and context of lime mud and represent low energy environment that demonstrate open marine. Barrier (BA); consist of Algal peloid grainstone (BA1), bioclast, and peloid graistone. Context of grainstone and very high energy condition indicates in barrier environment. Lagoon facies (L); consists of peloid bioclast packestone (L1). This facies is mainly consisted of benthic foraminifera such as Orbitolina sp., Nezzazzata sp., miliolids. The skeletal allochems are abundant and diversity. Benthic Foraminifera in context of limy mud represent Low energy environment that demonstrate Lagoon.
Figure-4. Microfacies column of the Sarvak Formation in Bahregansar oil field
Source: Ahmadi (2016)
Figure-5. Microfacies column of the Kazhdumi Formation in Bahregansar oil field.
Source: Ahmadi (2016)
8. CONCLUSION
Biostratigraphic study of the Sarvak Formation in Bahregansar oil field shows 13 genera and 9 species of benthic foraminifera and 5 genera and 8 species of planktonic foraminifera. As a result, 3 biozones were introduced including 1-Nezzazata- Alveolina Assemblage Range Zone, 2-Trocholina- Orbitolina Assemblage Range Zone, 3- Favusella washitensis Range zone.
Biostratigraphic study of the Kazhdumi Formation shows 9 genera and 3 species of benthic foraminifera and 3 genera and 2 species of planktonic foraminifera. As a result, 2 biozones were introduced including 1-Oligostegina facies 2- Favusella washitensis Range Zone. Those zones are according to introduced zones by Wynd (1965). Foraminiferal study indicated an Albian-Cenomanian for the Sarvak and Kazhdumi formations. The microfacies analysis of the samples taken from the Sarvak and Kazhdumi formations in Bahregansar oil field led to the recognition of 7 micrifacies for Sarvak Formation and 7 microfacies for Kazhdumi Formation, all microfacies divided into three facies belts (depositional environment) including open marine, bar and lagoon environments. The facies analyses represent the Sarvak and Kazhdumi formations in Bahregansar oil field were formed in a carbonate ramp.
Plate 1
Fig.-1.Reticulina reicheli,depth 2610m, Sarvak Formation.
Fig.-2.Salpingoporella sp., depth 2880m, Sarvak Formation.
Fig.-3.Cylindroporella sp., depth 2881m, Sarvak Formation.
Fig.-4. Nezzazzata simplex Omara 1956, depth 3739m, Sarvak Formation.
Fig.-5.Prealveolina cretacea Darchiaci 1837, depth 2706m, Sarvak Formation.
Fig.-6.Quniqueloculina, Axial section, depth 2720m, Sarvak Formation.
Fig.-7. Lenticulina sp. Lamark 1804, Axial section, depth 2870m, Sarvak Formation.
Fig.-8. Dicyclinasp., Munier–CHa lmas 1887, Equatorial section, depth 2754m, Sarvak Formation.
Fig.-9.Chrysalidina gragata Dorbigny 1839, depth 2690m, Sarvak Formation.
Fig.-10. Hemicyclammina signalimaynei, depth 2720m, Sarvak Formation.
Fig.-11. Cuneolina pavonia Drbigny, depth 2754, Sarvak Formation.
Fig.-12. Cylindroporella sp. Sarvak Formation.
Plate2
Plate-2.
Fig.-1.Trocholina Arabica Hensoni 1948, Axial section, depth 2687m, Sarvak Formation.
Fig.-2.Trocholina intermedia Hensoni 1948, Axial section, depth 2815m, Sarvak Formation.
Fig.-3.Trocholina sp. Paalzow 1922, Axial section, depth 2800m, Sarvak Formation.
Figs.-4-5.Orbitolina concava, Lamark 1816, depth 2815m, Sarvak & Kazhdumi Formation.
Figs.-6-7-8. Dictytoconus sp., depth 2616m, Sarvak Formation.
Fig.-9.Nezzazzata sp. Omara1956, Axial section, depth 2716m, Sarvak Formation.
Fig.-10.Nezzazzata sp., Omara1956, sub axial section, depth 2715m, Sarvak Formation.
Fig.-11. Ovalveolina ovum, Dorbigny, Equatorial section, depth 2716m, Sarvak Formation.
Fig.-12.Pesudolitunella reicheli ,Mariei 1955, depth 2845m, Sarvak Formation.
Plate 3
Plate-3.
Figs.-1, 5. Muricohedbergella rischi, Moullade 1966, Axial section, depth 2830m, Sarvak Formation.
Fig.-2. Heterohelix globulosa, Ehrenberg 1840, Axial section, depth 2678m, Ilam Formation.
Fig.-3. Heterohelix reussi, Cushmani 1938, Axial section, depth 2675m, Ilam Formation.
Fig.-4. Ticinella primula, Luterbacher 1963, Axial section, depth 3060m, kazhdumi Formation.
Figs.-6-7-8. Favusella washitensis, Carsey 1926, Axial section, Depth2840m, Sarvak Formation.
Fig.-9. Rotalipora subticinensis, Gandolfi 1957, Axial section, depth 2865m, Sarvak Formation.
Fig.-11. Rotalipo rareichli, Mornodi 1950, Axial section, depth 2713m, Sarvak Formation.
Fig.-12. Rotalipora gandolfii, Luterbacher & Permolisilva1962, Axial section, depth2716, Sarvak Formation.
Plate4
Plate-4.
Fig.-A. Benthic foraminiferal wackestone, Lagoon (C1), Sarvak Formation.
Fig.-B. Oligostegina pelagic wackestoe, Open marin (O3), Kazhdumi Formation.
Fig.-C. Peloid bioclast packestone, Lagoon (L1), Kazhdumi Formation.
Fig.-D. Bioclast peloid wackestone, Lagoon (C2), Sarvak Formation.
Fig.-E. Bioclast packestone, Open marin (O1), Kazhdumi Formation.
Fig.-F. Spicule sponge oligostegina wackestone, Open marin (O 5), Kazhdumi Formation.
Fig.-G. Bioturbated lim mudstone, Open marin (O2), Kazhdumi Formation.
Fig.-H. Oligostegina wackestone, Open marin (O4), Kazhdumi Formation.
Fig.-I. Algal peloid grainstone, Bar (Ba1), Kazhdumi Formation.
Fig.-J. Bioclast grainstone, Bar (B1), Sarvak Formation.
Fig.-K. Pelagic foraminiferal wackestone, Open marin (A3), Sarvak Formation.
Fig.-L. Oligostegina wackestone, Open marin (A1), Sarvak Formation.
Fig.-M. Dolomitized peloid grainstone, Bar (B2), Sarvak Formation.
Fig.-N. Pelagic foraminiferal packestone, Open marin (A2), Sarvak Formation
| Funding: This study received no specific financial support. |
| Competing Interests: The authors declare that they have no competing interests. |
| Contributors/Acknowledgement: All authors contributed equally to the conception and design of the study. |
REFERENCES
Afghah, M. and G. Farhoudi, 2012. Boundary between upper cretaceous and lower Paleocene in the Zagros mountain ranges of Southwesern Iran. Acta Geologica Sinica, 86(2): 325-338. View at Google Scholar | View at Publisher
Ahmadi, S., 2016. Microbiostratigraphy and sedimentary environment of the Sarvak and Kazhdumi formations in Bahregansar oilfield: PhD Thesis, Tehran University of Science and Research. pp: 5.
Bourgeois, F., 1969. Kuh-e Bangestan: A model for cretaceous structures in Iran, techincal memoir 89. Iranian Offshore Oil Company Technical.
Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. American Association of Petroleum Geologists, Memoir, 1: 108-121. View at Google Scholar
Flugel, E., 2010. Microfacies of carbonate rocks, analysis, interpretation and application. Berlin: Springer.
Husinec, A., L. Velic, L. Fucek, I. Vlahovic, D. Maticˇec, N. Os˘tric and T. Korbar, 2000. Mid cretaceous orbitolinid (Foraminiferida) record from the islands of cres and Los˘inj (Croatia) and its regional stratigraphic correlation. Cretaceous Research, 21(1): 155-171. View at Google Scholar | View at Publisher
James, G.A. and J.G. Wynd, 1965. Stratigraphic nomenclature of iranian oil consortium agreement area. American Association of Petroleum Geologists Bulletin, 49(12): 2182- 2245. View at Google Scholar | View at Publisher
Kalantary, A., 1969. Foraminifera from the middle Jurassic-Cretaceous successions of Koppet-Dagh region (NE Iran). National Iranian Oil Company, Geological Laboratories.
Khalili, M., 1974. The biostratigraphic synthesis of the Bangestan group in South West Iran. Report 1219, Tehran, Offshore Oil Company of Iran.
Madadi, A., 1986. Report 1842, Tehran, Offshore Oil Company of Iran, Unpublished Paper.
Motiei, H., 1992. Stretigraphy of Iran, Zagros stretigraphy. Geological Survey of Iran. pp: 232.
Tabiyejou, S., M. Afghah, R. Sarnadi and A.R. Yamini, 2014. The determination and identification microfacies collection and sedimentary environments of Ilam and Sarvak reservoir in Sarvestan oil field. Proceeding of 32 National and 1 International Geoscience Congress Petroleum Geology, Iran, Ahvaz, 174.
Wynd, J.D., 1965. Biofacies of the Iranian oil consortium agreement area IOOC Report No. 1082. Unpublished.