Facies variation, sedimentary environments, and sequence stratigraphy of the Sarvak Formation adjacent to the Hendijan–Bahregansar and Kharg–Mish Palaeohighs, northwestern Persian Gulf

Abstract

AbstractThe northwestern part of the Iranian sector of the Persian Gulf (NWPG) is a highly significant hydrocarbon offshore area, located within the southern Mesopotamian Foreland Basin. It lies in front of the Zagros Fold Belt along the subducting margin of the Arabian plate. Tectonic movements along the Hendijan–Bahregansar and Kharg–Mish palaeohighs influenced the Cretaceous stratigraphy of the NWPG. The reservoir significance of the Sarvak Formation makes data about microfacies, sedimentary environments, and sequence stratigraphy of the rock unit in areas adjacent to the palaeohighs (Hendijan, Bahregansar, Kharg, and Dorood oilfields) quite valuable. Rock samples and available palaeologs are used to define the microfacies and to interpret the depositional environments of the Sarvak Formation from the Dorood, Bahregansar, and Hendijan subsurface sections. Seven microfacies attributed to three sub-environments, namely, inner-, middle-, and outer ramp are designated. The absence of turbidites and continuous reefs indicates that carbonates of the Sarvak Formation in the studied area accumulated on a homoclinal ramp. Additionally, three third-order depositional sequences are identified. The first depositional sequence signifies that during late Albian–Cenomanian the area was structurally stable while in Cenomanian (the second and third sedimentary sequences) the Sarvak Formation comprises shallow facies as the result of the sea level fall. Finally, the late Cenomanian epirogenic movements and uplift along the Kharg–Mish and Hendijan–Bahregansar caused regression and erosion of the Cenomanian strata in adjacent areas.Keywords: Microfacies, Depositional environments, Sequence stratigraphy, Middle Cretaceous, Sarvak Formation, Persian Gulf. IntroductionThe Persian Gulf, 240000 km2 in the area, is bordered by the Abadan Plain and the southern part of the Dezful Embayment on the north, the Fars platform on the northeast, the Arabian Peninsula on the south, and Oman on the east (Ziegler 2001; Konyuhov and Maleki 2006; Ghazban 2007; Al-Qayim et al. 2018). Development of the Persian Gulf region occurred during the Late Cenozoic and after the formation of the Zagros Ranges (Ghazban 2007). The Hendijan–Bahregansar and Kharg–Mish are two major fault systems in northwestern the Persian Gulf. The Kharg–Mish palaeohigh was active from the Aptian whereas the former remained inactive till the latest Cenomanian when uplift along both resulted in favorable conditions for the formation of such oil traps as the Hendijan, Bahregansar, Kharg and Dorood oilfields. Of these, the Sarvak Formation is an important oil reservoir in the Zagros area (Motiei 1994; Van Buchem et al. 2011; Esrafili-Dizaji et al. 2015; Vincent et al. 2015). Tectonic movements along the Hendijan–Bahregansar and Kharg–Mish palaeohighs influenced the Cretaceous stratigraphy of the NWPG. The reservoir significance of the Sarvak Formation makes data about microfacies, sedimentary environments, and sequence stratigraphy of the rock unit in areas adjacent to the Hendijan–Bahregansar and Kharg–Mish palaeohighs in the Hendijan, Bahregansar, Kharg, and Dorood oilfields. Rock samples and the available palaeologs are used to define the microfacies and to interpret the depositional environments of the Sarvak Formation from the Dorood, Bahregansar, and Hendijan subsurface sections. Material & MethodsA total of 210 thin sections made of rock samples collected from the Sarvak Formation are examined regarding their petrographic, sedimentologic, and stratigraphic features. Additionally, available information, including palaeologs and unpublished internal reports documenting the Sarvak Formation in the Kharg Oilfield are consulted. The petrographic carbonate classification follows that of Dunham (Dunham 1962). Flügel (2010) facies belts and sedimentary models are also used. The sequence stratigraphic approach based on Sharland et al. (2001) model for the Arabian Plate and its subsequent updates (Bromhead et al. 2022; Van Buchem et al. 2011; Davies et al. 2002, 2019) are adopted herein. Discussion of Results & ConclusionsThe Sarvak Formation in the northwestern part of the Persian Gulf comprises 0–700 m carbonates with silty shales, dolomitized limestones, and dolostone intercalations. It succeeds the Kazhdumi Formation and precedes the Ilam or Gurpi formations. The stratigraphic distribution of foraminifera therein led to the introduction of some biozones/biofacies (Wynd 1965). These include Trocholina-Orbitolina assemblage biozone, Rudist debris biofacies, Nezazzata-alveolinids assemblage biozone and Oligostegina biofacies. According to the fossil content and also the lack of index fossils, the Sarvak Formation at the subsurface sections studied is dated as late Albian–Cenomanian.Seven microfacies attributed to three sub-environments, namely, inner-, middle-, and outer ramp are identified from the Hendijan, Bahregansar, and Dorood oilfields. Poorly fossiliferous dolomitized lime mudstone microfacies recognized in the Bahregansar and Hendijan oilfields are comparable with the tidal facies previously reported from the Kuh-e-Mond, Ahwaz oilfield, and the Sarvak Formation around Shiraz. The presence of peloidal miliolid grainstone, miliolid lime mudstone, and miliolid foraminifera grainstone microfacies shows that, for the most part, deposition occurred in such a low-energy environment as an inner ramp. These facies occurring in all wells studied are previously reported from the Masjed Soleiman, Kuh-e-Siah, Kuh-e-Mond, Kuh-e-Bangestan, Siri, and Ahwaz oilfields. Middle ramp sediments comprising rudist bioclast packstone-grainstone microfacies occur in the Dorood and Bahregansar oilfields and exhibit characteristics of high-energy environments. These facies have previously been reported from the Kuh-e-Siah, Kuh-e-Mond, Shivand stratigraphic section and Kuh-e-Bangestan. The outer ramp facies include benthic-planktonic foraminifera wackestone and planktonic foraminifera mudstone-wackestone. The main components of this microfacies are planktonic foraminifera accompanied by oligosteginids. The absence of turbidites and continuous reefs indicates that carbonates of the Sarvak Formation in the studied area accumulated on a homoclinal ramp.Additionally, three third-order depositional sequences are identified. The first is incomplete as its lower boundary occurs within the Kazhdumi Formation. The Transgressive System Tracts (TST) starts with a reduction in the arriving of fine-grained siliciclastic input (shale) from the Arabian Plate was associated with inner to outer ramp biofacies. The Maximum flooding surface (MFS) was associated with the microfacies 6 and 7 in the Bahregansar and Hendijan oilfields. In contrast, the MFS in other wells is located in the inner ramp (microfacies 3 and 4). The Highstand system tract (HST) is represented by a shallowing upward sequence in the middle to inner ramp facies with diverse benthic foraminiferal assemblages accumulated in a progradational pattern (microfacies 3). Comparison of the data from these wells with those from the Ahwaz oilfield (Bromhead et al. 2022), and well F in Kuwait shows that the MFS probably can be correlated with the K110 of Sharland et al. (2001) in other parts of the Arabian plate. Depositional sequences 2 and 3 are identified in the middle of the Sarvak Formation. The depositional sequence 2 represents the last one identified in the Bahregansar and Dorood oilfields due to tectonic activities along the Hendijan–Bahregansar and Kharg–Mish faults. The TST of this sequence starts with inner and outer ramp biofacies and the MFS was associated with microfacies 5 and 6. The sedimentary package representing stagnation (early HST) and the onset of sea level regression (late HST) includes inner ramp facies rich in benthic foraminifera with such porcelaneous and agglutinated representatives as Nezazzata, alveolinids, miliolids, and textulariids. The third depositional sequence (incomplete) is identified in the Kharg Oilfield. The TST starts with inner ramp biofacies and the MFS was associated with the microfacies 5. The HST is represented by inner ramp facies. Comparison of the data from these wells with those from the Arabian plate sequence stratigraphic model indicates that depositional sequences 2 and 3 are similar to the k120 and K130 identified from the Arabian plate.The first depositional sequence signifies that during late Albian–Cenomanian the area was structurally stable while in Cenomanian (the second and third depositional sequences) the Sarvak Formation comprises shallow facies resulting from the sea level fall. Finally, the late Cenomanian epirogenic movements and uplift along the Hendijan–Bahregansar and Kharg–Mish faults caused regression and erosion of the Cenomanian strata in adjacent areas.

Keywords

• microfacies
• depositional environments
• sequence stratigraphy
• middle cretaceous
• sarvak formation
• persian gulf