Hamzeh Mehrabi1 · Hossain Rahimpour‑Bonab1 · Elham Hajikazemi · Adeleh Jamalian
Received: 14 May 2015 / Accepted: 16 September 2015
© Springer-Verlag Berlin Heidelberg 2015
In the Zagros area and the Persian Gulf, Upper Cretaceous carbonate sequences are among the most important hydrocarbon reservoirs. In this study, facies analysis and stratigraphic interpretation of these sequences, Cenomanian-Santonian in age, have been carried out in subsurface sections from various parts of the Zagros area (including the Dezful Embayment and Fars Province) and the Persian Gulf. To have a better understanding of the facies variations at the regional scale, depositional facies of these formations have been determined and grouped as facies associations. Frequency analyses of depositional facies and their characteristics reveal considerable variations in the study area. These are interpreted to have resulted from the combined effects of paleoenvironmental conditions and platform configuration. The overall depositional model of these formations is that of a carbonate ramp, which was likely homoclinal for the Sarvak and distally-steepened for the Ilam Formation. The isopach maps of the studied intervals are depicted based on the available data from hundreds of drilled wells, surface sections, and seismic interpretations. Large scale variations in facies and thicknesses of the studied formations are interpreted to be controlled by regional tectonic evolution and sea-level fluctuations during the Upper Cretaceous
The Upper Cretaceous depositional sequences in the Middle East are characterized by extensive shallow-water carbonate platforms within which locally, and at different times, intra-shelf basins developed hundreds of kilometers across and with water depths of 50–100 m (Murris 1980; Razin et al. 2010; van Buchem et al. 2010; Fig. 1).
In the Zagros area and the Persian Gulf (Fig. 2), the sequences are characterized by considerable variations in facies and thicknesses (Motiei 1993). These changes are related to the sedimentation in a foreland basin along the NE margin of the Arabian Plate, which are substantially controlled by the combined effects of tectonic activities, eustatic sea-level fluctuations and palaeoclimatic conditions (Fig. 1; Alsharhan and Nairn 1997; Sharland et al. 2001; Mehrabi and Rahimpour-Bonab 2014).
This study focuses on the Cenomanian—Santonian intervals, which are well-known in the Middle East by different lithostratigraphic nomenclatures in different countries (Fig. 3). The general stratigraphy and paleogeography of these formations have been studied in detail in various areas of the Arabian Plate, notably in Iran, the United Arab Emirates and Oman (e.g., James and Wynd 1965; Murris 1980; Burchette 1993; van Buchem et al. 2006). A regional sequence-stratigraphic model for this interval was provided by Sharland et al. (2001) and high-resolution sequence-stratigraphic studies on the outcrops of the Sarvak Formation were published by Razin et al. (2010) in the High Zagros (SW Iran). These studies documented the detailed stratigraphic architecture of the shallow water platform and intra-shelf basinal deposits and discussed the relative influence of tectonics, eustasy, and climate on their formation and evolution. Seismic interpretation and seismic stratigraphic modeling confirmed and refined these sequence-stratigraphic models (van Buchem et al. 2001; Sepehr and Cosgrove 2005; Grelaud et al. 2006; Ahmadhadi et al. 2007; Farzadi and Hesthmer 2007).
Diagenetic history and reservoir properties of the Upper Cretaceous deposits were investigated in the context of tectonic setting and palaeoclimatic condition (Taghavi et al. 2006; Hajikazemi et al. 2010; Rahimpour-Bonab et al. 2012a, b, 2013; Mehrabi and Rahimpour-Bonab 2014; Mehrabi et al. 2014, 2015).
In this study, we investigate the controls of active tectonic setting and eustatic sea-level changes on the Upper Cretaceous carbonate platforms and their facies characteristics in the onshore and offshore of the Zagros area (Fig. 2).
To achieve this goal, facies analyses and stratigraphic interpretations are integrated using the new data from recently drilled wells and outcrops of the Sarvak-Ilam formations to interpret their regional stratigraphic and depositional models.
General geology and stratigraphy
The Zagros fold-thrust belt (ZFTB) of SW Iran (Fig. 2) is the result of a complex geodynamic history including a platform phase during the Paleozoic, rifting during the Permian and Triassic, passive continental margin of the Neo-Tethys ocean in the Jurassic- early Cretaceous, ophiolite emplacement (obduction) dated as late Cretaceous, and finally, collision and crustal shortening since the Neogene (Berberian and King 1981; Agard et al. 2005; Sherkati et al. 2006; Jahani et al. 2009). It has been divided into three tectonic zones from the NE to the SW: the High Zagros, the Zagros Simply Folded Belt, and the Zagros Foredeep Zone (Falcon 1974). The Simply Folded Zagros is subdivided according to its tectonic and sedimentary evolution into three domains; Lorestan, Izeh, and Fars areas (Motiei 1995). The study areas are located in the northern and southern Dezful Embayment (located in the Fore-deep Zone), the interior part of the Fars province and the Persian Gulf.
Major tectonic deformation in the ZFTB took place in the Late Cretaceous to the Late Pliocene (Koop and Stoneley 1982; Alavi 2004; Farzipour-Saein et al. 2009). Firstly, this orogenic belt is the result of initial closure of NeoTethys (Fig. 1), which led to thrusting, obduction and the creation of a foreland basin in the Late Cretaceous (Alavi 2004).
A regional uplift following the ophiolite obduction caused a deep erosion and/or non-deposition of the carbonate sequences (i.e., Sarvak and Ilam formations and their stratigraphic equivalents) in various parts of the Middle East (Fig. 3), particularly where the salt diapirism and basement fault reactivation have resulted in the formation of structural highs (Stoneley 1981; Jahani et al. 2007; Hollis 2011).
On the Arabian Platform, the effects of these activities, during the Cenomanian-Santonian, have led to three important phases of emergency and disconformity formation (see Harris et al. 1984; Alsharhan and Nairn 1997 for more details). Paleohighs and intrashelf basins are recorded in both onshore and offshore of south and southwestern Iran, including the Dezful Embayment, Fars region and the Persian Gulf (e.g., van Buchem et al. 2001; Sepehr and Cosgrove 2005; Alavi 2007; Farzadi and Hesthmer 2007; Rahimpour-Bonab et al. 2013). Palaeoclimatic proxies suggest that the Cretaceous was one of the warmest Phanerozoic periods (Barron 1983).
During this time, the NE margin of the Arabian Plate (including the Zagros area of SW Iran) was near the equator (Sharland et al. 2001; Keller et al. 2008). This study focuses on the Cenomanian–Santonian carbonate sequences of Bangestan Group (including the Sarvak and Ilam formations) that form the most prolific hydrocarbon reservoir units in south and southwest Iran, after the Oligo-Miocene Asmari Formation (Setudehnia 1978; Beydoun et al. 1992; Alsharhan and Nairn 1997). The sequences have been drilled by numerous exploration/production wells in many oil and gas fields of this area.
In the type section (Tang-e-Sarvak, Kuh-e Bangestan), the Sarvak Formation conformably overlies the Kazhdumi Formation with a transitional contact, but its upper contact with the shales and marls of the Gurpi Formation is sharp (Motiei 1993). However, in many oilfields (including those discussed here), the Sarvak Formation is overlain unconformably by carbonates of the Ilam Formation.