ORIGINAL ARTICLE

Paleoclimate and tectonic controls on the depositional  and diagenetic history of the Cenomanian–early Turonian carbonate reservoirs, Dezful Embayment, SW Iran Hamzeh Mehrabi

Hossain Rahimpour-Bonab
Received: 12 April 2013 / Accepted: 23 April 2013
Springer-Verlag Berlin Heidelberg 2013 Abstract  Integrated facies and diagenetic analyses within a sequence stratigraphic framework were carried out on mid-Cretaceous Sarvak carbonate reservoirs in five giant and supergiant oilfields in the central and southern parts of the Dezful Embayment, SW Iran. Results of facies analysis indicate a homoclinal ramp-type carbonate platform for this formation with the frequencies of different facies associations in six wells reflecting their approximate position in the sedimentary model.

Introduction

Integrated facies and diagenetic analyses within a high-resolution sequence stratigraphic framework are essential prerequisites for any systematic reservoir characterization study (Moore 2001; Schlager 2005; Roger 2006; Lucia 2007; Ahr 2008). Generally, sedimentary facies control primary poroperm distribution in carbonate reservoirs (Schlager 2005).

In the absence of significant diagenesis, reservoir parameters are mostly controlled by individual facies characteristics (micro-scale) and the distribution of sedimentary environments (macro-scale) (e.g., Ahr 2008). However, in most cases, due to the high sensitivity of carbonate rocks to diagenetic alterations, the primary distribution of poroperm (reservoir quality parameters) can be drastically changed. Additionally, diagenesis is the main factor controlling reservoir quality in most carbonate reservoirs of the world (James and Choquette 1984; Ehrenberg 2006; Ehrenberg et al. 2008).

Nonetheless, many of the diagenetic pathways are controlled directly (early diagenetic features) or indirectly (late diagenetic processes) by primary (facies) characteristics of sediments (that is faciescontrolled diagenesis). Meteoric diagenetic processes (eogenetic and telogenetic) are the essential factors governing the reservoir quality of unconformity-related carbonate reservoirs (Immenhauser et al. 2000; Moore 2001; Wang and Al-Aasm 2002; Ehrenberg et al. 2008; Weidlich 2010; Reinhold and Kaufmann 2010). Depending on several factors, notably climatic conditions, duration of exposure and mineralogy, these processes may improve or destroy
H. Mehrabi,
H. Rahimpour-Bonab (&) Department of Geology, College of Science, University of Tehran, Tehran, Iran e-mail: hrahimpor@gmail.com; rahimpor@khayam.ut.ac.ir

reservoir quality (Mazzullo and Chilingarian 1992; Sun and Esteban 1994; Weidlich 2010). To make a correct and logical correlation between facies and diagenetic features, construction of a sequence stratigraphic framework can be useful (Tucker 1993; Schlager 2005).

Global, regional, and local relative sea-level changes together with tectonic activities are the main factors shaping sedimentary sequences (Sharland et al. 2001; Hofmann and Keller 2006; Razin et al. 2010).
Correlation of facies and diagenetic features within a sequence stratigraphic framework has resulted in the generation of geological models for carbonate reservoirs that can be used in reservoir simulations (Roger 2006; Masse and Masse 2011).
The Sarvak Formation in SW Iran has been the subject of several studies dealing with facies and paleoenvironments, diagenetic features, sequence stratigraphy, and reservoir quality of this second, most important hydrocarbon reservoir in Iran (e.g., Taghavi et al. 2006; Beiranvand et al. 2007; Farzadi and Hesthmer 2007; Ghabeishavi et al. 2010; Hajikazemi et al. 2010, 2012; Razin et al. 2010; RahimpourBonab et al. 2012a, b).
However, there has been a lack of integrated sedimentological and stratigraphical studies of the Sarvak Formation in the giant/supergiant oilfields in the Dezful Embayment, which has led to inconsistencies in reservoir modeling. Moreover, the lateral and vertical facies changes in this unit in this area, along with its variable and complex diagenetic history, are not well understood. The main target of this research is to investigate the controls of paleoclimate and tectonics on the facies characteristics, diagenetic history, and reservoir quality of the mid-Cretaceous reservoirs in the Sarvak Formation of the Dezful Embayment in SW Iran, with coming from six wells from five giant/supergiant oilfields (Fig. 1)

Methods

For sedimentological analysis and interpretations, about 1,370 m of core from six exploratory or appraisal wells drilled in five giant/supergiant oilfields, located in central and southern parts of the Dezful Embayment (including Abteymour, Ahwaz, Gachsaran, Marun, and Rag-e-Safid oilfields), were examined in detail to describe the various facies and diagenetic features. In addition, petrographic studies and microscopic image analyses on 750 thin-sections were carried out. The modified Dunham (1962) textural scheme of Embry and Klovan (1972) was used together with sedimentary structures and fabrics, grainsize, and rock composition. Facies analysis used standard models and microfacies descriptions (e.g., Wilson 1975; Buxton and Pedley 1989; Pedley 1998; Flu ¨gel 2004).

From the data, a regional conceptual depositional model and sequence stratigraphic framework was constructed for the
upper Sarvak in the studied oilfields. On the basis of statistical analysis of the facies associations and their frequencies in the studied wells, their approximate positions in the depositional model were determined. Results of facies analysis (e.g., vertical facies variations), biostratigraphic analysis, diagenetic studies and log data (especially gamma-ray logs) were used for determination of the thirdorder sequences in the studied intervals. Finally, facies characteristics and diagenetic features were correlated within the sequence framework (systems tracts). Additionally, these third-order sequences were correlated with the other studies in Iran and neighboring areas.

Paleoclimate, paleogeography, and tectonic setting

The middle Cretaceous (Cenomanian–Turonian; 98–92 Ma) is generally described as a warm period across the world and in the Middle East (Huber et al. 2002; Hay 2008;
Fig. 1 a Location map of the Dezful Embayment in SW Iran and Mesopotamian Basin of Iraq. b Schematic map of studied oilfields and paleohighs that existed in various parts of SW Iran in middleupper Cretaceous. The Abteymour, Gachsaran, and Rag-e-Safid oilfields are located above the paleostructures FaciesKeller et al. 2008). At this time, the NE margin of the Arabian Plate (i.e., Zagros Basin) was located in the northern hemisphere close to the equator (Setudehnia 1978; Glennie 2000; Sharland et al. 2001; Ziegler 2001; Huber et al. 2002) (Fig. 2). Thus, the middle Cretaceous Sarvak platform was under the domination of a warm and humid tropical climate with heavy rainfall. Tectonically, in the Neotethys, the middle Cretaceous is the beginning of oceanic crust subduction beneath central Iran and Oman (Glennie 2000; Sepehr and Cosgrove 2004; Bordenave and Hegre 2005; Heydari 2008). Evidence for this event is recorded by ophiolites obducted onto the continental crust in the Zagros suture zone (Neyriz and Kermanshah) and Oman Mountains (Motiei 1993; Alsharhan and Nairn 1993; Sharland et al. 2001) (Fig. 2). At this time, the NE margin of the Arabian Plate changed from a passive continental margin into an active tectonic one (Sepehr and Cosgrove 2004; Alavi 2004, 2007).
Reactivation of basement faults along with halokinetic movements, related to the Hormoz salt series, marked the middle Cretaceous as one of the most tectonically active periods in the geological history of the Middle East (Glennie 2000, 2010; Sepehr and Cosgrove 2004; Sherkati et al. 2005; Ahmadhadi et al. 2007; Motamedi et al. 2011).
These tectonic activities led to the generation of multiple paleohighs in different parts of the region including SW Iran and the Dezful Embayment (Figs. 1b, 2). Moreover, the presence of these paleohighs resulted in great variations in facies and diagenesis of contemporaneous carbonate formations (Sepehr and Cosgrove 2004; Alavi 2004, 2007; Rahimpour-Bonab et al. 2012a).
The presence of several paleohighs is recorded on seismic lines in SW Iran (e.g., van Buchem et al. 2001). One of these important paleohighs is located below the reservoir anticline of the Gachsaran Oilfield (Fig. 1). Other well-known paleohighs in the Dezful Embayment are located in Rag-e-Safid and Abteymour oilfields (NazarAghaei 1986; Ashrafzadeh 1999; van Buchem et al. 2001).
Eustatic sea-level fluctuations from the Albian to Turonian (Fig. 2c), accompanied by the humid climate, exerted strong controls over the reservoir quality by recurring subaerial exposure and formation of disconformities in the sedimentary record of the Middle East and SW Iran (van Buchem et al. 1996; Sharland et al. 2001; Immenhauser et al. 2000; Sadooni and Aqrawi 2000; Razin et al.

Stratigraphy

The Dezful Embayment of southwest Iran (Fig. 1a) is one of the most prolific hydrocarbon provinces of the world. Several giant and supergiant oilfields are located in this province that produce oil (and gas, with less importance) mainly from the Mesozoic Khami Group (Fahliyan and Dariyan Formations) and Bangestan Group (Sarvak and Ilam Formations) and Cenozoic Asmari Formation (Fig. 2). The Sarvak Formation (late Albian?–middle Turonian) of the Bangestan reservoirs is the second most important reservoir succession (after the Oligo-Miocene Asmari Formation) in the Zagros Basin (Motiei 1993; Fig. 2).
Lithologically, it is composed mainly of limestones that are dolomitic in parts. In most parts of the Dezful Embayment (and other neighboring areas in the Fars province and the Persian Gulf basin), the lower parts of the Sarvak Formation are composed of pelagic, argillaceous limestone that has a lower reservoir quality than its upper parts. In these areas, some thin intervals of marl and shale (mostly less than 10 m thickness) are also present (Motiei 1993). The upper part of the Sarvak shows higher reservoir quality due to the effects of the above-noted disconformities (Taghavi et al. 2006; Beiranvand et al. 2007; Hajikazemi et al.
2010; Rahimpour-Bonab et al. 2012a, b). For this reason, most of the exploratory and appraisal wells in the Dezful Embayment oilfields have only drilled the upper part of this formation and it is five of these that are the focus of this study (Fig. 1b). The studied interval is equivalent to the Mishrif Formation in Iraq, Natih Formation in Oman, Mishrif, Ahmadi, and Rumaila Formations in UAE and Kuwait (Fig. 2b), and Derder Formation in SE Turkey.
Seemingly, all these units were deposited on a ramp-like carbonate platform on the NE margin of the Arabian Plate. In its type section, the Sarvak Formation lies upon the Kazhdumi Formation with a transitional contact and is overlain by shale and marl of the Gurpi Formation with a sharp contact (Motiei 1993). However, in several parts of the Dezful Embayment (including studied oilfields) carbonates of the Ilam Formation rest upon the Sarvak with a disconformable contact (Fig. 2a, c).
Depending on the presence and numbers of unconformities, the Sarvak Formation can be divided into two (lower and upper; where only one disconformable surface is present) or locally three parts (lower, middle, and upper, where two important disconformities are present) (Rahimpour-Bonab et al. 2012a) (Fig. 2c). These parts show considerable changes in their geological and petrophysical characteristics and consequently have different reservoir potentials. This study is focused on the Sarvak interval above the local C-T unconformity, which includes its middle and upper parts.
the Sarvak carbonates together with the trend of relative sea-level from eustasy and regional tectonics (SW Iran)
of the Bangestan group in the studied oilfields showing the position of the disconformities within the Sarvak carbonates together
Schematic map of studied oilfields and paleohighs that existed in various parts of SW Iran