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Coupled imprints of tropical climate and recurring emergence on the reservoir evolution of a mid-Cretaceous carbonate ramp, Zagros Basin, southwest Iran

H. Rahimpour-Bonab, H. Mehrabi, A.H. Enayati-Bidgoli,
M. Omidvar Department of Geology, College of Science, University of Tehran, Iran
The Cretaceous Sarvak Formation (and its equivalents on the Arabian platform) is one of the most important carbonate reservoirs to have developed under the warm equable climate. This formation in the Abteymour Oil field contains a ydrocarbon reservoir in a carbonate ramp formed under a humid tropical oceanic climate during the middle Cretaceous in the far southwest margin of Zagros Basin, in the transition zone of the Mesopotamian Basin and Dezful Embayment.
Detailed petrography assisted by image analysis resulted in the recognition of 12 microfacies grouped in six facies associations indicating a homoclinal ramp-type platform. The importance of climatic control over the facies types and patterns and diagenetic history is elaborated in this study.
Interactions of sea-level fluctuations, local tectonic activity, and climate are reflected by the substantial dissolution and development of reservoir quality in different horizons of this mud-dominated carbonate unit.
The history of reservoir formation and its evolution during post-depositional events are revealed by a high-resolution study of primary and secondary textural features and their relationships. Accordingly, the relative timing of these processes and their paragenetic sequences indicate a complex reservoir history.
From the Late Albian to the Mid Turonian three phases of emergence originated from eustatic changes and movements of basement blocks (palaeohigh formation), which led to extensive leaching and karstification. The Mid Cenomanian eustatic sea-level fall led to the first disconformity.
A second emergent period and disconformity was the result of Late Cenomanian Early Turonian movement of basement blocks. An Early e Middle Turonian sea-level rise or local subsidence of the palaeohigh led to the inundation of this area and deposition of the upper division of the Sarvak Formation.
This buried the middle Sarvak Formation to shallow depths and caused some cementation and mechanical compaction. A widespread Mid Turonian fall in sea level and basin-wide exposure lasting several million years led to the formation of an important regional disconformity in the upper Sarvak unit, represented by major diagenetic dissolution and an over-mature karst profile. Concurrently, deep circulation of meteoric waters led to another phase of non-fabric selective leaching beneath the CET disconformity (telogenetic diagenesis) in the middle Sarvak Formation.
These two stages of diagenesis (eo- and telogenetic) resulted in considerable improvement of the reservoir in the middle Sarvak succession. Generally, these diagenetic features show a good correlation with depositional facies and sequence stratigraphic positions (systems tracts).
As shown in this study, they are predictable in the sequence stratigraphic framework of the formation. Three third-order sequences in the Mid Cenomanian e Mid Turonian interval are recognized.
These are clearly correlated over the Abteymour Oil field and other areas in the Zagros Basin and Arabian Platform (Oman) and provide a suitable framework for reservoir correlations and further modeling.


The Bangestan Group of Albian-Campanian age contains some of the most prolific reservoirs of the Arabian Platform and Zagros Basin hydrocarbon provinces, predominantly within neritic carbonates of the Sarvak Formation and equivalent units (e.g., the Mishrif Formation in Iraq).
Domination of a humid tropical climate in the mid Cretaceous of the Middle East (e.g., Murris, 1980;Beydoun, 1991; Beydoun et al., 1992; Fluteau et al., 2007; Keller et al., 2008; Hollis, 2011) is reflected in the tropical-type carbonate platforms and facies assemblages associated with respective diagenetic features, especially extensive karst development (e.g., Alsharhan and Nairn, 1993, 1997; Hollis, 2011).
This * Corresponding author.
E-mail address: rahimpor@khayam.ut.ac.ir (H. Rahimpour-Bonab).

situation, accompanied by frequent sea-level fluctuations, resulted in extensive eogenetic diagenesis and general reservoir improvement in the succession throughout the Middle East (Shano et al.,1993; Haq et al., 1988; Immenhauser et al., 2001; Sadooni, 2005;Taghavi et al., 2006; van Buchem et al., 2011; Hollis and Sharp,2011).
Repeated sea-level fluctuations and leaching in the warm humid climate led to extensive micro- and mega-karst development and to major reservoir improvement, immediately beneath the local and regional unconformities (Razin et al., 2010; ollis,2011). Purdy and Waltham (1999) considered that unconformity-related reservoirs form 20e30% of the total hydrocarbon reserves of the world.
Generally, the various effects of diagenetic processes on reservoir quality depend on several factors including original mineralogy and texture of depositional facies, climate (temperature and water supply) (Harris et al., 1984; James and hoquette, 1984; widlich,2010).
In addition, the duration of the emergence modifies the pore systems and, hence, the poroperm values. In most cases, short periods of exposure produce better results (Harris et al., 1984;Weidlich, 2010).
In this regard, description of the paragenetic sequence of diagenetic events aids determination of the history of poroperm evolution (Dunnington, 1967; James and Choquette, 1984; more, 2001; Al-Habshi et al., 2003; Rahimpour-Bonab, 2007; Ahr,2008; ollis, 2011) and its relationship to the sea-level fluctuations.
In this paper, the temporal distribution of various facies assemblages of the Sarvak Formation in the Abteymour Oilfield, which is located in the transition zone between the Mesopotamian Basin and the Dezful Embayment in southwest Iran (Fig. 1), is determined.
To recognize the major factors that govern reservoir quality, an integrated approach that included depositional facies analysis and a detailed reconstruction of the diagenetic history was carried out within a sequence stratigraphic framework (e.g., Moore, 2001; Roger, 2006; Gomes et al., 2008).
Comprehensive facies analysis revealed primary (depositional) pore-size distribution and reservoir quality. The diagenetic history indicates that secondary alteration and hence rearrangement of pore spaces shaped the ultimate reservoir characteristics.
To generate a regional view of diagenetic impacts, this study is accompanied by a reconstruction of the depositional environment (using high-resolution facies analysis) and biostratigraphy in a sequence stratigraphic framework.