Hamzeh Mehrabi, Maryam Mansouri, Hossain Rahimpour-Bonab, Vahid Tavakoli, Maryam Hassanzadeh
a.Department of Geology, College of Science, University of Tehran, Tehran, Iran
b.Abdal Industrial Management Company, Geology and Geophysics Division, Tehran, Iran
Chemical compaction features are important diagenetic imprints that generally interrupt reservoir quality in most carbonate reservoirs. In the Permian-Triassic reservoirs of South Pars Field, they are recorded in various types and intensities.
High- to low amplitude stylolites and solution seams are concentrated within the K1–K4 units of the upper Dalan and Kangan formations, depending on their facies characteristics, diagenetic alterations, and dominant lithology.
In these units, there are some meaningful trends between the chemical compaction zones and sequence stratigraphic positions including the systems tracts of third-order sequences. Control of chemical compaction on reservoir characteristics is evaluated by using the results of petrographic studies, petrophysical logs and core poroperm measurements.
Internal reservoir architecture of the studied reservoirs is determined using both hydraulic ﬂow units and stratigraphic modiﬁed Lorenz plot approaches. Identiﬁed bafﬂe and barrier units show a close correspondence with chemically-compacted zones. This indicates that zones with a high concentration of stylolites and solution seams can be considered as small-scale vertical ﬂow barriers in the Permian-Triassic reservoirs of South Pars Field.
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Besides facies characteristics and tectonically-induced features, diagenetic processes are among the main factors that control reservoir quality and ﬂuid ﬂow in carbonate reservoirs (Lucia, 2007; Ahr, 2008).
As important diagenetic features, products of chemical compaction process, including the stylolites and solution seams, are complex column-and-socket structures that form as a result of pressure dissolution in both carbonate and clastic rocks during their residence in burial realms (e.g., Nenna and Aydin, 2011;Croize et al., 2013; Heap et al., 2014).
They generally form when sediments experience high burial depth (around 0.8 to 42 km)due to an increase in overburden pressure and temperature (Railsback, 1993; Mallon and Swarbrick, 2008). In some cases, they have formed by tectonic activities. Tectonic-induced stylolites mostly occur as oblique, or sometimes, perpendicular to the bedding surfaces (Andrews and Railsback, 1997).
Under such a high pressure-temperature condition, mineralogically unstable to me-tastable grains being dissolved at inter-grain junctions in forms of solution seams and stylolites (Ehrenberg et al., 2007). Dissolved materials and other ﬂuids can migrate along the stylolites-paths to long distances and provide materials for other diagenetic features, such as burial cement. In this point of view, they can be considered as ﬂow paths or conduits for ﬂuid migration (Hassan,2007; Heap et al., 2014).
On the other hand, they can act as ﬂow barriers, when in-soluble residual materials (such as clays, pyrite, and oxides) accumulate in their paths and provide effective seals especially in the vertical direction; i.e., perpendicular to the stylolite’s plane (Burqess and Peter, 1985; Dutton and Willis, 1998; Alsharhan and Sadd,2000).
Therefore, characterization of stylolites, including their frequency, amplitudes, and tightness, can help to get a better understanding about the reservoir heterogeneity in carbonate sequences (Oswald et al., 1995; Ehrenberg et al., 2007).
In the Persian Gulf, Permian-Triassic reservoirs were subjects of several studies that investigate their geological and structural history, sedimentary environment, sequence stratigraphy, hydrocarbon potential, and internal reservoir architecture (e.g., Pollastro,03; Insalaco et al., 2006; Maurer et al., 2009; Rahimpour-Bonab al., 2010; Mehrabi et al., 2015).
In most of these researches, chemical compaction features are reported as diagenetic features these reservoirs. However, in the South Pars Field.
The main purpose of this paper is to describe the chemical compaction features and investigate their relationships with lithological variations, facies associations, depositional sequences and reservoir heterogeneities in the Permian-Triassic reservoirs of South-Pars gas ﬁeld in the Persian Gulf. their detailed characteristics and controls on reservoir quality of concerned units are not yet thoroughly understood, especially
2. Geological setting and stratigraphy
Along with its southern extension (North Dome), the South Pars Field is the world’s largest gas reserve that accumulated in the NNE–SSW trending Qatar Arch (QA) structure in the Persian Gulf (Fig. 1; Szabo and Kheradpir, 1978; AL-Jallal, 1995). This arc has bounded to the north and northeast by the Zagros fold and thrust belt (ZFTB) and is placed in the interior part of the Arabian Platform (Konert et al., 2001).
The Persian Gulf has been inﬂuenced by this regional gentle or broad anticline structure (Alsharhan and Nairn, 1997). During the late Precambrian to Early Cambrian, tectonic movements in central Saudi Arabia caused activation of fault systems and uplift in this area, i.e. the Qatar Arch. During the Silurian, source-rock intervals were deposited in the western and eastern parts of QA.Plate <
Deposition of Permian-Triassic shallow marine carbonates and evaporites was initiated by an extensive marine transgression on the Arabian /span>during the Late Permian. This transgression was related to the opening of the Neotethys Ocean and the creation of a passive margin in the northeastern part of the Arabian Plate (Edgell, 1996; Sharland et al., 2001).
These widespread carbonate-evaporite intervals were deposited as a blanket within which some individual horizons are correlatable over wide distances throughout the Persian Gulf. The Late Permian to Early Triassic carbonate-evaporite units of the South Pars Field have been divided into the Dalan and Kangan formations (Khuff equivalents), respectively (Fig. 2; Kashﬁ, 2000).
Due to Late Triassic tectonic movements in the QA, there was no sedimentation in these sub-basins from the latest Triassic to Middle Jurassic times (Murris, 1980). Stratigraphic successions in the subsurface of the QA consist of (Fig. 2):
- The post-Precambrian to the pre-Permian clastic sediments(including Silurian source rocks);
- The Permian-Triassic sequences of thick limestones, dolomites, and anhydrites, (Khuff equivalents) in the Arabian Plate (as reservoir rocks);
- The post-Triassic section, predominantly marine sequences of limestones, dolomites, shales, and evaporites with local sandy lastic sequences.
Stratigraphically, the Dalan Formation (Late Permian) is commonly divided into four units named as K3, K4, Nar member, and K5.
Among these, the K4 and K5 units are mainly composed of dolomite and limestones with some anhydritic intervals. The K3 unit consists of dolomite and dolomitic limestone. The Early Triassic Kangan Formation is divided into K2 (limestone and dolomite) and K1 (anhydrite, dolomite, and limestone) units (Fig. 2).
The main reservoir intervals of these formations are K2 and K4 units, in which high energy shoal facies (mainly ooid grainstones to packstones) provided thick, porous, and permeable intervals that produce the main portion of gas in the South Pars Field (Ehrenberg,2006; Tavakoli et al., 010). This study focuses on the K1–K4 units of these reservoirs that correspond with the upper Dalan and Kangan formations.
Fig. 1. A-Location map of the Persian Gulf and South Pars Field on the Qatar-Arc structure. B-Detailed location map of the studied ﬁeld in the Persian Gulf of Southern Iran.
Fig. 2. A-Stratigraphic column of the South Pars Field including the Permian-Triassic Dalan and Kangan formations (after Alavi, 2004). B-Detailed stratigraphy of the Permian-Triassic reservoirs including their lithology, reservoir units, and sequence stratigraphy.