PRECIPITATION IN PERMO-TRIASSIC CARBONATES AT THE SOUTH PARS GASFIELD, OFFSHORE IRAN CONTROLS ON RESERVOIR QUALITY
H. Rahimpour-Bonab+*, B. Esrafili-Dizaji*, and V. Tavakoli*
Dolomitization and related anhydrite cementation can complicate the characterization of carbonate reservoirs. Both processes have affected the Permo-Triassic Upper Dalan – Kangan carbonates, the main reservoir at the South Pars gasfield, offshore Iran.
The carbonates were deposited in the shallow-marine ramp or epeiric platform and, according to previous studies, underwent intense near-surface diagenesis and minor burial modification. Detailed petrographical and geochemical analyses indicate that dolomitization and anhydrite precipitation can be explained in terms of the sabkha/seepage-reflux models.
The early dolomites then re-equilibrated or re-crystallized in a shallow burial setting. Evaluation of poroperm values in different reservoir intervals indicates that replacive dolomitization in the absence of anhydrite precipitation or with only patchy anhydrite has enhanced the reservoir quality. Where anhydrite cement is pervasive and has plugged the rock fabric, poroperm values are significantly decreased.
As emphasized in previous studies and confirmed here, dolomitization and anhydrite cementation, together with original facies type, are the major factors controlling reservoir quality in the Dalan – Kangan carbonates at South Pars. When associated with minor anhydrite cementation, replacive dolomitization has enhanced reservoir quality by increasing permeability.
However, porosity in fabric-retentive dolomite was apparently inherited from the precursor rock and therefore reflects the original depositional environment. Low-temperature dolomitization is commonly fabric-selective and partially fabric-retentive.
Whole rock stable isotope thermometry indicates that fabric-destructive dolomites in the reservoir rocks formed at temperatures above 22ºC, whereas fabric-retentive dolomites and associated anhydrides formed in surface and near-surface conditions. Fabric-destructive dolomite or dolomite neomorphism post-date fabric-retentive dolomite and continued to form in deep burial conditions (~1400m). These observations may explain why fabric-retentive dolomite and anhydrite fabrics are traversed by stylolites.
Dolomitization and related anhydrite precipitation commonly affect platform carbonates and can exert significant control on reservoir quality. These processes have been investigated in major hydrocarbon reservoir rocks such as the Permian Khuff and Jurassic Arab Formations (Cantrell et al., 2004; Lindsay et al., 2006; Ehrenberg, 2006;Ehrenberg et al., 2006, 2007; Rahimpour-Bonab 2007; Ehrenberg et al., 2009; Maurer et al., 2009; Rahimpour-Bonab et al., 2009).
The importance of these diagenetic processes on reservoir heterogeneity is widely recognized but the precise impact of dolomitization and anhydrite precipitation on poroperm evolution is a matter of debate (e.g. Warren, 2000; Lucia, 2004; Machel, 2004; Warren, 2006; Esrafili-Dizaji and Rahimpour-Bonab, 2009).
In most cases, dolomite porosity is inherited from the precursor limestone but is frequently reduced due to cement precipitation (Lucia, 2004), although instances are known in which dolostones have higher porosities than the precursor limestones (Machel, 2004). Dolomitization can, therefore, have a variable effect on poroperm values (Purser et al., 1994; Machel, 2004) and can enhance or reduce porosity depending on the mode and timing of the dolomitization process (Mazzullo, 1992).
Dolomitization is frequently associated with anhydrite precipitation, a process that commonly has a negative effect on reservoir quality. However, Lucia and Ruppel (1996) and Lucia (1999) noted that replacive nodular and poikilotopic anhydrite had only a minor influence on poroperm values in the South Cowden carbonates (West Texas), and Lucia et al. (2004) showed that anhydrite fabrics can enhance reservoir quality.
Thus the combined effects of dolomitization and associated anhydrite precipitation on reservoir quality are complex and variable. Permo-Triassic carbonates (“Khuff equivalent”) form the most important producing reservoir in the Persian Gulf and surrounding area (Ehrenberg et al., 2007; Bordenave, 2008). More than 25 non-associated gas reservoirs producing from this interval are located on- and offshore Iran.
The South Pars gasfield (Table 1) was discovered in 1990 and mainly produces from the Upper Dalan Member and Kangan Formation (lateral equivalents of the Upper Khuff Formation) on a north-plunging anticline in the Qatar Arch (Fig. 1). South Pars together with its extension in Qatar (North field) forms the largest natural gas accumulation known.
Fig. 1. Location map of the South Pars and North Dome fields where the world’s largest gas accumulation is present in the Upper Dalan Member – Kangan Formation. The location of 15 exploration/appraisal wells in the South Pars field is shown. Data from ten of these wells is used in this paper.
Fig. 2. Sediments thicknesses in the subsurface of the Qatar Arch and Western and Eastern sub-basins, as estimated from regional isopach maps. Permian to Oligo-Miocene data are from Bahroudi and Talbot (2003), and Silurian data is from Bordenave (2008). The map above shows the boundaries of the Infracambrian Hormuz salt basin (in white).
Previous studies have shown that carbonate reservoir rocks can be extremely heterogeneous in nature (e.g. Ehrenberg, 2006; Rahimpour-Bonab, 2007; Rahimpour-Bonab et al., 2009; Esrafili-Dizaji and Rahimpour-Bonab, 2009).
More than 60% of the reservoir rocks at South Pars are dolomitized and anhydrite occurs in close association with the Dolomites, reflecting the influence of hypersaline depositional conditions on both calcium sulphate precipitation and dolomitization (Ehrenberg, 2006). The purpose of this paper is to investigate the origin and nature of dolomitization and anhydrite precipitation of the reservoir carbonates at South Pars and to assess their impact on reservoir quality.
LOCATION AND REGIONAL GEOLOGY
The Qatar Arch, an NNE-SSW-trending positive tectonic feature of Infracambrian origin, divides the Persian Gulf into two troughs (the eastern and the western Hormuz Salt sub-basins) (Fig. 2).
The structure of the arch was inherited from the Amar and Najd tectonic systems (Al-Husseini, 2000). Both the Arch and the adjacent troughs have been rejuvenated and uplifted repeatedly since the Early Silurian (Murris, 1980; Alsharhan and Nairn, 1997; Konert et al., 2001; Pollastro, 2003) (Fig. 2).
The troughs had different subsidence rates and depositional histories during the Phanerozoic. Sediments tend to thicken east- and westwards, away from the Qatar Arch. Sediments in the western subbasin are 6.7 km thick, those in the eastern sub-basin are about 5.2 km thick (Fig. 2). At the present day, the eastern flank of the Qatar Arch forms a gently dipping monocline. The western side is bounded by faults (Kazerun Fault) and a series of steep-sided anticlines (e.g. Konert et al, 2001; Alsharhan and Nairn, 1997; Ziegler, 2001).
Tectonic movements during the Late Precambrian–Early Cambrian (Najd fault system) in central Saudi Arabia caused reactivation of pre-existing fault systems resulting in regional uplift and may have gently elevated structural features including the Qatar Arch (Al-Husseini, 2000; Murris, 1980). Both sub-basins were rejuvenated during the Silurian, resulting in the deposition of thin source-rock intervals (Bordenave, 2008).
Thinning of Permian sediments may indicate the existence of a syn-depositional structural high to the SE of the Zagros, or a block-faulted horst in the Qatar Arch area (Edgell, 1977; Kashfi, 1992). PostPalaeozoic tectonic activity may have revived this structural high, as indicated by the erosion of Late Triassic units (Murris, 1980; Kashfi, 1992).
The Arch was a positive structure during the Palaeozoic and gradually subsided during the Jurassic (Saint-Marc, 1978). It was then active periodically throughout the Mesozoic and Cenozoic, including the late Tertiary when sediments currently exposed at the surface were deposited (Konert, 2001; Alsharhan and Nairn, 1997).
The relatively small thickness of the sedimentary cover in the Qatar Arch (some 4 km) compared to adjacent areas (i.e. 7 to 14 km in the Zagros fold belt) indicates that it has been a palaeohigh during most of the Phanerozoic. The stratigraphic succession in the subsurface of the Qatar Arch is well-documented. Analogous rocks crop out in the Zagros Mountains, Central Arabian Arch, and the central and northern Oman Mountains (Sharland et al., 2001; Alsharhan and Nairn, 1997). Most of the subsurface succession was deposited in shallow-marine conditions (limestones, dolomites, shales, and evaporites) (Fig. 3).
Fig. 3. Generalized stratigraphy of the South Pars field showing lithostratigraphic units and depositional environments. The thickness of the stratigraphic column is approximately 4 km; it is dominated by shallow-marine limestones, dolomites, shales, and evaporites. Megasequences I-XI are after Alavi (2004, not to scale). There is uncertainty about the presence of Cambro-Ordovician and Silurian sediments in the Qatar Arch. The stratigraphic position of the Upper Dalan and Kangan carbonates, the focus of this paper, is hghlighted.