Journal of Petroleum Science and Engineering 141 (2016) 90–113

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Journal of Petroleum Science and Engineering

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Sedimentological and diagenetic controls on reservoir properties in the

Permian–Triassic successions of Western Persian Gulf, Southern Iran
Javad Abdolmaleki a,n, Vahid Tavakoli a, Ashkan Asadi-Eskandar b
a
School of Geology, College of Science, University of Tehran, Tehran, Iran
b
Pars Oil and Gas Company (POGC), Tehran, Iran

art ic l e i nf o a b s t r a c t

Article history: The Permian–Triassic intervals in the Arabian Plate, namely the Khuff Formation and its equivalents,
Received 29 June 2015 Dalan and Kangan formations, are among the most important gas reserves in the word. In Iran, they were
Received in revised form the subject of several studies, which mainly focused on the supergiant South Pars Field. This study
11 January 2016
provides informations on the facies characteristics, diagenetic features and reservoir properties of these
Accepted 15 January 2016
intervals in the western Persian Gulf (Golshan Field). As well, characteristics of the lower Dalan Member
Available online 21 January 2016
were evaluated for the first time from the Iranian sector of the Persian Gulf. Results of this study revealed
Keywords: the combined effects of depositional processes and diagenetic overprints on the reservoir quality of these
Reservoir characteristic intervals. Among the various diagenetic processes, anhydrite and calcite cementation has intense de-
Permian–Triassic
structive effects on the reservoir quality of precursor porous facies. On the other hand, dolomitization has
Lower Dalan Member
different effects on the reservoir quality. It has a positive effect, if it is not concomitant with over-
Western Persian Gulf
Golshan field dolomitization and anhydrite cementation. Results of this study reveal that there is a close relationship
South Pars between the depositional facies and diagenetic processes in the Dalan–Kangan sequences. Therefore,
different reservoir/non-reservoir units of these intervals can be traced in a sequence stratigraphic fra-
mework. In order to constructing a sequence stratigraphic model, as a basis for geological reservoir
model, various orders of sequences (third to fifth/sixth) were differentiated and correlated across the
giant Golshan Field and supergiant South Pars Field.
& 2016 Elsevier B.V. All rights reserved.

1. Introduction properties of these intervals were also investigated in some stu-

dies (e.g., Rahimpour-Bonab, 2007; Esrafilli-Dizaji and Rahimpour-
The Middle Permian–Early Triassic Dalan and Kangan forma- Bonab, 2009; Rahimpour-Bonab et al., 2010; Tavakoli et al., 2010).
tions and their time-equivalent (Khuff Formation) are the most Moreover, Insalaco et al. (2006) established the sequence strati-
important gas reservoirs in the world (Insalaco et al., 2006; Eh- graphic framework of these successions throughout the Zagros
renberg et al., 2007; Tavakoli et al., 2010; Asadi-eskandar et al., area. Also, reservoir governing factors of the Early Triassic Kangan
2013). Since the 1948, discovery of prolific gas reservoirs in Bah- Formation were carried out in the eastern parts of the South Pars
rain, the Permian–Triassic successions are the main targets for Gas Field (Moradpour et al., 2008; Peyravi et al., 2010).
natural gas exploration, which led to discovering of several giant In this study, for the first time, the Permian–Triassic sequences
and supergiant gas fields in the Persian Gulf (Bashari, 2005). of the western Persian Gulf (Golshan Field) have been focused in
The most studies of the Permian–Triassic successions in the view of their facies properties, diagenetic evolution and reservoir
Iranian parts of the Persian Gulf are reported from the supergiant characteristics (Fig. 1). Also, the characteristics of less known
South Pars Gas Field (Fig. 1). Depositional environments and facies Lower Dalan Member (Lower Khuff) have been explored in the
characteristics have been studied by some researchers (e.g., In- Persian Gulf and correlated with the adjacent areas in the Arabian
salaco et al., 2006; Esrafilli-Dizaji and Rahimpour-Bonab, 2009; territories.
Rahimpour-Bonab et al., 2009; Asadi-eskandar et al., 2013). The For evaluation of factors governing the reservoir characteristics
controls of depositional and diagenetic processes on the reservoir of these sequences (Permian–Triassic in age) in the selected field,
results of facies and diagenesis studies are combined with re-
n servoir characterization analysis. To construct a geological-based
Corresponding author.
E-mail addresses: J.maleki3@ut.ac.ir, J.maleki3@yahoo.com (J. Abdolmaleki), reservoir model and prediction of reservoir quality variation
vtavakoli@ut.ac.ir (V. Tavakoli). among the studied wells, sequence stratigraphic framework of the

http://dx.doi.org/10.1016/j.petrol.2016.01.020
0920-4105/& 2016 Elsevier B.V. All rights reserved.
 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113 91

Fig. 1. Location map of the studied area and the South Pars gas field in the Persian Gulf, southern Iran.

Dalan–Kangan formations is established and used as a basis for equivalents on the Arabian Plate started in the Mid–Permian,
correlation of reservoir and non-reservoir units, in the field scale. which accompanied by the detachment of Cimmerian terranes
To evaluate the larger scale variations in reservoir quality, the from the Pangea supercontinent and deposited as post-rift cover
various orders of determined sequences were correlated with su- on a passive continental margin of the newly formed Neo-Tethys
pergiant South Pars Field. The constructed sequence stratigraphic Ocean to the Early Triassic (Konert et al., 2001). During this time,
model can be used for future regional investigations. the palaeoclimate is interpreted as transitional from icehouse to
greenhouse along with sea-level oscillations of moderate wave-
length and amplitude (Al-Jallal, 1995; Sharland et al., 2001). The
2. Geological setting and stratigraphy climate regime was probably similar to the arid conditions, which
is closely comparable to the present-day Persian Gulf carbonate
The Permian–Triassic carbonate platforms of the Arabian Plate system (Alsharhan and Kendall, 2003).
can be considered as a flat epeiric ramp extending for more than The Dalan and Kangan formations (Khuff Formation) are often
2500 km in SE-NW strike-direction and more than 1500 km in separated into two upper and lower members by the widespread
SW–NE dip-direction that led to the formation of a layer-cake type middle anhydrite or Nar member (Fig. 2; Szabo and Kheradpir,
platform with certain m-scale marker beds traceable hundreds of 1978; Alsharhan, 2006), which regionally separated the strata into
km across the Plate (e.g. Aigner and Dott, 1990; Al-Jallal, 1995; two different hydraulic regimes (Alsharhan, 2006). The Kangan
Insalaco et al., 2006; Esrafilli-Dizaji and Rahimpour-Bonab, 2009; and Dalan formations that unconformably overlie the non-marine
Tavakoli et al., 2010). Deposition of the Khuff Formation and its or extremely shallow marine clastic of the lower Permian or older
92 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113

Fig. 2. Chronostratigraphic and sequence stratigraphic subdivisions of the Dalan and Kangan Formations (Khuff Formation) in the Arabian plate and Persian Gulf area
(compiled from Strohmenger et al., 2002; Sharland et al., 2001, 2004; Insalaco et al., 2006; Koehrer et al., 2010). The studied intervals are shown by red and blue bars. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

strata in the most of the Arabian Plate are capped by the im- (e.g. Buxton and Pedley, 1989, Wilson, 1975, Flugel, 2004) and to
permeable anhydrite and shale successions of the Triassic Dashtak describe facies, a modified Dunham (1962) texture scheme (Embry
Formation and its equivalent Sudair Formation (Szabo and Kher- and Klovan, 1971) was used. These data were integrated with se-
adpir, 1978; Bashari, 2005). The organic-rich Silurian Hot Shale quence stratigraphic and reservoir characterization analyses to
(Sarchahan Fm. and/or Qusaiba Member) is considered the hy- investigate depositional and diagenetic controls on the distribu-
drocarbon source rock for these reservoirs (Bordenave, 2008). tion of reservoir (flow) units and non-reservoir units in these lu-
crative formations.

3. Materials and methods

4. Facies analysis and depositional environment
A systematic geological based reservoir characterization study
applied on the Permian–Triassic gas reservoirs of the Persian Gulf Detailed macro- (on cores, slabs and plug samples) and mi-
to elaborate on factors governing reservoir quality in these lucra- croscopic (thin section and SEM) petrographic analyses, have led
tive sequences. To achieve this goal, petrographic analysis of core to identification of 17 facies types (Figs. 3 and 4), which are
samples and thin sections from two giant and super-giant fields of grouped into six facies associations (Table 1). Depositional en-
the Persian Gulf (i.e. Golshan and South Pars gas fields) were used vironments of facies range from the outer ramp to inner ramp and
together with other data including the petrophysical wire logs and sabkha settings. However, the most of recognized facies belong to
routine core analysis. In this way, more than 470 m of cores and the inner ramp depositional settings. In order to recognition of
1300 thin sections from the South Pars Field (one well) and 850 m depositional settings of facies associations and reconstruction of
of cores and more than 3400 half-stained thin sections, conven- palaeoenvironmental condition, standard models of ancient and
tional well logs and poroperm data from the Golshan Field (wells recent carbonate-evaporate environments were used as analogs
A and B) were used. In addition, several samples were analyzed by (Purser, 1973; Wilson, 1975; Shinn, 1986; Burchette and Wright,
SEM technique to determine their mineralogical composition, 1992; Selley, 1996; Alsharhan and Kendall, 2003; Flugel, 2004;
textural relationships and pore characteristics. Warren, 2006). The sedimentological characteristics and some
High resolution petrographic analyses together with image aspects of reservoir properties of determined facies of the Dalan–
analyses and quantitative analysis of rock components were used Kangan formations are summarized as follows. The effects of di-
to determine the depositional facies (and microfacies) and diage- agenetic overprints on the primary reservoir quality of these facies
netic features of the Dalan–Kangan formations. Facies analysis is will be evaluated in the ongoing parts.
carried out using standard models and microfacies descriptions F1) Massive and clean anhydrite: The facies is a clean and tight
 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113 93

Fig. 3. Photomicrographs and core photos from detected facies in the Dalan–Kangan successions. (A, B) Massive and clean anhydrite; (C–E) Laminated or aligned dirty
anhydrite; (F, G) Fenestral dolomudstone; note to mud cracks and anhydrite filled fenestral pores; (H) Mud/dolomudstone; (I, J) Nodular mud/dolomudstone;
(K) Stromatolite boundstone; (L) Bioclast-peloid mud/wackestone.

anhydrite free of any impurity such as organic matters (Fig. 3a and aligned anhydrite nodules are residual selenite gypsum previously
b). The dominant fabrics of this facies are chicken-wire and en- reported from the Khuff Formation and main concentration of this
terolithic, and it is interpreted to have deposited in a sabkha en- facies was within the local pond and temporal hypersaline lagoons
vironment (Kendall and Skipwith, 1969; Warren, 2006). developed during the lower order regressive stages occurred in a
F2) Laminated or/and aligned dirty anhydrite: This anhydritic higher order transgressive phase (see Warren, 2006).
facies has a dirty appearance due to the presence of impurities F3) Fenestral dolomudstone: This dolomicritic facies is char-
such as organic matters (OM's) and/or dolomicrites. The main acterized with fenestral fabrics and mud cracks is both core slab
macroscopic evidence of this facies is its layered nature with and thin sections (Fig. 3f and g). These features are representing
aligned evaporite crystals (Fig. 3c, d and e). The aligned crystals are supratidal to upper intertidal environments for this facies (Shinn,
commonly separated from each other by OM's and/or dolomicrites 1986, Flugel, 2004).
(Fig. 3c, d and e). The facies has formed in the sabkha settings as F4) Mudstone/dolomudstone: this facies represented by pure
well as hypersaline lagoons and ponds (Table 1). Seemingly, mudstone to dolomudstone, which has dolomite lithology in most
94 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113

Fig. 4. Other examples of detected facies in the Dalan–Kangan succession. (A) Bioclast-peloid wacke/packstone, (B) Bioclast-oncoid-peloid grainstone, (C) Peloid/Ooid
grainstone, (D) Ooid grainstone, (E) Coarse intraclast-bioclast grainstone, (F) Coarse intraclast wacke/grainstone, (G) Coarse intraclast wacke/grainstone- Bioclast-intraclast
pack/wackestone; note to the hummocky cross-stratification, (H) Bioclast-intraclast pack/wackestone; with score surface, (I) Fossiliferous-bioturbated mudstone/wack-
estone, (J) Thrombolite, (K and L) Shale or shaly mudstone.

cases. Rare bioclasts (lagoonal fauna), disperse bioturbations and source of hypersaline fluids and overdolomitization (the effect and
lamination are the other important features (Fig. 3h). Dolomicrite intensity of dolomitization on the reservoir quality will be dis-
recrystallization has increased the low poroperm values of this cussed in the diagenesis section).
facies. F6) Stromatolite boundstone: The facies is marked by its obvious
F5) Nodular mudstone/dolomudstone: This facies contained casts lamination, which is recognizable on both core slab and in the thin
and nodules of anhydrite and often has dolomitic lithology (Fig. 3i sections (Fig. 3k). In the most cases, this facies is interpreted to be
and j) that is deposited in hypersaline lagoon and local ponds deposited in intertidal setting (Esrafilli-Dizaji and Rahimpour-Bo-
(Warren, 2006). The low reservoir quality of this facies (with cal- nab, 2009), but in the Triassic strata and especially near to the
citic lithology) was improved by diagenetic processes. In the in- Permian–Triassic boundary this facies can be formed in different
tertidal setting (local ponds), this facies had lower reservoir quality settings as a result of favorable condition after the Permian–
than lagoonal environment that is related to its distance from the Triassic boundary mass extinction (see Kershaw et al., 2012). This
 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113 95

Table 1
The main characteristics of defined facies in the Dalan and Kangan formations.

Facies code Facies name Remarks Depositional environment

F1 Massive and pure anhydrite Clean and light feature, chicken-wire and enterolithic fabrics Supratidal (sabkha)
F2 Laminated or/and aligned Polluted by organic matters, composed of dolomudstone seams, aligned no- Supratidal to intertidal (Salina/Pond/)
anhydrite dules that often their contacts are visible and hypersaline lagoon
F3 Fenestral dolomudstone Dolomudstone with fenestral fabric and mud crack Supratidal to upper intertidal
F4 Mudstone/dolomudstone Bioturbated or laminated (Hypersaline) Lagoon
F5 Nodular mudstone/ With anhydrite nodule Supratidal to intertidal (Salina/Pond)
dolomudstone and hypersaline lagoon
F6 Stromatolite boundstone Lamination Lower intertidal
F7 Bioclast-peloid mudstone/ Often bioturbated Low energy lagoon
wackestone
F8 Bioclast-peloid wackestone/ Often bioturbated and contained micritized grains Higher energy lagoon
packstone
F9 Bioclast-oncoid-peloid Some micritized grains with lagoonal fauna Lower intertidal (Landward intertidal)
grainstone Shoal complex (leeward)
F10 Peloid-ooid grainstone Often highly dolomitized and cemented by anhydrite Lower intertidal (Landward intertidal)
Shoal complex (leeward)
F11 Ooid grainstone Distinct cross-bedding Shoal complex (central)
F12 Coarse Bioclast-intraclast High isopodous cement, open marine fauna as dominate grains Shoal complex (seaward)
grainstone
F13 Coarse intraclast wackestone/ Contained intraclasts with various sizes (sand to pebble size), texture, shape
packstone and diagenetic features:
A: angular to sub-angular, mictitic/dolomicritic fragments in a anhydritic ce- A: Supratidal to upper intertidal
ment (or matrix) with brecciated texture, associated with exposure surfaces
such as fenestral fabrics and mud cracks, upper intertidal to supratidal
B: sub-rounded to rounded, in association with hummocky cross stratification B: Mid ramp (between FWWB and
and other storm related facies (especially in mid-ramp setting) SWB)
C: angular to rounded unusual intraclast. This facies is confined to the Kangan C: Inner to outer ramp
Formation.
F14 Bioclast-intraclast wackestone/ Hummocky cross stratification, alternated with adjacent (off-shoal) facies, Mid ramp (between FWWB and SWB)
packstone mixed by lagoonal and open marine fauna and other reworked grains
F15 Fossiliferous bioturbated mud- Bioturbated, sponge spicule, echinoderm and bryozoan debris Outer ramp (bellow SWB)
stone/wackestone
F16 Thrombolite Clotted fabric with ostracoda and claraia, near to the PTB Lower intertidal to subtidal
F17 Shale or shaly mudstone Dark feature, laminated Lagoon

should be considered in the sequence stratigraphic interpretations. interparticle and moldic pores had led to some improvements in
The facies is partly to wholly dolomitized in most cases and it reservoir quality.
caused of more poroperm values than calcitic lithology. F11) Ooid grainstone: This facies is substantially composed of
F7) Bioclast-peloid mudstone/wackestone: This mud-dominated ooids (Fig. 4d) and, in many cases, could be recognizable as oo-
facies is indicated lagoonal low-energy situation. It is mainly bio- molds and cross beddings on the core slabs. This facies is attrib-
turbated and comprising lagoonal biota (e.g. gastropods; Fig. 3l). uted to a high energy setting such as central and/or seaward shoal.
Although, it has fair primary poroperm values but it can obtain Good reservoir properties of this facies have mainly resulted from
higher values of poroperm (especially permeability) during intense leaching and consequent development of moldic porosity.
dolomitization. In this facies, separated moldic pores have provided porous in-
F8) Bioclast-peloid wackestone/packstone: This facies represents tervals with high porosity and low permeability values. In some
higher energy conditions in the lagoonal setting, where tidal cases, dolomitization and fracturing can enhanced its permeability
channels were developed. It mainly composed of peloids and mi- values.
critized lagoonal biota (e.g., some benthic foraminifera, ostracod, F12) Coarse intraclast-bioclast grainstone: The open marine in-
and gastropods; Fig. 4a). In most cases, the facies has low por- dicators such as bryozoan, brachiopod and echinoderm debris are
operm values. This facies shows higher reservoir quality, where it prominent components in this facies (Fig. 4e). The relative fre-
subsequently underwent dolomitization process. quency of skeletal grains of this facies is higher in the South Pars
F9) Bioclast-oncoid-peloid grainstone: In the Dalan Formation, Field than the Golshan Field in which the intraclasts are pre-
this facies is mainly composed of skeletal debris (e.g., foraminifera, dominant. Interparticle and intraparticle pores have enhanced
ostracod, gastropods, echinoderms bivalves and algae) and non- reservoir quality to some extent. Often, early marine cementation
skeletal grains (e.g., peloids and some intraclasts). But, in the reduced the poroperm values. Dolomitization had not important
Kangan Formation, oncoids are dominant grain types. In the effect on the reservoir properties of this facies.
landward intertidal zone, this facies is mainly affected by dolo- F13) Coarse intraclast wackestone/packstone: This facies is
mitization and anhydrite cementation (Fig. 4b), which have re- composed of intraclasts with various grain sizes (sand to pebble)
sulted in reservoir quality to be increased or decreased, respec- and shapes, which comprises different textures ranges from
tively. Sometimes in the leeward shoal settings, preserved inter- wackestone to packstone. Based upon these features, it can be
particle and vuggy pores have increased the reservoir quality. subdivided into three sub-facies:
F10) Peloid- Ooid grainstone: This facies is mainly composed of Sub-facies- A: This group of intraclastic facies is including the
peloids and fine ooids (Fig. 4c), which are deposited in two dif- angular to sub-angular mictitic/dolomicritic fragments in an an-
ferent depositional settings with various reservoir potentials. In hydritic cement (or matrix) with brecciated texture. Some features
the landward intertidal setting, dolomitization and anhydrite ce- such as fenestral fabrics and mud cracks are also present, which
mentation caused some variations in reservoir properties. As well, are indications of subaerial exposure (Fig. 4f).
in the leeward and central shoal complexes, preservation of Sub-facies- B: The group including intraclasts, which are often
96 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113

sub-rounded to rounded in shape, juxtaposition with hummocky depositional settings. This sub-facies is composed of unusual in-
cross stratification and other storm feature bearing facies. It has traclasts (Wignall and Twitchett, 1999), which have worldwide
been recorded mainly in association with shoal and mid-ramp existence and interpreted as anachronistic facies in the Early
facies. This group is more frequent in the Early Triassic intervals Triassic sequences next to the Permian–Triassic boundary mass
(i.e. storm deposit) (Fig. 4g). extinction (refer to Wignall and Twitchett, 1999; Pruss and Bottjer,
Sub-facies- C: In this group, angular to rounded intraclasts are 2004; Pruss et al., 2005 for more details).
visible in different depositional settings. Facies of this group are In term of reservoir quality, mud dominated textures of this
coinciding with all or several unusual features including changes facies are very tight with low poroperm values. In some cases, due
in cementation rate in underlying and overlying facies. In more to the dolomitization and fracturing, the facies shows higher re-
cases, hosted facies show intense cementation, coeval with bioe- servoir properties. In the shoal facies, the F13-B has fair inter-
vent and also erosional and/or dissolved surfaces (truncation particle porosity values.
bottom surfaces in some cases) in the early Triassic Kangan For- F14) Bioclast-intraclast packstone/wackestone: This facies is
mation. Facies of this group also show evidence of high energy marked by a mixture of shallow (such as gastropod, ostracod and

Fig. 5. Two conceptual depositional models are presented for the Permian–Triassic Dalan and Kangan Formations. A: A 3D model with relative position of the defined facie.
B: Distribution of detected facies, main diagenetic processes and reservoir quality across the reconstructed depositional environment. Generally, from the landward setting
towards open marine, the intensity of dolomitization is decreased. Dissolution and cemenation processes are dominant in the seaward and landward intertidal. In general,
the best reservoir quality is mainly related to the seaward intertidal (shoal complex) and to some extent in the landward intertidal.
 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113 97

Fig. 6. Main detected diagenetic processes in the Dalan–Kangan interval. (A) ooid grainstone with isopachous (bladed) rim cementation and good poroperm, (B) highly
cemented bioclastic facies with isopachous (marine) and equant/dog teeth calcite cementation that led to very low permeability spite of good porosity, (C) a completely
plugged facies with various calcite cements including isopachous, equant/dog teeth and blocky, (D) a close view of dolomitized rim cement, (E) pore filling anhydrite
cementation and low poroperm values, (F) SEM image of a calcite crystal that affected by hypersaline fluids, note to fine dolo-crystals and anhydrite cement.

foraminifera) and open marine biota (such as echinoderms and Kangan formations, there are several meaningful relationships
brachiopods) together with scored or erosional surfaces, storm between the facies associations, their depositional settings and
deposits and hummocky cross stratification (Fig. 4g and h). Due to diagenetic alterations (e.g., Rahimpour-Bonab et al., 2010; Tavakoli
the mud-dominated nature and lesser influence of dolomitization, et al., 2010; Asadi-eskandar et al., 2013). This has led to reservoir
this facies has low reservoir quality. quality trends to be predictable in the sequence stratigraphic fra-
F15) Fossiliferous bioturbated mudstone/wackestone: this facies is mework (Ahr, 2008; Slatt, 2006; Asadi-eskandar et al., 2013). The
composed of fine bioclasts including brachiopods, sponge spicules, main diagenetic alterations and their controls on the reservoir
echinoderms and bryozoan (Fig. 4i) in which bioturbation is pre- properties of the Permian–Triassic sequences are elaborated in
sent in form of burrow fills. It is interpreted to have deposited in details and presented as follow.
the outer ramp depositional setting. This facies has similar re-
servoir properties as the latter facies (F14).
5.1. Cementation
F16) Thrombolytic facies: This facies shows clotted fabric (Fig. 4j)
and is limited to a single stratigraphic position. It was deposited
This is one of the most important diagenetic features affected
just above the Permian–Triassic boundary and composed of fine
the reservoir quality in the Dalan and Kangan formations. Miner-
bioclasts such as ostracods and claraia bivalves. Based on accom-
alogically, there are two types of cements in the studied intervals,
panied facies, it is interpreted to have occurred in the intertidal to
which are including carbonate and anhydrite cements. The ce-
subtidal lagoonal settings. The thrombolytic interbeds have low
ments show several fabrics, which can be formed during the dif-
poroperm values and can be acted as intra-formational barriers
ferent stages of diagenetic history. Most of these cements are re-
(Insalaco et al., 2006; Rahimpour-Bonab et al., 2009), which are
correlatable in large scale (Insalaco et al., 2006). However, frac- lated to the syn-depositional to shallow burial environments (see
turing can increase the permeability of this facies, which has re- Rahimpour-Bonab et al., 2010; Tavakoli et al., 2010). Here, a sum-
corded in some samples of the studied intervals. marized description of these is presented and their controls on
F17) Shale or shaly mudstone: This laminated and dark colored reservoir quality are elaborated in details. Cement's classification is
facies is limited to the upper parts of the Kangan Formation. Based based on their effects on reservoir parameters (i.e. poroperm
on the facies characteristics (i.e. absence of deep marine fauna and values).
mud-dominated fabric) and facies association, the facies represents
shallow marine and low-energy lagoonal setting (Fig. 4k and l). 5.1.1. Rim (isopachous and bladed) cement
Results of our observations combined with previous studies In most cases, these types of cements have aragonitic mineralogy,
show that the overall depositional model for the Dalan and Kan- which formed in the high energy settings such as shoal complexes,
gan formations is that of a homoclinal carbonate ramp (Fig. 5; e.g., especially their seaward parts (Flugel, 2004). Subsequently, this pri-
Insalaco et al., 2006; Esrafilli-Dizaji and Rahimpour-Bonab, 2009). mary aragonitic mineralogy is replaced by calcite and dolomite during
diagenesis. Although the rim cements have minor effects on the total
porosity values (Fig. 6a and b), but they can resulted in poroperm
5. Diagenetic processes and reservoir quality reduction in conjunction with other types of cements (Fig. 6c). Dis-
solution of grains during meteoric diagenesis, left these rim cements
In carbonate reservoirs, diagenesis has important control on and isolated molds, which can be subsequently linked together and
the primary poroperm distribution and modify the pore networks provided touching vugs (with high poroperm values) during the dis-
(e.g., Lucia, 2007; Ahr, 2008; Tavakoli et al., 2010). In the Dalan and solution and/or dolomitization (Fig. 6d).
98 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113

5.1.2. Interparticle cements resulted in extensive dolomitization and anhydrite cementation

These cements are recognizable in the studied intervals with (Fig. 6e). In the earlier stages of percolation, hypersaline brines
various mineralogy (e.g., calcite, and anhydrite) and fabrics (e.g., with the high corrosivity can dissolved calcite crystals and re-
poikilotopic anhydrite, blocky and equant calcite). placed it with anhydritic cements (Fig. 6f). The facies with such
In many cases, their primary calcitic mineralogy changed to the cements are commonly grain supported facies that formed in the
dolomite during diagenesis. The equant and blocky calcite cements landward settings, close to the brine's source.
occupied the inter-grain spaces in most of the high energy facies.
They can also form in the latter stages of diagenesis in the meteoric 5.2. Dissolution
and/or burial realms (Flugel, 2004). There is also a close relationship
between the anhydrite cementation and dolomitic intervals. They This diagenetic process took place during the meteoric diag-
are more frequent in the grain dominated facies developed in the enesis and mainly affected the unstable (aragonitic) grains in-
landward intertidal areas; close to the source of hypersaline fluid. cluding ooids (Fig. 7a–d) and bioclasts (Fig. 7e and f). As a con-
These will be discussed in details in the following parts. sequence, moldic pores formed during the meteoric dissolution
have improved the reservoir storage capacity. The shallow de-
5.1.3. Intraparticle cements positional settings of the Permian–Triassic successions, especially
These types of cements are mostly anhydritic but calcitic ones the Dalan Formation, together with low oscillations of sea level
are also recorded. They are occupied the moldic pore spaces and had led to short-term subaerial exposures of shoal complexes. In
annihilated the reservoir quality in this way (Fig. 6c and e). During many cases, dissolution of unstable grains left moldic pores in
the meteoric diagenesis, unstable allochems, such as aragonitic form of oo-moldic (very common) and bio-moldic (in some cases)
ooids, were dissolved. Then, in the shallow burial realm, seepage- porosities. Such pore spaces have resulted in high porosity values.
reflux brines percolated trough the unconsolidated sediments and But, in the absence of dolomitization and fracturing, they have

Fig. 7. Examples of highly porous facies due to dissolved aragonitic allochems (ooid and bioclast). (A–D) dissolved ooids and oo-molds, (E–H) dissolved primary aragonoitic
bioclasts.
 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113 99

commonly low permeability. sequences of the Persian Gulf.

The mentioned gradients in dolomitization have various effects
5.3. Dolomitization on poroperm distribution within the different depositional facies,
especially in mud-dominated facies that inherited low reservoir
Petrographic and petrophysical investigations revealed that potential from their depositional settings. Based on the petro-
dolomitization has a major role on the reservoir characteristics of graphic evidence and poroperm measurements, the most im-
Permian–Triassic sequences in the studied fields. Considerable portant factor governing reservoir quality in the dolomitized mud
parts of the studied intervals are partly to completely dolomitized, dominated facies of the Dalan–Kangan formations seems to be
depending on the facies associations and depositional settings. dolomite crystals sizes. Conversely, in the grain dominated facies,
Depending on several factors including the model and timing of this role is minor.
dolomitization, it can variably alter the poroperm distribution in Although, in some intervals, dolomitization has no significant
the carbonate sequences (Mazzullo, 1994; Purser et al., 1994). In direct effect on reservoir quality enhancement, but it can provided
the Dalan–Kangan formations and their equivalents in the neigh- a suitable background (brittle lithology) for subsequent events
boring areas (i.e. Khuff Formation), dolomitization and anhydrite such as fracturing that can increase reservoir quality. Here, the
precipitation can be explained in terms of the sabkha and seepage- main controls of dolomitization on the reservoir characteristics of
reflux models (Alsharhan, 2006; Rahimpour-Bonab et al., 2010; the Dalan–Kangan formations are discussed.
Tavakoli et al., 2010). In these models, the hypersaline fluids are
mainly considered to be originated from the sabkha (for sabkha- 5.3.1. Direct effect of dolomitization on reservoir quality
type dolomitization model) and, to some extent, hypersaline la-
goons/ponds located within the supratidal to subtidal settings (for – Grain dominated facies
seepage-reflux model) (Saller and Henderson, 1998; Purser et al., In most parts of the studied intervals, the landward intertidal
1994; Warren, 2006). Therefore, it is expected for the density of grain dominated facies (close to source of hypersaline fluid)
hypersaline fluids to be reduced with increasing distance from the endured higher degrees of dolomitization and anhydrite ce-
source in both vertical and horizontal directions (Saller and Hen- mentation than their neighboring seaward shoal complexes. On
derson, 1998; Warren, 2006). Such decreasing trend in the density the other hand, limestone facies with moldic porosity have
of hypersaline fluids has led to the lower intensity of dolomitiza- lower permeability values (Figs. 6b and 7a and b) than the
tion in a similar trend (i.e. away from the origin of hypersaline dolomitized facies in which dolomitization affected inter-molds
fluids). This is accompanied by the remarkable changes in textural calcitic cements and enhanced the permeability values (Fig. 6d).
characteristics of dolomite crystals (i.e. crystal sizes and clearness). Seemingly, in the grain dominated facies, dolomitization has
Away from the source, dolomite crystal's sizes have increased and improved reservoir quality where it was not accompanied by
they become clearer due to the decrease in the numbers of anhydrite cementation.
available nuclei and impurities along with the lower saturation of – Mud dominated facies
hypersaline waters (Saller and Henderson, 1998). The similar
trends are visible in the studied intervals of the Permian–Triassic Mud-dominated facies of the studied intervals show various

Fig. 8. Un-dolomitized and dolomitized mud-dominated facies and their pore types (upper row: polarizing microscope photomicrographs; lower row: SEM photos). (A and
D); un-dolomitized (wholly calcitic) facies with no visible porosity. (B–F); dolomitized facies with intercrystalline pores. Partly dolomitized and recrystallized facies have
higher reservoir properties (i.e. F, C).
100 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113

intensities of dolomitization (Fig. 8). They are recorded as un-do- in reservoir quality has occurred when the dolomitizing fluids do
lomitized to over-dolomitized facies in which the reservoir prop- not enter the additional ions (i.e. Mg2 þ ) into the system. In the
erties are considerably different, ranging from low- to relatively absence of diagenetic processes that improved the reservoir
high reservoir quality facies. These variations can be explained as quality, the over-dolomitization of facies during the earlier phases
follows: of brine percolation has resulted in the reservoir quality to be
In the earlier stages of brine's percolation into the shallow- decreased (Warren, 2006). Away from the source, decrease in the
buried sediments, higher density of brine has resulted in intense brine's density has resulted in lower intensities of dolomitization.
dolomitization (over-dolomitization). Away from the brine's As a consequence, the dolomite crystals become larger in size and
source, the density of brine and, consequently, the intensity of the inter-crystalline pore spaces have increased (see Fig. 8).
dolomitization have decreased (Warren, 2006; Saller and Hen-
derson, 1998). The dolomite crystals have higher density than the 5.3.2. Indirect effect of dolomitization on reservoir quality
calcite. Therefore, dolomitization of calcitic facies has resulted in In some cases, dolomitization had no important effect on the
the generation of some spaces, which can provide reservoir units reservoir quality of hosting facies, but it provided a brittle lithol-
with high poroperm vales. Also, the crystal's growth during diag- ogy for subsequent fracturing and controls the reservoir properties
enesis can provide a resistant porous and permeable framework in this way. The preferential fracturing of dolomite lithology than
within the dolomitized intervals (see Saller, 2004). Such increase the limestones is related to its higher crystallinity and brittle

Fig. 9. A- Thickness ratio chart of fractured and un-fractured intervals in the studied well. This shows higher frequency of fractures in the dolomitized intervals. B- FMI log,
lithology column and poroperm values from a 60 m thick fractured zone within the studied interval. Most of the selected interval is from the mud-dominated facies with low
primary poroperm values. As shown, fractures with lower aperture sizes are mostly occurred within the calcitic intervals and have a negligible effect on the permeability. A
reverse trend is observed within the dolomitized intervals that has increased the permeability. D- The percentages of the fractured and un-fractured intervals within the
various depositional sequences of the studied wells. As shown, the total thickness of fractured interval is higher in the well-B than the other studied well.
 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113 101

Fig. 10. Porosity vs. permeability diagram of mud-dominated facies in the studied interval and distinguished areas based on the poroperm relationships.
102 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113

nature (see Purser et al., 1994; Aguilera, 1980, Ortega et al., 2010). dolomitic parts. On the other hand, fracture's frequencies are
In the studied formations, fractures are more frequent in the do- considerably different among the studied wells. As shown on
lomitic intervals. Also, they have larger aperture sizes in the do- Fig. 9C, the fractures are more frequent in well-B than the well-A.
lomitic parts. In dolomites, fractures have a greater chance of Because of the studied filed is located on a salt dome, Therefore,
being preserved than the limestones. This can be explained by a such differences can be attributed to the different locations of
concept known as “Emergent Threshold” (Lonergan et al., 2007). selected wells on this dome. However, detailed analysis of frac-
In a wide range of lithological units with synkinematic ce- tures and their effects on reservoir quality of the Dalan–Kangan
mentation, there is a threshold value of the kinematic fracture formations needs to be more explored using a complete dataset
opening (separation between two previously adjacent points that can be the subject of future studies.
across the fracture regardless of later mineral filling) above which Also, FMI logs from the Khuff Formation in Abu Dhabi reveal
fracture porosity is preserved and below which fractures are that nearly 81% of the open fractures are in dolomitic intervals, 18%
completely filled. This fracture aperture size has been termed the in limestone units and only, 1% of all open fractures in anhydrite
emergent threshold (Laubach, 2003). If cementation rate exceeds cemented intervals (Loutfi and Abul Hamd, 1989).
opening rate, the fractures are expected to be sealed. The emer- The other positive effect of dolomitization on reservoir quality
gent threshold for limestones is about 1 cm but in dolostones it is of carbonate sequences is its higher capability of preservation
about 1 mm (Olson et al., 2007). Therefore, it is proved that similar primary porosity during the burial. In this view, dolomitized facies
aperture fractures of a given set of fractures are more likely to have are more resistant than the limestones against the burial com-
preserved in dolomites than the limestones, because their lower paction and subsequent porosity reduction (Schmoker and Halley,
emergent threshold (see Lonergan et al., 2007 for more details). 1982; Purser et al., 1994).
A similar trend is also recorded in the Dalan and Kangan for-
mations. To understand relationship between the fractures and
lithology, the FMI (Formation Micro Imager) log of a 60 m-thick 6. Depositional facies and reservoir quality
fractured interval from the studied formations is presented along
with poroperm measurements of this interval (Fig. 9). To get a In this part, reservoir properties of depositional facies of the
better understand about the fractures and their effects on per- Dalan–Kangan formations will be evaluated in the studied wells of
meability, most of the selected intervals are from the mud-domi- the Golshan Field, in western Persian Gulf. In addition, impact of
nated facies with low primary poroperm values. In those intervals diagenetic alterations on primary reservoir quality of these facies
of calcite mineralogy, the fractures have lower aperture sizes and, will be considered. As shown in Figs. 10–12, it is possible to classify
consequently, show lower permeability measurements (Fig. 9B). depositional facies of the studied intervals into three and some-
On the contrary, larger aperture size of fractures in the dolomitic times four distinct groups based upon their reservoir properties.
intervals has resulted in higher permeability values (Fig. 9B). Ap- These reservoir facies groups (RFG's) have gradual boundaries and
parently, high density closed (cemented) fractures have provided overlap each other (Figs. 10–12). The following is a brief descrip-
flow barriers in some parts of the studied formations with domi- tion of these groups.
nant calcitic mineralogy. Such phenomenon is not recorded in the RFG1: Facies of this group have low porosity (2–5%) and

Fig. 11. Porosity vs. permeability diagram of grain dominated facies in the studied interval and distinguished areas based on the poroperm relationships.
 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113 103

RFG1: the most abundant facies of this group are highly ce-
mented grain-dominated facies in which inter-grains and intra-
grains calcite cements are predominant diagenetic features.
Moreover, dolomitized grain-dominated facies with frequent an-
hydrite cements are also fall within this reservoir group.
RFG2: Grain-dominated facies with moldic porosity as their
dominant pore-types are the most frequent facies of this group. In
some cases, fracturing causes these moldic pores to be connected
and permeability increases. However, in many cases, intense dis-
solution (mainly meteoric) has increased reservoir quality of these
facies by changing the pore types from moldic to vuggy pores. As
well, dolomitization of rimmed cements can also improve the
permeability, to some extent.
RFG3: In this group, fractured facies with both calcite and do-
lomite lithologies are predominant and un-fractured, dolomitized,
grain-supported facies are subordinate.
RFG4: This group is particular of grain-supported facies. The
Fig. 12. Porosity vs. permeability diagram of subordinate facies in the studied in-
terval and distinguished areas based on the poroperm relationships. common feature in facies of this group is their dominant pore type
that is moldic. Due to this pore type, samples of this group are
placed in a distinct location in the poroperm cross plot (see
permeability (commonly less than 0.1 mD) values. This group Fig. 11). In these facies, the most frequent cement types are inter-
comprises facies with lowest reservoir quality. grain and rimmed cements, respectively. The dolomitized grain-
RFG2: A wide range of poroperm measurements is visible in dominated facies have lesser frequency in this group.
this group. There is a linear relationship between porosity and
permeability in facies of this group. This means that the perme- 6.3. Reservoir quality of subordinate facies
ability increases with increasing porosity.
RFG3: Facies of this group have relatively low (less than 10%) Subordinate facies in the studied intervals of the Dalan–Kangan
porosity values. Porosity values are mostly lower than 2%, but the Formations are including anhydritic facies (F1, F2), microbial facies
permeability has a wide range (0.001–100 mD). (F6, F16) and shale (F17). These facies have low frequency within
RFG4: This group substantially comprises grain dominated fa- the studied successions and have poor reservoir quality in almost
cies. Porosity values are between 10% and 30% and permeability all cases. In some cases, dolomitization and fracturing has im-
measurements vary from 0.1 to nearly 10 mD. proved their reservoir quality, especially within the dolomitized
stromatolitic facies (Fig. 12).
6.1. Reservoir quality of mud-dominated facies

Porosity-permeability cross plot for the mud-dominated facies 7. Reservoir controlling factors in the Dalan–Kangan forma-
of the Dalan–Kangan formations is depicted in Fig. 10. In a general tions in Golshan field
view, it is apparent that the dolomitized facies have better re-
servoir quality than the other non-dolomitized ones. Therefore, it In the previous parts, reservoir quality variations of deposi-
can be concluded that dolomitization was the main factor gov- tional facies of the Dalan–Kangan formations were evaluated by
erning reservoir quality in the mud-dominated facies. These facies using the poroperm cross-plots. As noted, the facies can be clas-
fall within the facies reservoir groups (RFG's) 1–3 introduced sified into two main groups as; mud-dominated and grain-domi-
earlier for the studied formations. Here, the main characteristics of nated. Depending on several factors including facies characteristics
these groups are discussed. (i.e. texture) and diagenetic overprints these two main group show
RFG1: The most frequent facies in this group are non-dolomi- considerable variations in reservoir quality. The main reservoir
tized, mud-dominated facies belonging to the outer ramp and la- controlling factors in the studied intervals of Dalan–Kangan for-
goonal settings (Fig. 10). The other facies of this group are over- mations are shown in Fig. 13. As is evident, these governing factors
dolomitized and fractured limy facies in which fractures are are substantially different in the mud-dominated and grain-
mostly sealed by cementation. dominated facies.
RFG2: Dolomitized mud-dominated facies formed away from The mud-dominated facies with lowest reservoir quality are
the brine's source (see diagenesis part) in lagoonal setting are un-dolomitized to partly dolomitized (Fig. 13) deposited in low
frequent facies within this group. The facies are not over-dolomi- energy settings (i.e. outer ramp and lagoon). Some fractured facies
tized (e.g., Facies 7; Fig. 10). Facies with open micro- fractures are in which the fractures are thoroughly filled by cements (mainly in
also included in this group. limestone parts) have also very low poroperm values. Dolomiti-
RFG3: Dolomitized and fractured mud-dominated facies are the zation (sabkha-type and/or seepage reflux model) and fracturing
most frequent facies in this group. In these facies, fractures are can be considered as the main reservoir controlling factors in the
commonly open with high aperture sizes. The other facies of this mud-dominated facies of Dalan–Kangan formations (Fig. 13).
group are dolomitized and non-fractured facies in which dolomi- In the grain-supported facies, three main scenarios have oc-
tization has resulted in some improvements in reservoir quality. curred. 1- The lowest reservoir quality is recorded in highly ce-
mented in which inter-grain cements are prevailing features
6.2. Reservoir quality of grain-dominated facies (Fig. 13). 2- The facies with abundant moldic pores isolated by
rimmed cements show high porosity values, but their permeability
In Fig. 11, poroperm cross plot is depicted for grain-dominated values are low. In some cases, dolomitization and fracturing have
facies of the Dalan–Kangan intervals in the studied wells. As increased permeability in these facies. 3- Intense (meteoric) dis-
shown, these facies are distributed within all four facies reservoir solution has resulted in the formation of moldic and vuggy pores
groups (RFG's) previously introduced for the studied formations. in high energy shoal facies that provided thick units (up to 10 m)
104 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113

Fig. 13. Important reservoir controlling factors in the studied intervals and their effects on reservoir quality (i.e. poroperm values). Right: mud-dominated facies; Left: gran
dominated facies.

of high reservoir quality in the Permian–Triassic successions of 8.2. Fourth-order sequences

western Persian Gulf, Golshan Field.
These sequences are surrounded by supratidal to upper inter-
tidal facies at the top and the base (Fig. 14). The MFSs are mainly
8. Sequence stratigraphic framework determined by presence of the outer ramp facies (Fig. 14). General
stacking patterns and facies changes and components are the
The lateral distribution of sedimentary facies depends on de- same as third order sequences. Mainly third order sequences are
positional environments while their vertical stacking pattern is composed of one or more fourth order sequences (Fig. 14). These
dictated by sea-level fluctuations and reflected in their sequence sequences are correlatable with detected fourth order sequences
stratigraphic framework (Schlager, 2005). Employing sequence presented by Insalaco et al. (2006).
stratigraphy for 3-D visualization of facies distribution in a re-
servoir body makes it possible to superimpose patterns of diage- 8.3. Third-order sequences
netic overprinting on this framework model (see Rahimpour-Bo-
nab et al., 2012a, 2012b; Asadi-eskandar et al., 2013). Insertion of Regional stratigraphic analyses (Strohmenger et al., 2002; In-
facies and diagenetic features within a sequence stratigraphic salaco et al., 2006; Koehrer et al., 2010, 2012) suggested that the
framework has resulted in the generation of geological-based Upper Khuff strata (equivalents of the upper Dalan and Kangan
models for carbonate reservoirs that can be used in reservoir si- sequences) can be subdivided into four third-order sequences
mulations (Slatt, 2006; Asadi-eskandar et al., 2013). namely KS4 to KS1 (see Fig. 1). However, there are some dis-
In the previous sections, the roles of depositional and diage- crepancies about the ages of sequence stratigraphic surfaces. This
netic processes on reservoir evolutions of the Permian–Triassic study shows that depositional sequences of the upper Dalan and
sequences of the Persian Gulf were discussed. As noted, there are Kangan formations in the selected fields have more harmonization
some meaningful relationships between the depositional facies with sequence stratigraphic schemes presented by Insalaco et al.
and diagenetic alterations in the Dalan–Kangan formations among (2006) and Koehrer et al. (2010). Depositional sequences of the
the studied wells. Thus, they can be studied in a sequence strati- lower Dalan Member show close correlation with KS6 sequence
graphic framework. (Strohmenger et al., 2002). The sequence boundaries of these se-
In this study, we used the T-R sequence stratigraphic procedure quences are mainly indicated by sabkha anhydrites and other su-
(see Embray and Johannessen, 1992; Catuneanu et al., 2011) to pratidal facies with fenestral and mud crack features (Fig. 14). The
define depositional sequences of the upper Dalan and Kangan maximum flooding surfaces are often coinciding with domination
formations. Accordingly, the studied intervals are subdivided into of open marine (off-shoal) facies (Fig. 14). Here, the KS4–KS1 de-
fifth (or sixth), fourth and third-order sequences. However, due to positional sequences and their reservoir properties are detailed in
the importance and performance of third order sequences in re- the Permian–Triassic reservoirs of the Persian Gulf. They can be
servoir zonation and modeling of carbonate successions, they are used as a reliable framework for ongoing reservoir modeling.
fully described for the studied intervals. In order to detection of
lateral facies and lithological changes, lower order sequences are 8.3.1. KS4
investigated. In the Golshan Field, only the HST of this sequence was cored,
which is coincide with HST hemi sequence of a fourth-order se-
8.1. Fifth/sixth-order sequences quence (Figs. 14 and 15). Conversely, it is completely cored in the
wells drilled in the South Pars Field (Fig. 14). This systems tract is
In the previous studies, lower order sequences (fifth and sixth composed of one fourth-order HST, two complete and one fifth/
order) are not discussed in the Iranian parts of the Permian– Sixth order hemi sequence (Fig. 15). In the studied wells, this
Triassic strata. In the Arabian part of this platform, it is suggested systems tract starts with outer ramp or off-shoal facies (MFS) and
that the 5th order sequences are probably record a 100,000 year the maximum drop in the relative sea level (SB) is marked by the
and are represent Milankovitch signals (Koehrer et al., 2010). presence of chicken wire anhydrites (type-1 SB). This third-order
These sequences are composed of several shoaling upward sequence contains a large volume of high-energy shoal facies in its
cycles with supra-tidal to off-shoal facies (Fig. 14). They are de- early to middle HST (Fig. 14). In both studied wells, the late HST is
fined based on facies changes and vertical stacking patterns composed of lagoonal, leeward intertidal and sabkha facies that, in
without any dating. They considered here as fifth-sixth order se- well-B, the lagoonal facies are more frequent than the well-A in
quences, because their detail differentiation was not possible using which the sabkha facies are predominant (Fig. 15). On the other
the available data. hand, the common pore types are substantially different in the
 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113 105

Fig. 14. Correlation of detected depositional sequences (third, fourth and fifth orders) among the studied wells and the South Pars gas field.
106 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113

Fig. 15. The KS4 third order sequence and its fourth- and fifth/sixth depositional sequences of the upper Dalan Member in the studied wells. Poroperm variations are also
shown within the distinguished sequences.

Fig. 16. The KS3 third order sequence and its fourth- and fifth/sixth depositional sequences of the upper Dalan Member in the studied wells. Poroperm variations are also
shown within the distinguished sequences.

shoal facies of the studied wells. In the well-A, shoal facies are equivalent in the well-A. It has mainly rooted from its calcitic li-
characterized by intra-granular porosity that are connected to- thology that has resulted in fractures to be cemented in almost all
gether in almost all cases. In well-B, the shoal facies have lower cases (Fig. 15). In addition, higher intensity of marine cementation
intra-granular porosity and the rare present moldic pores are also and disconnected moldic pores has also resulted in different re-
cemented. As well, the dolomitization of mud-dominated facies is servoir quality in two studied wells, especially in the early HST
higher in the well-A than the other studied well (well-B; Fig. 15). (Fig. 15). The other responsible for decreased reservoir quality in
In the well-A, due to the lesser cementation and lower fre- the well-B (especially in the late HST) was the lacking or scarcity of
quencies of closed (cemented) fractures, the reservoir quality is dolomitized lagoonal facies. Such dolomitized facies are more
totally higher and lower heterogeneities are recorded within the frequent in the well-A in a similar stratigraphic position (Fig. 15).
grain-dominated interval (Figs. 15 and 21). In the early HST, higher In the latest parts of this systems tract, the dolomitized mud-
reservoir quality in the facies of shoal complexes has resulted from dominates facies of lagoonal and intertidal settings have higher
the presence of connected moldic pores, which have resulted in permeability and low porosity values. On the other hand, high
higher poroperm values (Fig. 15). The lower poroperm values of energy facies of intertidal setting show higher porosity and lower
these facies are also accompanied with the isolated inter-granular permeability measurements than the well A.
pores as the dominant pore types. In the late HST of this sequence Generally, this systems tract has a better reservoir quality and
(in the well-A), opposite trends are visible in porosity and per- lower heterogeneity in the well-A than the well-B (Figs. 15 and 21).
meability measurements (increasing upward for permeability and The main cause of this is the existence of the dense and closed
a reverse trend for porosity) in the dolomitized facies of lagoon fractures, disconnected moldic pores and lower thickness of do-
and intertidal settings (Fig. 15). Below the SB (sequence boundary), lomitized intervals of this systems tract in the well-B.
a non-reservoir anhydritic layer (to a 1.5 m thick) separates this
systems tract from its upper sequence of this well (A). 8.3.2. KS3
In the well-B, despite the higher fracturing of this systems tract, This sequence is composed of a fourth-order sequence and four
it has lower reservoir quality and higher heterogeneity than its fifth-order sequences, surrounding by sabkha related anhydritic
 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113 107

layers (type-1 SB) at its lower and upper boundaries (Fig. 16). The and fractured mud-dominated facies. In this well (B), the sequence
MFS of this sequence is detected based on the domination of outer is terminated to a non-reservoir unit composed of sabkha-type
ramp (off-shoal) facies (Fig. 16). In most cases, fifth-order se- anhydrites with a 2 m thickness.
quences are composed of outer ramp and shoal facies around the The average porosity of this sequence is higher in the well-A
MFS surfaces. In this order of sequences, the SBs are represented than the well-B. Higher frequency of grain-dominated facies with
by lagoonal or sabkha facies (Fig. 16). The thickness of this se- frequent moldic pores in the well-A can be considered as the main
quence is different among the studied wells. It contains the cause of this general trend. Unlikely, the average permeability of
highest development of shoal complexes around the MFS (late TST the KS3 is higher in the well-B (Fig. 21). This has been resulted
and early HST). On the other hand, the lagoonal and intertidal from the higher frequency of fractures in this well (Fig. 9). Gen-
facies are predominant in the early TST and late HST (Fig. 16). This erally, the reservoir heterogeneity is lower in the KS3 of the well-
sequence is thicker in the South Pars than the Golshan Field. But, A, than the other studied well (Fig. 16).
the shoal facies are more frequent in the KS3 sequence of the
Golshan Field (Fig. 14). 8.3.3. KS2
In the well-A, this sequence, and its fourth-order counterpart, This sequence has lower reservoir properties and higher facies
begins with the fifth-order KS3-4 sequence that comprises two variations than the most of defined sequences throughout the
separate reservoir units bounded by dirty anhydritic facies (F2; Persian Gulf and encompasses the Permian–Triassic boundary
Fig. 16). In the thinner lower unit, the dolomitized and fractured (Fig. 14). It is deposited above a chicken-wire anhydritic interval
mud-dominated facies have higher reservoir quality than the up- and terminates to a fenestral and mud crack bearing unit. How-
per unit that comprises both grain- and mud-dominated facies ever, in the South Pars field, it is enclosed by two chicken-wire
showing lesser dolomitization and lower fracturing. Upwards anhydrite units (Fig. 14). The MFS of this sequence is located at
(toward the MFS), there are some layers with relatively good re- different positions among the studied fields. In the South Pars
servoir quality and low heterogeneity that are mainly composed of field, it is established at top the seaward shoal strata. But, in the
grain-dominated facies (Fig. 16). In the well-A, the layer with best wells A and B, it is located at top of the shoal facies and above an
reservoir quality are concentrated around the MFS of this sequence outer ramp interval, respectively (Fig. 17). This sequence can be
(late TST and early HST). The HST of this sequence is hetero- considered as two fourth-order and five fifth-order depositional
geneous than the TST (in the well-A). The late HST is composed of sequences. The upper and lower parts of the KS2 are dolomitic.
lagoonal fenestral dolo-mudstones with a relatively high reservoir The thrombolytic facies are recorded around the MFS of this se-
quality. Finally, in the well-A, this sequence terminates to a 1.5 m quence (Fig.17). They can be acted as a regional intra-formational
thick, non-reservoir anhydritic layer below the SB (Fig. 16). barrier/cap rock in the Permian–Triassic reservoirs (Insalaco et al.,
In the well-B, this sequence, and its fourth-order counterpart, 2006). In the studied fields, factors governing the reservoir quality
begins with the fifth (or sixth)-order KS3-4 sequence that com- in this sequence are considerably different. In the South Pars field,
prises one reservoir unit bounded by dirty anhydritic facies (F2; dissolution has improved the reservoir properties, but in the other
Fig. 16). Upwards in the TST, the reservoir quality has decreased wells, dolomitization was more important. This inconsistency can
and reaches to its minimum just below the MFS, in the outer ramp be attributed to the position of studied fields on the Khuff platform
facies. This is in apparent contrary to which that recorded in the and the Qatar Arch, which has led to different facies characteristics
well-A in similar stratigraphic position (Fig. 16). In the HST, the and consequent different diagenesis history.
best reservoir facies are recorded in the grain-dominated facies In the well-A, the TST of this sequence in mainly composed of

Fig. 17. The KS2 third order sequence and its fourth- and fifth/sixth depositional sequences of the Kangan Formation in the studied wells. Poroperm variations are also
shown within the distinguished sequences.
108 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113

Fig. 18. The KS1 third order sequence and its fourth- and fifth/sixth depositional sequences of the Kangan Formation in the studied wells. Poroperm variations are also
shown within the distinguished sequences.

dolomitized to un-dolomitized mud-dominated facies of lagoonal Generally, same as the former sequence, the average porosity is
setting with relatively high to low reservoir quality, respectively higher in the well-A and the average permeability is higher in the
(Fig. 17). In this well, in some intervals of this systems tract, the well-B (Figs. 17 and 21). This is because of higher frequency of
grain-dominated facies have developed with high reservoir qual- fractures in the well-B (Fig. 9).
ity. Around the MFS, in the fifth-order sequence of KS2a-2, alter-
nations of grain- (belonging to the shoal complexes) and mud- 8.3.4. KS1
dominated (lagoonal) facies have developed with commonly low This sequence starts with fenestral dolomudstones in the Gol-
reservoir quality (Fig. 17). Intense cementation has occurred in shan Field, and with sabkha-type anhydritic facies in the South
grain-dominated facies after the Permo-Triassic boundary (see Pars Field. In the both studied wells, it terminates to anhydritic
Woods et al., 1999). In middle parts of the HST, reservoir facies facies at its upper SB. In addition, the shoal facies are recognized as
have concentrated within the dolomitized and fractured mud- the MFS (Fig. 14). The KS1 is composed of three fourth-order and
dominated facies and, with a lesser importance, in the grain- ten fifth-order depositional sequences (Fig. 14). In the fourth and
supported facies (Fig. 17). In the late HST of this third-order se- fifth-order sequences, the SBs are characterized by the presence of
quence (in the well-A), which corresponds with the HST of fifth/ hypersaline lagoonal facies and sabkha anhydrites and MFSs are
sixth order sequence of KS2a-1, the over-dolomitized facies are mainly recognized by the appearance of thin layers of shoal facies
recorded. In some fractured intervals of this part, the reservoir (Fig. 14). High frequency of lagoonal and peritidal facies represents
quality has increased (Fig. 17). a very shallow water environment with local hypersaline lagoons
In the well-B, this sequence can be differentiating into two and high energy sub-environments. In the wells A and B, most
parts. The first part covers the most interval of the TST (especially parts of the sequence are dolomitized expect for the middle parts
around the MFS of the fifth/sixth order sequence of KS2a-3), in (Fig. 14).
which the Phi/K ratios are higher in the reservoir units than the The dolomitization is very extensive within this sequence
other parts of this sequence (Fig. 17). In this part, alternatives of (Fig. 14). In addition to the intertidal and sabkha settings as the
dolomitized mud-dominated facies with high reservoir quality and sources for dolomitizing brines, they could also originate from the
un-dolomitized ones (with low reservoir quality) are recorded. hypersaline lagoons. In many cases, an extensive dolomitization
Moreover, a lower thickness of grain-supported facies with rela- has affected the shoal facies. Such dolomitized shoal facies are
tively low reservoir quality is distinguished within this part. The mainly cemented by anhydrites (Fig. 14).
second (upper) part of the sequence begins form the latest parts of Although the most parts of this sequence are dolomitic in the
TST and continues to the end of sequence. It comprises facies with both studied wells, their fracturing intensity and frequency are
Phi/K ratios that are lesser than the lower part, in most cases considerably different (Fig. 9). The fractures are more frequent in
(Fig. 17). Also, in many cases, high permeability values are mea- the well-B. Such discrepancies over the area of study reveal the
sured within the low porosity, grain-supported facies that have importance of tectonic conditions in various parts of this region.
resulted from the higher concentration of open fractures in this The openings of fractures are generally low in this sequence.
part. In the well-B, fracturing was the main cause of reservoir In the well-A, after a 2 m-thick stromatolitic interval, there are
enhancement (mainly permeability) in most of the reservoir units some alternations of extremely dolomitized, grain-dominated fa-
of this sequence. cies with anhydritic cements (belonging to the shoal and leeward
 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113 109

Fig. 19. Distinguished depositional sequence of the upper Lower Dalan Member and its correlation with other sequence stratigraphic schemes in the adjacent areas of the
Persian Gulf.

intertidal settings) and anhydritic layers towards the end of fifth/ reservoir importance. Whereas, surveying of the lower Dalan
sixth order sequence of KS1b-4. The alternations show a medium Member is also necessary for basin analysis and evaluation of its
to very low reservoir quality, respectively (Fig. 18). reservoir potential (see Strohmenger et al., 2002; Bendias et al.,
In the earlier parts of this sequence, in the fourth order se- 2012; Forke et al., 2012; Walz and Aigner, 2012). Another im-
quence of KS1C, fracturing exerts a major role in controlling the portant aspect is that the Lower Khuff sequence (KS5 and KS6)
reservoir quality, especially in the well-B. It has been causes im- shows marked facies and thickness variability and possibly is not
provement in permeability even in the low porous intervals layer-cake, which makes its sequence stratigraphic correlations
(Fig. 18). challenging (Koehrer et al., 2010, Bendias et al., 2012; Walz and
In the TST of KS1b-3 sequence, two intervals of medium re- Aigner, 2012). Here, depositional environments, diagenetic process
servoir quality and different thicknesses are present in the well-A. and reservoir characterization of the Lower Dalan Member are
The lower reservoir interval is composed of shoal facies associa- evaluated in a sequence stratigraphic framework. At the end, de-
tion and bounded by the anhydritic layers and non-porous la- fined depositional sequences are compared with other places in
goonal facies at its lower and upper parts, respectively. This unit the Arabian Plate.
has a thickness of about 7 m and it seems to be developed in the This sequence is substantially differing from the previously
other studied well (i.e. well-B). But, in the well-B, the gross discussed sequences in view of its environmental characteristics
thickness of this reservoir interval reduced to 3 m (Fig. 18). The and diagenetic features. It has deposited in a shallower deposi-
upper reservoir interval comprises mainly back-shoal facies and is tional setting with more frequent shoal complex and landward
very thin in the well-B. intertidal (more grain-dominated) facies than the other deposi-
Around the MFS (i.e. late TST – early HST), the reservoir quality tional sequences of upper Dalan and Kangan formations (Fig. 21).
has changed in both of the studied wells as a result of deeper In view of the diagenetic imprints, dolomitization is its notable
depositional setting and different diagenesis history. In these in- diagenetic feature. Due to the higher frequency of fractures in
tervals, the high reservoir quality is originated from the presence dolomitic intervals, it has a major control over the reservoir quality
of shoal facies with lesser anhydritic cements. Unlike their similar in these intervals (Figs. 9 and 21).
facies (high-energy, grain-dominated shoal facies), a higher het-
erogeneity is recorded in the well-B as a result of closed fractures 9.1. Correlation with other places in the arabian plate
in the limestone intervals (Fig. 18) and a lower heterogeneity is
recorded in the facies of these intervals in the well-A (Fig. 18). In the studied fields, the upper part of the lower Dalan Member
In the HST of this sequence, lagoonal, intertidal and shoal facies was cored just below the Nar Member (Middle anhydrite;
are frequent facies in both studied wells, respectively. The re- Figs. 1 and 19). The most prominent sequence stratigraphic
servoir intervals of this systems tract are different among the schemes of the Arabian Plate are presented in the (Fig. 19) and
studied wells. In the well-A, these intervals are thicker than the correlated with depositional sequences defined in the Lower Khuff
other well (Fig. 18). In these intervals, two main factors are re- strata and its equivalents. Totally, determined sequences can be
sponsible for inconsistencies in reservoir quality: the first con- correlated with the KS6 of Strohmenger et al. (2002), SQ3 of
trolling factor was the development of non-reservoir layers (e.g. Weidlich and Bernecker (2003) and lower part of the KS5 of
stromatolitic) in the palaeo- environment location of the well-B (Koehrer et al., 2010).
that caused reservoir compartmentalization. The other factor was
the fracturing that causes heterogeneity in the intervals of this 9.2. KS6 sequence in the lower Dalan member
systems tract in the well-B.
On the whole, the lower frequency of open fractures along with The lower boundary of this sequence is located above a 0.5 m-
the higher development of non-reservoir facies (e.g., stromatolite) thick chicken-wire anhydrite layer and its upper limit corresponds
were the main causes for lower values of mean porosity and per- with the lower boundary of the Nar Member, which is not cored in
meability of this sequence in the well-B than the well-A (Fig. 9). the studied wells. However, the upper sequence boundary is char-
acterized by the occurrence of sabkha facies (i.e. F13; Fig. 20). The
MFS of this sequence corresponds with the upper limit of a shoal
9. Lower Dalan member grainstone layer (Fig. 20). This grainstone layer shows evidence of
deeper depositional settings than the other shoal facies associations
Almost all previous studies of the Dalan Formation are focused in the studied intervals and contained some open-marine fauna
on its upper part (known as upper Dalan Member due to it higher (e.g., brachiopod and echinoderm) at top of this layer.
110 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113

Fig. 20. Detected depositional sequences (third, fourth and fifth/sixth orders) in the upper part of Lower Dalan Member.
 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113 111

mud-dominated facies in which poroperm values have increased

as a result of fracturing. Towards the end of this third-order se-
quence (i.e. the HST of KS6a-1 fifth/sixth order sequence), the fa-
cies of low reservoir quality are developed. They are including
nodular and fenestral dolomudstones in which the permeability
has increased, in some cases, as a result of fracturing (Fig. 20).

10. Conclusions

In this study, the Permian–Triassic Dalan and Kangan forma-

tions were studied in two exploration/production wells from the
western parts of the Persian Gulf (Golshan Field). Determined
depositional sequences in the Golshan Field were correlated and
compared with their counterparts in the central Persian Gulf
(South Pars Field). The ramp-type depositional model of the Dalan
and Kangan formations has controlled the primary distribution of
reservoir quality under the arid climatic condition. Afterward,
different diagenetic processes overprinted the Dalan–Kangan in-
Fig. 21. Porosity vs. permeability diagram of depositional sequences (Well A and B).
tervals and alter their primary reservoir potential. In general, most
of the diagenetic features are controlled directly and/or indirectly
by the facies characteristics and distribution. Among the diage-
In the TST of this sequence, which corresponds with the TST of netic processes, dolomitization and, to some extent, dissolution
a fourth order sequence, after the deposition of a 0.5 m-thick an- are more important in view of their effects on reservoir quality.
hydritic layer, the cycle of lagoon-shoal-intertidal facies is re- They are mostly occurred in certain facies associations and, con-
peated. The maximum flooding surfaces of fifth-sixth order se- sequently, in specific sequence stratigraphic position. In all se-
quences in this systems tract are mainly characterized by the de- quences, fractures can be seen with different density and aperture
velopment of shoal facies and their sequence boundaries are sizes. However, in the dolomitized intervals, fractures show higher
marked by the nodular dolomudstone facies (Fig. 20). However, frequency and density and larger aperture sizes.
the lagoon-shoal is the dominant depositional cycle in this sys- The sequence stratigraphic framework of the studied intervals
tems tract. is established and a geological model (facies and diagenesis in the
The HST of this third-order sequence comprises a complete and sequence stratigraphic framework) is presented that can be used
a TST of fourth order sequence. This systems tract is characterized as a basis to predict the changes in reservoir quality and construct
at its lower part (HST of fourth-order sequence) by the lagoonal, a robust reservoir model. As well, determined sequences of this
and with a lesser importance, shoal and intertidal to supratidal study are traceable throughout the Persian Gulf and, therefore, are
facies (Fig. 20). In this systems tract (HST), corresponds with TST of applicable in the basin-scale reservoir models. The factors gov-
fourth-order sequence, high energy shoal facies are more frequent. erning reservoir quality are comparatively different among the
Within the late HST of third-order sequence (corresponding to the studied wells from the Golshan Field. However, they can be
HST of fourth-order sequence), the shoal facies were less frequent summarized as follows:
and thicker intervals of lagoonal facies have developed. In this In the KS4 and KS3 sequences, dissolution and dolomitization
system tract (HST), in the lower order (fifth-sixth) sequences, the improved the reservoir quality. In the two studied wells, the KS2
maximum flooding surfaces are mostly coinciding with the shoal shows low reservoir quality, due to the dominance of mud domi-
dominated layers. The sequence boundaries mostly correspond nated and highly cemented facies. In the KS1 with shallow water
with the fenestral or nodular dolomudstones and anhydritic or facies (anhidritic and stromatolite) and intense effects of dolomi-
brecciated facies (largely in the latest parts of the systems tract) of tization and close fracturing, there are more poroperm hetero-
intertidal to supratidal settings (Fig. 20). geneities than the other sequences.
In the lower parts of this sequence (i.e. the KS6b-11 fifth/sixth The studied interval of lower Dalan Member corresponds with
order sequence and early TST of the KS5b-10 sequence), high re- the KS6 sequence of the Arabian Plate (Strohmenger et al., 2002).
servoir quality is developed within the grain-dominated facies The facies and diagenetic evidences indicate its shallower de-
(with intergranular porosity) and dolomitized mud-dominated positional setting in comparison to the other sequences. Pervasive
facies (Fig. 20). In the middle parts of the TST (to the end of the dolomitization and fracturing are main characteristics in the KS6
KS5b-7), lower poroperm values are recorded that coincide with sequence of the Lower Dalan Member. As a consequence, they
undissolved grain-dominated facies in which closed (cemented) have main controls on the reservoir quality of this Member in the
fractures have developed. Golshan Field.
From the lower SB of the KS6b-7 sequence to the middle parts
of HST of fourth-order KS6b sequence, intervals with the best re-
servoir quality have developed that show the lowest heterogeneity Acknowledgments
within this sequence (Fig. 20). The main responsible for this high
reservoir quality was the fracturing. Dense open fractures have We thank Y. Arghavani for her supporting. T. and R. Abdolma-
resulted in high poroperm values that are not consistent with their leki and Z., H.A and S.H Sheykhlar are also thanked for their useful
facies characteristics. The intertidal to sabkha facies are recorded helps. Also, Many thanks to H. Rahimpour-Bonab, H. Mehrabi, A.
in the late HST of KS6b fourth-order sequence that in some in- Enayati-Bidgoli and B. Esrafili-Dizaji for their useful helps and
tervals, fracturing has increased permeability of these facies suggestions.. The University of Tehran provided facilities for this
(Fig. 20). Towards the MFS of KS6a-1 fifth/sixth order sequence, a research for which the authors are grateful. We would like to ac-
variety of facies have developed that are including both grain- and knowledge the POGC for sponsoring and data preparation.
112 J. Abdolmaleki et al. / Journal of Petroleum Science and Engineering 141 (2016) 90–113

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