Kangan and Upper Dalan formations are forming reservoir sequence of the South Pars field. Litology of the kangan formation is carbonate and (limestone and dolomite with anhydrite intervals), so digenetic processes were very active in this formation and these processes have changeed the reservoir quality. Importances of the digenetic process are including dissolution, calcite cements, dolomitization, anhydritization, physical and chemical compaction and fracturing

Ashkan Asadi Esksndar

Gholamreza Hosseinyar

Hossein Rahimpour-Bonab

Masoud Asgharian Rostami

English

Nature Precedings

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Survey of the diagenesis process and effect these process on 
reservoir quality of the Kangan formation in South Pars Field 
Gh. Hosseinyar1; H. Rahimpour-Bonab2; A. Asadi Esksndar3  ,M. Asgharian Rostami4 
1, 2,4: School of Geology- University College of Science, University of Tehran-Iran 
3: Expert in geology- Pars oil and gas co.-tehran-Iran 
Abstract 
Kangan and Upper Dalan formations are forming reservoir sequence of the South Pars field. Litology of the 
kangan formation is carbonate and (limestone and dolomite with anhydrite intervals), so digenetic processes were 
very active in this formation and these processes have changeed the reservoir quality. Importances of the digenetic 
process are including dissolution, calcite cements, dolomitization, anhydritization, physical and chemical 
compaction and fracturing. 
Keywords: Diagenesis, Reservoir quality, Kangan, Triassic. 
Introduction 
Diversity of reservoir quality in shallow carbonate is usual (Lucia, 2007; 
Moore, 2001). Diversity reflects the effect of sedimentary environment process and 
diagenetical process. This paper focuses on the Kangan formation in the Persian 
gulf in Iran (figure 1). Upper Dalan (upper Permian) and Kangan (lower Triassic) 
formations (equivalent to the upper khuff) are main gas reservoir section in Middle 
East region that include about 18% of world gas reserve. These formations are 
usually subdivid into the 4reservoires zone; the Upper Dalan subdivide into the K4, 
K3reservoirs and Kangan subdivide into the K2 and K1reservoirs (Insalaco et al., 
2006).  
Methodology  
For identify facies and sedimentary environment, in this project, we use 2050 
thin section for petrographic studies that have supplied from 3 wells of South pars 
field in Persian Gulf (fig 1). Thin sections cut from horizontal plug samples (with 
35cm from each other) and all of them stained by Alizarin Red-S (relevant Dickson, 
1965) to identify calcite from dolomite mineralogy. We survey the diagenetical 
processes completely and 15 samples from Kangan formation had selected for 
scanning electron microscopy (SEM) observation.      
  2 
 
Figure 1: Location map of study area 
The microfacies of Kangan formation are including: Bioclast grainstone –
packstone microfacies, Coarse to medium Oncoid-intraclast-bioclast 
grainstone(dolograinstone) microfacies, medium to coarse ooid-bioclast grainstone 
to packstone (and dolostone) microfacies, Ooid grainstone to packstone 
microfacies(and dolostone), peloid-ooid-bioclast packstone to grainstone 
microfacies, peloid-bioclast packstone to wackstone microfacies, Mudstone to 
wackstone (and dolostone) microfacies, Thrombolite microfacies, Dolomudstone 
with anhydrite microfacies and layered to massive anhydrite microfacies. 
Discussion 
Diagenetical processes that acted in this section have been formed in all of 
diagenetic environments such as marine, meteoric and burial environments 
(Scholle and Scholle, 2003; Flügel, 2004). Some of these processes interpret 
below: 
1- Micritization: in the Kangan, micritization is common process and has 
formed in more of the facies. This process has affected ooids, peloids and some of 
sceltal fragments and envelope rim of the grains (whole of the grain in several 
samples). Micrite envelope has replaced by dolomite on the part of section (Figure 
2- A). 
2- Calcite cementation: calcite cements are very important and also more 
effective diagenetic processes in the Kangan formation and have different kinds 
that summarized below: Isopachous Cement, Circumgranular Bladed Cement, 
  3 
Equant Calcite Cement, Drusy Calcite Cement and Coarse Crystalline Calcite 
Cement. Equant calcite cement is frequent than the others, that is interpreting here. 
•  Equant Calcite Cement: this cement type form on meteoric and burial 
diagenetic environments and filled the void spaces in grain-dominated facies 
(Scholle and Scholle, 2003). Association of this cement with creation of moldic 
porosity indicates that provide from solution flour of grains on meteoric 
environment. Equant calcite cement, in Kangan formation, is observe in  coarse to 
medium Oncoid-intraclast-bioclast grainstone(dolograinstone) microfacies, 
medium to coarse ooid-bioclast grainstone to packstone(and dolostone) 
microfacies, Peloid-ooid-bioclast packstone to grainstone microfacies and ooid 
grainstone to packstone microfacies. Filling the interparticle  porosity by this 
cement type and creation mold voids associate it lead to very decreasing on 
permeability values (Figure 2- B). 
3- Mechanical-chemical compaction: same process usually in the 
muddominated facies (or dolo) is more effective than graindominated ones and 
decreases porosity and permeability. In the Kangan formation, this process lead to 
creation presser-solution situation in facies. Form of the stylolites through the 
section (commonly in facies boundary), presser-solution on dolomite crystals and 
grain contacts with each other are instances for this topic (Figure 2- C).   
4-  Dolomitization: Jones and Xiao, 2005  Lucia, 2007 hypothesized that the 
porosity in dolostones is a function of a) the porosity of the precursor limestone, b) 
source of the co3-- , c) compaction, d) cementation by later dolomite flour and e) 
the amount of overdolomitization. Dolomitization connect the voids with each 
other and increase the permeability. 
From the models that have suggested for dolomitization of strata, 
dolomitization based the evaporative model, the seepage-reflux model, the Dorag 
model and subsurface burial dolomitization and dolomite cementation have acted 
in this section. The replacement of dolomite in facies is possible retain the original 
fabric (fabric retentive dolomite) or destruct it (fabric destructive dolomite) and/or 
select the fabric of rocks (fabric selective dolomite (Figure 2- D and E)), (Machel, 
2004). In dolomitized facies in Kangan formation, we observe all of dolomite 
fabrics and also observe planner (e and s) and non-planner dolomite crystals (Sibly 
and Gregg, 1987). Planner dolomite crystals (e and s) have formed in fabric 
destructive dolomite on sand shoals facies and non-planner dolomite crystals 
present in facies have dolomitized by replacement on primary diagenesis time in 
most of the facies (mostly in dolomudestones(figure 3)).  
  4 
 
Figure 2: a) Micritization on ooids in ooid grainstone with pore filling 
anhydrite cements; B) ooid grainstone microfacies with equant calcite 
cement and moldic porosity assocate it; C) physical-chemical compaction 
(evaporate veins staple on stylolite ُ s strike; D) fabric selective dolomite 
that create by replacement; E) overdolomitization and fabric destructive 
dolomite type; F) oomolde filling dolomite cement (burial) 
 
Figure 3: Scanning electron microscopy image: a) Planner-s dolomite crystal (scale 20 µ); b) 
Planner-e dolomite crystal (scale 10 µ) 
Most of the dolomitic cements have formed on somedeal depth to depth stage 
and indicate oil window in reservoirs. Dolomitic cements are including: 
•  Filling cements: these type cements are idiomorphic and coarse 
crystal that have filled molds and some of interparticle pores in grain-
dominated facies (Figure 2- F). 
  5 
•  Rim dolomite cements: this cement have formed on side of some 
ooids and peloids and some of the bioclasts. The frequent of the rim cement 
is very low in Kangan formation. 
•  Fin crystal dolomite cement: these cements have formed on strike of 
the stylolites and indicate burial origin for them. Some of these cements 
have filled pores partly and our surveys show that permeability decrease in 
same facies. 
5- Anhydritization: anhydrite (CaSO4) and gypsum (CaSo4.2H2O) are 
common minerals in dolostones reservoirs. Most of the gypsum changed to 
anhydrite; so we interpret the anhydrite cements in here. Distribution of anhydritic 
cements in Kangan formation is very much and have destructed  the quality of 
reservoir on parts of the section. Anhydrite present in most of the facies and has 
filed pores such as interparticle, intercristalin, moldic and fenestral pores. The time 
of form of the anhydrite had widespread and we can observe primary to burial 
original anhydrite through the section. Anhydrite fabrics have  identified(based 
lucia, 2007), that have summarized below: 
Poikilotopic Anhydrite, Pore-Filling Anhydrite or Pervasive Anhydrite, Bedded 
or Layered Anhydrite, Nodular Anhydrite, Single and sporadic crystals anhydrite, 
evaporate veins(Figure 2- A, C and D). From these fabrics, poikilotopic and pore-
filling anhydrite are the most common form of anhydrite in the par of dolostone in 
our studding reservoir. 
6- Dissolution: The observations suggest that solution process in carbonate 
facies lead to creation mold and vuggy pore space (Saller and Henderson, 1998; 
lucia, 2007). In effect of this process on meteoric phase in the Kangan formation 
facies, ooid grains and some skeletal fragments (such as gastropods, foraminifer 
and some bivalves) have solved in grain-dominated facies and so, the moldic 
porosity is very distributed. Although creation of new pore spaces through this 
process has increased the porosity, but cementation by the calcite associate it has 
occluded interparticle pores and permeability of the facies decline. 
7- Fracturing: fractures are phenomena that increase reservoir quality 
(especially permeability). This phenomenon has created through diagenes in the 
Kangan formation but most of them have occluded by anhydrite and calcite 
cements. So are vacant of fractures. 
Conclusion: 
From the present diagenetic processes in Kangan formation, calcite cementation 
(especially equant and drusy calcite cements) and anhydritization and compaction 
had most effect on decline the porosity and permeability in reservoir. Whereas 
  6 
dissolution and fracturing have caused that reservoir quality improve. 
Dolomitization has bilateral effect; in some cases dolomitization has connected 
pore space with each other and has increased relevance them (increasing reservoir 
quality), but in some of facies amount of dolomitization had more and growth of 
the crystals lead to occluding the pores (decreasing reservoir quality). Dolomite 
cementation cased reservoir quality decline, too. 
 
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Dolomitization, anhydrite cementation, and porosity evolution in a reflux system: Insights from reactive transport models,

Microfacies of Carbonate Rocks: analysis, interpretation and application.

Upper Dalan Member and Kangan Formation between the Zagros Mountains and offshore Fars, Iran: Depositional system, biostratigraphy and stratigraphic architecture: GeoArabia.11,

Survey of the diagenesis process and effect these process on reservoir quality of the Kangan formation in South Pars Field

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Survey of the diagenesis process and effect these process on reservoir quality of the Kangan formation in South Pars Field

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