EarthDoc-55774

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Third Arabian Plate Geology WorkshopPermo-Triassic (Khuff) Petroleum System of the Arabian Plate

Kuwait City, Kuwait, 28 November - 1 December 2011

Reservoir Rock Characterization, Classification and Pore Typing in

the Dalan/Kangan Carbonates (Khuff Analogues), South Pars Field

Behrooz Esrafili Dizaji (University of Tehran, MAPSA Co., Iran

), H. Rahimpour-Bonab (University of Tehran) and F. Kiani

Harchegani (Azad University of Khorasgan, Iran)

The South Pars Field, discovered in 1990, is part of the world's largest single gas

accumulation located in the Gulf. The Iranian part of this immense gas accumulation accounts

for 5% of the worlds and 60% of Iran's total gas reserves. This Field produces from

Dalan/Kangan carbonates (Khuff analogues). These PermianTriassic carbonate reservoirs in

the field are highly stratified in nature and display layer-cake geometry. Combined core

analysis and detailed thin section studies are used for facies analysis. Accordingly, 14 major

facies were recognized in these carbonates. Facies analysis shows that their depositional

setting was located along the inner part of an epiric carbonate system that extended from a

peritidal setting to a shallow subtidal zone (back-shoal setting), passing over to a high energy

shoal and fore-shoal facies (Figure 1).

Petrographical and geochemical evidences indicate that these facies were mainly exposed to a

shallow diagenesis and minor subsequent burial. Major diagenetic processes and events

affecting the Dalan/Kangan carbonates include: micritization and marine cementation,

anhydrite nodule formation, early dolomitization and dolomite neomorphism, dissolution

and/or neomorphism of aragonite, anhydrite plugging and calcite cementations, mechanical

and chemical compaction, fracturing. Hypersaline and meteoric diagenetic realms were two

well-identified zones in the reservoir intervals. Three major diagenetic environments affected

reservoir intervals.

Generally, carbonate reservoirs consist of different rock types and various fluid-flow units.

Significance of reservoir rock classification for heterogeneity characterization in these

reservoirs is now widely recognized. Our studies indicated that depositional facies and early

diagenesis (intensity and extent of dolomitization, dissolution and cementation) has controlled

the first order reservoir heterogeneity in these reservoirs (Figure 1).

In this research a new reservoir rock classification scheme is established, which is based on

poroperm controlling factors (Table 1). By integration of petrographic examination and

petrophysic data, twelve major rock types are recognized. These rock types ranked in various

reservoir qualities (from poor to good). Therefore, the best reservoir quality can be found

commonly in dolomitized and non-dolomitized grain-dominated rock types. Other good

reservoir rocks are sucrosic and recrystalized dolomite (with high intercrystalline poroperm)and fractured rock types but these rock types are not common in the reservoir.

Furthermore, pore typing analyses are carried out using thin sections and image analysis,

SEM, MICP, X-ray CT, core poroperm data and flow unit equations (FZI, RQI and R35).

Some 152 samples are selected from reservoir intervals that have commercial poroperm

values (units with > 5% and K > 0.1 mD) for production. Image analysis (2-D in thin

section photographs) of these samples indicated seven basic pore types. These pore types and

their relative percents are intergrain (14%), fenestral (3%), moldic (43%), intercrystalline

(8%), fracture, vuggy or solution enlarged (16%), fracture (13%), and stylolitic (3%). Relative

proportions of these pore types for each sample is plotted on the ternary diagenetic gradient

(Figures 2 and 3). Pore scale examinations showed that pore system properties were modified

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during diagenesis to varying degrees. Considering general diagenetic history and porosity

evolution of this reservoir, three main porosity generation stages are recognized as pre-

meteoric, meteoric and post-meteoric. Although a significant amount of secondary porosity

has formed during meteoric diagenesis, later diagenesis has effectively improved reservoir

quality. The results reveal that relative abundance of moldic pores has the main effect on pore

system connection in these carbonate rocks. In conclusion, localized late diageneticalterations played a key role in the porosity evolution and imposing reservoir heterogeneity

(particularly in the pore scale).

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Figure 1: Idealized field-scale facies model for reservoir rocks (Upper PermianLower

Triassic Dalan and Kangan formations) of the South Pars Field. Depositional attributes,

petrographical parameters, facies, facies-related early diagenetic processes, and reservoir

characteristics are shown. Poroperm values of each facies indicating effects of the

environmental energy on the distribution of reservoir properties.

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Table 1: Reservoir rock classification scheme applied for carbonate reservoir of the South

Pars field. Typical thin section photo-micrographs from these rock types are added here. In

this scheme, fractured rock types (RT-1) are units with dominant fracturing and microfracture pore types. Anhydritic dolomites are rock units that contain more than 30% anhydrite.

Anhydrite in these rocks show two main fabrics, patchy (nodular and poikilotopic

cementation) and uniform (layered and pore-filling type). Cementation Index (CI) and

Dissolution Index (DI) definitions are based on the cements percents (with various

mineralogies) and dissolution porosity, respectively. Crystal size (CS) is an important factor

in the classification of non-mimic dolomites.

Fracturing Mineralogy Fabric Texture Rock Type

Fractured RT1

CI10% RT3

DI>5% RT4Limestone Mud-dominatedDI100 m RT8Dolomite

Non-mimic

Type CS

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porosity generation (diagenetic sequence). These pores are dominant pore types in the South

Pars Field. Two main diagenetic trends are recognized between triangular axes.

Figure 3: Six Pore type classes (PTC) are distinguished in the reservoir rocks (n=152).

Typical SEM microphotographs of these classes are also shown in this figure. As revealed,

pore system show high variety in this reservoir.

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