Sarvak Reservoir Facies Characteristics in One of the South West Field in Iran

Sarvak Formation (Late Albian-Early Turonian) as main reservoir in the field is one of the carbonate units of Bangestan Group in Zagros Basin with average thickness of 640 m. This formation conformably overlays the Kazhdumi Formation while the upper boundary is an erosional unconformity which is covered by Ilam Formation. There is a significant lateral and vertical heterogeneity in the reservoir layers that causes main challenge in reservoir characterization. In this paper, reservoir properties evaluation and construction of depositional model have been done based on lithotype study, sedimentary environment classification, petrophysical interpretations and SeisWorks. Five facies types (lithotypes) in the Sarvak Formation with particular rock properties are identified in 8 wells and 200 thin sections. The main facies association elements and relevant depositional settings have been interpreted by extracted Paleolog, facies and fossil association data which are related to depositional setting variations.


Introduction
The carbonate platform deposits of the Sarvak Formation were deposited in Late Albian to Early Turonian age [1] [2] [3] as the second most important carbonate reservoir in the Zagros Basin.This formation is a thick carbonated unit that is deposited in Neotethys Southern Margin of Zagros Basin.In the past, this rock unit was called Hipporite limestone, Rudist limestone and Leshtegan limestone, but with sectional measurement in Sarvak rock unit at Bangestan mountain, the Sarvak Formation substituted former names.The Sarvak Formation includes mostly carbonate in lithology and was composed of sequence of thin to medium-bedded limestone and massive limestone [1].This formation predominantly characte-rized by cyclic stacks of shallow-marine carbonates and has recently been investigated in detail as a reservoir rock [4] [5].The Sarvak Formation attains about 800 m thin, and medium to thick carbonate layers in thickness in its type section (Bangestan Mountain in vicinity of Izeh area).It conformably overlies the Kazhdumi Formation and unconformably covers by the argillaceous Gurpi Formation [3].
There are several investigations on the Sarvak Formation sedimentary and reservoir properties [6]- [12].The present study has been focused on the eight wells with special emphasis on the lithotypes analysis, sedimentary environments, and reservoir properties evaluation of the Sarvak Formation.

Material and Methods
According to lithotype study, sedimentary environment, petrophysical interpretations and SeisWorks of the Sarvak Formation, eight wells in one of the south west field in Iran were studied.A total of 200 thin sections of these wells were analyzed.Thin sections were stained with Alizarin Red-S and potassium ferricyanide [13] to determine mineralogical and qualitative elemental composition of the studied carbonates.Carbonate microfacies is described based on [14] with the modifications of [15] classification.Facies belts and sedimentary models of [16] and [17] were also applied.Lithotypes of this oil field reservoir formation including Sarvak Formation have been studied and classified using all available facies data and relevant petrophysical interpretations of drilled wells and Paleo logs.The available Paleologs from appraisal wells and some scattered core analysis reports have been reviewed and used as the main source for the following lithotype classification of Sarvak reservoir layers.The following parameters were used for introducing the lithotype identification: • The main rock lithology, achieved by petrophysical interpretations and sedimentological core review.
• Facies associations, interpreted by Paleolog data and sedimentological core review.
Reservoir quality was identified by petrophysical interpretation, shale volume and porosity values.The sedimentary environment reconstruction and depositional model analysis have also been individually done in this paper for Sarvak Formations, on the basis of the lithotype distributions and sedimentary environment classifications.
The seismic data from this study field shows a significant lateral and vertical seismic heterogeneity along the different reservoir layers, caused by lateral and vertical lithofacies changes.Based on the available interpreted acoustic impedance profiles, the distinct heterogeneities occurred in the Sarvak carbonate successions, influenced by seismic channelized system and significant facies changes, caused by eustatic sea level changes.It is stressed that the AI index can be influenced by many seismic properties and rock volume contents.For instance, the shale volume has a big effect on the AI values, which causes noticeable decrease of the rock density.In such condition, the shale volume is consi-dered as one of the main criteria in order to classify the discussed lithotypes for each reservoir layer.One of the major observations in the interpreted seismic profiles is particular channelized feature, which is concentrated in the Upper Sarvak reservoir unit.

Lithotype Analysis
Lithotype classification of the Sarvak Formation in the study field has been performed on the basis of the available geological subsurface data, particularly paleolog interpretation.At least five facies types (lithotypes) were identified in the Sarvak Formation with particular rock properties, consisting of individual facies associations, shale volume, reservoir quality and petrophysical signatures.The discussed lithotypes were identified and followed in number of selected this field wells.The main facies association elements and relevant depositional settings have been interpreted by extracted Paleolog facies and fossil association data, which is related to depositional setting variations.It is attempted to define a lithotype with particular sedimentary environment and distinct reservoir quality, ranged from non to high values.There is a geological/petrophysical lithotype classification that should be completed and revised as the next step by adding reservoir data.The microfauna assemblages have also been reviewed in order to interpret the major sedimentary environment for each Sarvak reservoir zone.
The Sarvak reservoir interval is characterized by the following correlatable lithotypes: • Lithotype 1: Argillaceous muddy facies to fossiliferous (low to non-reservoir rock).
The lithotype 1 is characterized by muddy carbonate facies that has a small contribution in the Sarvak interval and is also classified as low to moderate reservoir quality.This facies was more developed in the upper and intra Sarvak intervals.The noticeable gamma ray radiation (due to clay contents) and low porosity index are the main criteria in order to distinguish this type of lithotype.
In terms of depositional setting, it is more related to the restricted muddy carbonate platform (lagoonal to peritidal settings), affected by noticeable karstification and consequent dissolution processes.

Core Review
In this paper, we used core data from number of wells represent a distinct relationship between the set of porosity and permeability relationship in the Upper Sarvak unit that was cored in the above mentioned wells, representing different relation and showing a varied types of porosity distribution (Figures 6-9).The following porosity/permeability cross plots indicate that the reservoir characteristics and relevant porosity types and its distribution can vary in different reservoir zones and wells, affected significantly by sedimentary environment changes, local digenetic events, facies properties and vertical/lateral heterogeneity.
In well A, core review represents that the cored interval is placed around Sarvak zone-3 showing the high porosity rock (20% -25%) and moderate to low permeability (between 1 to 10 md), representing a carbonate stack with development of vugular disconnected porosity, caused by leaching digenetic process (Figure 6).The well B, core review represents that the cored intervals are placed around Sarvak zones-3 and 4 showing an ascending relationship between porosity and permeability, representing a carbonate stack with development of connected interparticle porosity, caused by accumulation of the Rudist debris with considerable interpores, accompanied with subsequent meteoric digenetic process (Figure 7).The well C, core review demonstrates that the cored intervals are placed around Sarvak zones-3 and 6 illustrating two different data communities in terms of porosity and permeability relationship, representing two carbonate stacks with different type of reservoir characteristics and porosity type.
Based on this cross plot, Sarvak zone-3 is classified as the best reservoir quality and the Sarvak zone-6 ranked as moderate to low reservoir quality (commonly the rate of permeability is less than 1 md), enhanced by small size connected pores and k/phi ascending relationship (Figure 8).The well D, core review reveals that the cored intervals are concentrated around Sarvak zones-2, 3 and 4 illustrating two different data communities in terms of porosity and permeability relationship, representing two carbonate stacks with different type of reservoir characteristics and pore type.Based on this cross plot, Sarvak zone-3 can be considered as the best reservoir quality with dominant connected porosity and large size of vuggy pores, Sarvak zone-4 indicates more heterogeneity that mixed by a part of zone-3 data showing disconnected vugular porosity, the zone-2 is also placed as low to non reservoir quality (Figure 9).There are some figures of the selected core images (Figure 10) from Sarvak cored intervals in the study cored wells, representing different litho types, pore types and direct influence of the meteoric diagenesis to improve the reservoir quality.There are some figures illustrating a comparison between the core intervals and the relevant reservoir characteristics that have been carried out on this project in some selected study field cored wells (Figure 11 and Figure 12).

Lithotype Heterogeneity
The achieved results represents that the Sarvak lithotypes can be followed roughly across the study Field and adjacent areas.There are some lateral heterogeneities particularly in the upper Sarvak interval, which has occurred by sedimentary environment fluctuations.One of the main important sedimentary fea-

Depositional System
The following schematic depositional models can be proposed for the Sarvak reservoir formations in the study Field on the basis of the vertical and lateral distributions of the lithofacies types.The introduced depositional models have been proposed according to the review of all geological data from the cored wells and available Paleolog interpretations.Their depositional systems have been interpreted and reviewed, based on these major depositional settings as below: • Coastal setting: different types of sandstones and reddish claystone.
• Inner ramp/shelf setting: shallow marine carbonate with restricted microfauna and more mud-supported facies and partly dolomitized.

Conclusions
There are geological/petrophysical lithotypes as Dunham classification that have been revised by received core data.There are distinct affinities between defined lithotypes in terms of facies type, main components and the relevant reservoir Figure 1.AI seismic profile among the number of wells in the study field.

Figure 2 .
Figure 2. Channelized features in one of the study field seismic profile.

Figure 3 .
Figure 3. Sarvak channelized Rudist bearing facies type in the study field.

Figure 6 .
Figure 6.Core porosity and permeability cross plot in the Sarvak Fm. (zone 8), well A.

Figure 11 .
Figure 11.A comparison between cores and log analysis, Upper Sarvak interval, zone 3, heavily oil-stained with large rudist debris.

Figure 13 .
Figure 13.Patchy distribution of bioclastic build-ups in the Sarvak Formation, Abadan Plain region.

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Mid ramp/shelf setting: bioclastic grain-supported facies with varied types of bioclast and large debris of the reef builders such as Rudist and Corals.• Outer ramp/shelf setting: bioclastic carbonate with a mixture of mud-supported and grain-supported facies, more diverse open marine microfauna and lesser digenetic impacts.• Hemipelagic setting: a mixture of benthic and planktonic microfauna, increasing of shale volume, marly facies.• Pelagic setting: planktonic microfauna are dominant with noticeable volume of the clay contents, marly to argillaceous facies.The vertical and lateral depositional systems interpretation for each reservoir formation has been done by use of Paleolog bearing wells in the study Field that are shown in brief in the Figure 15 and Figure 16.

Figure 15 .
Figure 15.Schematic depositional model of the upper Sarvak interval in the study field.

Figure 16 .
Figure 16.Schematic depositional model of the upper Sarvak interval in the Abadan Plain area.