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Introduction

Carbonate and siliciclastic deposits are the main hydrocarbon producer successions in the Arabian plate and SW Iran. Although most of Iran’s oil production comes from carbonate reservoirs in the Oligo-Miocene Asmari Formation and the Albian – Campanian Bangestan Group, the Siliciclastic reservoirs such as Kazhdumi, Gadvan, and Razak formations host the second oil and Gas reserves in Iran. Most Iranian hydrocarbon fields were located in the Zagros fold belt and the Persian Gulf during the Permian to Miocene geological time. Some of the main hydrocarbon fields in SW Iran and the Persian Gulf are shared filed and the estimation of the shared sector is important for knowing reservoir characteristics and hydrocarbon in place. To that, the geological history and setting are powerful keys for petroleum geologists to know the stratigraphical continuity and structural trends. The Conceptual Geological Model (CGM) contains and represents geological knowledge which are including the history of strata during the geological time. This model includes essential information regarding the structural, stratigraphical, and sedimentological that were used in reservoir characterization and interpretation. In this study, we mentioned the necessity of CGM for all disciplines in upstream studies, especially for petroleum geologists both in the study and operation sectors with an example in shared fields.

The major challenges in building the CGMs are the availability of subsurface information, data from different disciplines with the variable scale of resolution (micro to mega), lack of enough geological knowledge, and proper data integration. In this procedure, the main common uncertainties in the geological studies are the limited data and its quality, multiple possibilities of the geologic interpretations, reservoir geometry, areal extent, and thickness variations which lead to uncertain reservoir characterization.

Without a realistic geological framework and an appropriate Geological Model workflow, no 3D reservoir model can be used reliably as a predictive tool for well location definition, geosteering, hydrocarbons In-place, recoverable resources, and production forecasts.

This study conduces to the comparison of two main reservoir succession in the Carbonate and Siliciclastics system that describes in the following paragraph.

Summary

South Pars, Farzad and Azadegan fields were subjected to using the CGM for the reconstruction of the shared sector. The necessity of a reliable model that includes the shared sector in these fields is important for new operational issues and reservoir management. In point of Geological overview, the influence of carbonate and siliciclastic successions were highlighted in this study.

1. Carbonate Depositional System;

The carbonate reservoir is one of the main hydrocarbon reservoirs in Iran. Most of the oil and Gas reservoirs have been founded in this type of succession. Two main depositional carbonate sequences were highlighted in the reference text as a Ramp and Shelf System. The differentiation between these two types of Carbonate platforms is characterized by shelf edge break in comparison with Ramp System. A carbonate ramp is a gently dipping sedimentary surface on the sea floor. The facies belts are controlled primarily by energy levels (fair-weather wave base and storm wave base), variations in ramp topography, and material transport by storms, waves, and tides. The depositional slope gradient from the shallow-water shoreline or lagoon to the basin floor is of the order of a few meters per kilometer and usually less than 1°. Consequently, when we work on this type of reservoir succession, in the upstream studies and operational point of view, we do not have any significant change in terms of thickness and facies association.

Kangan-Dalan Reservoir as an example from Carbonate Ramp System

The Middle Permian to Early Triassic Dalan- Kangan Formations (the lithostratigraphic equivalent of the Khuff Formation) comprises a second-order transgressive-regressive carbonate sequence with a major transgression within the Lower Dalan, a highstand located around the base of the Upper Dalan, and a sequence boundary at the top of the Kangan. The Kangan and Dalan Formations are lithologically similar and difficult to distinguish at outcrop, and in the subsurface form a continuous unit in terms of reservoir pressures and properties (Motiei, 1993). The formations are dominated by shallow-marine carbonates deposited diachronous during a transgression from the north over the NE Arabian Plate. the Kangan and Dalan carbonates and evaporites were deposited in an epeiric carbonate platform following a major sea level rise in the Permian. The Nar Member consists of massively bedded anhydrite, anhydrite dolomite, and dolomites, and acts as an efficient seal for upper Dalan reservoirs. The Permo-Triassic carbonate reservoir in the supergiant South Pars-North dome field is an example of this system.

Stratigraphical and Structural Influences; South Pars and Farzad Shared Fields

South Pars and Farzad Fields are two types of example of carbonate successions that contains the effects of Stratigraphical and structural activities. As we described earlier, in the carbonate system, especially in Ramp System which is deposited in the Permo Triassic succession, no major fluctuation and stratigraphical thicknesses have been proposed, so, generally, based on CGM’s model, the shared sector of this structure has been prepared. This model consequently applicant for Farzad Structure. But the main concerns of this structure are the effects of Structural growth and circulation due the geological time. The Farzad structure is clearly developed along an E-W structural alignment which is also an elongated doubly plunging anticline with a negative gravity anomaly due to basement faulting and rupture of Hormuz Salt beds caused deep-seated diapirism and consequent structural growth. The stratigraphical framework reconstruction and isochore thickness maps were used to estimate the share sector with respect to CGM’s Model.

1. Siliciclastic System

Siliciclastic input created some of the most prolific hydrocarbon reservoirs on Earth. The siliciclastic reservoirs in the Arabian plate, host’s the world largest offshore oilfield, Safaniyah in Saudi Arabia and second largest oil field Burgan in Kuwait. In south west of Iran and Zagros fold belt, the Gadvan Formation and their equivalent Burgan, Nahr Umr are significant siliciclastic reservoir which is under exploration and development stage.

Gadvan Sandstone member, Azadegan-Majnoon Field

Understanding the controls on their occurrence can be insightful to predict their distribution and significance as key elements of petroleum systems. Moreover, understanding the regional controls on siliciclastic input can lead to insights for new exploration plays.

The Azadegan Oil filed located in SW of Iran close to Iraq Border line. The Siliciclastic member of Gadvan Formation are nominated for new exploration and development drilling campaign.

This filed is located in In the Mesopotamian Basin. Based on CGM’s Model (Distal or proximal locations) and the effects of main controlling factor containing Tectonic, climate and eustatic lowstand through the deposition of this strata, the prediction of Sandy layers through the Abadan plain were distinguished and predicted. This model shows the gradual transition from siliciclastic sandy layers to open marine carbonate from west to east.

The general trends show a clear reduction in the proportion and thickness of siliciclastic shales from more proximal locations in the west to more distal locations in the east.

This overall thickness distribution of siliciclastics member provides further evidence that their provenance was from the west or southwest.

Conclusions

The conceptual Geological Model is a critical role in subsurface studies. this model prepared based on integration of geologic and engineering data/interpretation studies of geological features from the basin scale to reservoir or field scale. Therefore, CGM is a critical input for the structure and reservoir characteristics distribution in 3D static reservoir model building and allows for bridging the gap between reservoir geology and 3D numerical reservoir modelling activity. This model also could be used as a guideline model in operational campaign.

Acknowledgements

The authors would like to thank PPL for permission to publish this manuscript.

References

• Flugel, E. (2010) Microfacies of carbonate rocks, Springer-Verlag, Berlin, p. 976.
• Motiei, H., 1993, Geology of Iran. The stratigraphy of Zagros: Geological Survey of Iran, Tehran [in Farsi].