پديد آورندگان :
دارايي، مهدي نويسنده دانشجوي دكتري رسوب‌شناسي و سنگ‌شناسي رسوبي، دانشگاه تهران، ايران , , اميني ، عبدالحسين نويسنده استاد رسوب‌شناسي و سنگ‌شناسي رسوبي، دانشگاه تهران، ايران , , صديق، محمد نويسنده كارشناس ارشد زمين‌شناسي، شركت ملي نفت مناطق مركزي ايران، ايران , , نيك‌انديش، عباسعلي نويسنده كارشناس ارشد زمين‌شناسي، شركت ملي نفت مناطق مركزي ايران، ,
كليدواژه :
كيفيت مخزني , ماسه‌سنگي اهواز , دياژنز , سازند آسماري , عضو , فروافتادگي دزفول
چكيده لاتين :
Introduction
The Oligo-Miocene Asmari Formation is the most prolific reservoir rock of Iran, containing more than 90% of recoverable oil of the country. The formation is dominantly composed of carbonates, but with two distinct mixed lithologic members known as the Ahwaz Sandstone Member (SW Zagros) and Kalhur Evaporitic Member (NW Zagros). In a regressive trend, the formation overlies the pelagic-dominated Pabdeh Formation and underlies the evaporitic to siliciclastic-dominated Gachsaran Formation, in most places. This study investigates the role of depositional and diagenetic controls on the reservoir quality of the Asmari Formation, where the significance of the Ahwaz Sandstone Member gives mixed carbonate-siliciclastic nature to the formation.
Material & Methods
The study is based on sedimentological and petrophysical data from two wells of a field located in the Dezful Embayment, where the Ahwaz Sandstone Member is present, alongside with some comparable sedimentological data from NW Zagros, where the Kalhur Evaporitic Member is extended. A total of 600 red-stained thin sections, 198 blue-dyed thin sections, and 908 poroperm values were the main data included in this study.
Discussion of Results & Conclusions
Facies analysis shows the Asmari Formation in the studied area is composed of 11 facies, representing three depositional systems. Most of the area was occupied by a carbonate depositional system with a ramp physiography. Meanwhile, in SW Zagros, a marginal marine deltaic system prevailed, and in NW Zagros a tectonically driven evaporitic intrashelf sub-basin was created by a combination of arid climatic condition, sea-level fluctuations and tectonics. In these two sub-basins, the deposition of Ahwaz Sandstone Member and Kalhure Evaporitic Member occurred, respectively.
Based on the findings, the main diagenetic processes affecting the Asmari Formation are micritization, dolomitization, dissolution, cementation, compaction, and minor fracturing. Micritization is a common process in the shoal and lagoonal facies, leading to a more susceptible facies to the later dolomitization. Dolomitization is the most pervasive diagenetic process of the formation, most of which occurred due to early diagenetic evaporative models (seepage-reflux and sabkha dolomitizations). Dissolution is another early diagenetic event in the strata, which probably happened by evaporitic brines. This process has dominantly produced moldic pores in the formation. Cementation as the main porosity destruction process has taken place in variable mineralogies and fabrics. Dolomite cement is the most widespread cement, precipitated in early diagenesis. Anhydrite cement is the more effective cementation event, influencing the reservoir quality of the formation and normally is the latest generation of the process in the Asmari Formation, succeeding other generations, commonly early diagenetic pore-lining dolomite cement. The cement in most places is in the form of poikilotopic and has patchy distribution, but in some other spaces has occurred as pervasive and pore-filling cement with even distribution, leading to a notable reduction in porosity. Compaction has taken place as both physical and chemical ones in the formation. Fitted fabrics and stylolites are the most prominent evidence for the chemical compaction, which are traceable in the shoal and grainy facies.
According to the results, the reservoir quality of the siliciclastic-dominated part of the formation was primarily controlled by depositional factors, especially lithology, but the carbonate part by diagenetic factors, particularly compaction, dolomitization, dissolution and anhydrite cementation. Generally, owing to moldic and vuggy porosities created by diagenetic dissolution, the carbonate-dominated part of the Asmari Formation has high storage capacity, where the pores make a separate network, because of the lacking of connection between them. Consequently, dissolution has had no effect on permeability and thus on flow capacity. In the strata, high flow capacity is extremely related to fracturing. In other words, where fracturing has occurred, the capacity is high and vice versa. The main porosity destruction process in the field is anhydrite cementation, which is more common in the oolitic shoal facies.
Keywords: Asmari Formation, Dezful Embayment, Ahwaz Sandstone Member, Diagenesis, Reservoir Quality
