Effect of diagenetic processes on the distribution of reservoir and non-reservoir units in the Asmari Formation, Ahvaz Oilfield (Wells No, X, Y, Z)

Document Type : Original Article


1 Department of Geology, Faculty of earth Siences, Shahid Beheshti University, Tehran, Iran

2 Department of Geology, Geology Department of National Iranian South Oil Company, Ahwaz, Iran


Introduction: The Asmari Formation in southwest Iran is the youngest and most important hydrocarbon reservoir in Iran. Asmari Formation in Ahvaz oil field in the studied wells consists of limestone, dolomite, dolomitic limestone, sandy dolomite and sandstone, siltstone and shale. In some cases, similar facies are subjected to similar diagenesis processes, but most of the time due to the high impact of diagenetic processes the porosity-permeability distribution is completely reversed. Therefore, it is necessary to carry out detailed diagenesis studies and investigate the effects of diagenesis on reservoir changes and evolutions in all systematic and fundamental studies of carbonate reservoirs and hybrid reservoirs (Lucia, 2007; Ahr, 2008).
Materials and methods: This study was conducted on wells No. X, Y and Z of Ahvaz oil field. In this study, 1100 thin sections of the drilling cores of the mentioned wells were studied. The types of facies and their governing diagenesis processes have been identified using microscopic thin-section studies of drill cores. Types of microfacies were named based on Dunham's method (Dunham, 1962). In order to distinguish limestone from dolomite, all thin sections were stained with alizarin red solution according to Dickson's method (1965). From digital well and porosity data (resulting from helium gas core testing) and core permeability (resulting from mercury saturation core testing) in order to distribute different units (reservoir and non-reservoir) in the studied wells and drawing the two-dimensional model of their distribution using Geolog, Excel software was used. This field was studied using flow hydraulic units in any rock type.
Results and discussion: Most of the diagenesis processes of the Asmari Formation in the studied field have occurred under conditions where the diagenesis environment was affected by the sedimentary environment. Dissolution and dolomitization, have played the greatest role in increasing the reservoir quality. These two processes, the development of which depended on the sedimentary environment, are the most important diagenetic processes controlling the reservoir quality. Dolomitization in the Asmari reservoir is very early and late diagenetic processes that and has had a dual effect (constructive and destructive) on the reservoir quality of the Asmari Formation, raising the reservoir quality in the limestone and in the Sandstones are in the form of cement Anhydrite cement is the most important diagenetic process with a negative impact on reservoir quality. The expansion of coarse-grained clastic facies are the best reservoir-prone facies, and the diagenesis processes of these facies have been mainly affected by their sedimentary environment. In the studied field, the higher the lateral expansion and thickness of the sandstone facies, the better its reservoir quality. The contribution of environmental conditions in controlling the reservoir quality of carbonate facies is less than that of clastic and mixed facies.
Conclusion: In the survey, eight hydraulic flow units were identified, the best of which are the 7th and 8th hydraulic flow units. These hydraulic flow units correspond to the rock type of loose sand. The worst hydraulic flow units 1 and 2, which corresponds to the dolomudstone rock type.


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