Dolomitization models and related fluid evolution in the carbonate platform of the Asmari Formation

Document Type : Original Article

Authors

1 National Iranian South Oil Company, Ahvaz, Iran

2 Department of Petroleum and Sedimentary Basins, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran

3 School of Geology, College of Science, University of Tehran, Iran

4 Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran

10.48308/esrj.2023.232397.1186

Abstract

In this research, dolomitization models of the Asmari Formation with 363.9 meters thickness were investigated in the Shadegan Oil Field in the Dezful Embayment, Zagros Basin. In the Shadegan Oil Field, the Asmari Formation consists mainly of limestones and porous dolostones associated with sandstones, and shales. In the SG-11 well, The Asmari Formation with Oligocene and lower Miocene in age conformably overlies the marly and shaly Pabdeh Formation and is, in turn, conformably overlain by the evaporites of the Gachsaran Formation. The Oligocene-Miocene Asmari reservoir is dominated by heterogeneity in various aspects, especially porosity and permeability caused mainly by dolomitization. Therefore, dolomitization is one of the important diagenetic processes in the Asmari formation, which has played an important role in controlling the reservoir quality of this formation. This formation has undergone complex diagenetic alteration represented mainly by multiple generations of dolomites. Dolomite types includes (i) very finely-crystalline, fabric-retentive dolomite (D1), (ii) fine to medium-crystalline, fabric-retentive dolomite (D2); (iii) medium to coarse-crystalline, fabric-destructive dolomite (D3); and (iv) very coarsely-crystalline, non-planar saddle dolomite (D4). Petrographic and geochemical evidence, and especially interrelation of facies with dolomite distribution, are consistent with the dolomitization by five different mechanisms/models in the carbonate platform of the Asmari Formation in the Shadgan Oil Field. D1 substituted by micritic matrix in mud-supported facies before early compaction. D2 and D3 were formed by the reflux of warmer and more saline fluids during intermediate burial, and also by the recrystallization of D1.

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