Effects of dolomitization on porosity – Permeability distribution in depositional sequences and its effects on reservoir quality, a case from Asmari Formation, SW Iran

Abstract

The Oligocene-Miocene Asmari reservoir is dominated by heterogeneity in various aspects, especially porosity and permeability caused mainly by dolomitization. The consequences of petrographic studies of core samples from five cored wells lead to four depositional environments to be identified: inner ramp, mid ramp, outer ramp and basin. This succession is arranged in six third-order depositional sequences. The formation has undergone complex diagenetic alteration represented mainly by multiple generations of dolomites. Dolomite types comprise (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). 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. D4 was precipitated from hot, saline fluids during deep burial. Fine-to medium-crystalline, fabric-retentive dolomites occur in the upper part of the formation, whereas coarse-crystalline fabric-destructive dolomites mostly occur in the middle and lower parts. This causes distribution of porosity and permeability in the depositional sequences, thereby affecting the reservoir quality. Accordingly, dolomitization resulted in enhanced porosity and permeability and improved reservoir quality.