The role of relative sea-level fluctuations on dolomitization of carbonate reservoirs Case study: the Oligocene–Miocene Asmari Formation

The Oligocene–Miocene Asmari reservoir is dominated by heterogeneity in various aspects, especially porosity and permeability caused mainly by dolomitization. The Asmari Formation has been deposited along a homoclinal ramp-type platform with a gentle slope divisible into an inner ramp, mid ramp, outer ramp and basinal settings. The distribution of dolomite through the Asmari carbonate platform in the Shadegan Oil Field is not uniform nor is it random. All 12 facies associations are variably affected by dolomitization and have influenced reservoir quality. Inner ramp facies associations are the most dolomitized  while, the mid- and outer ramp facies associations are moderate to least dolomitized intervals. Six third-order depositional sequences were recognized within the Oligo–Miocene succession. They are bounded by sequence boundaries (SB) with significant evidence of subaerial exposure, diagenetic alteration, oxidizing conditions, and an abrupt change in facies or facies bathymetry. The stratigraphic distribution of facies associations proves more dolomite percent is formed near the sequence boundary as well as in the high stand system tract (HST) deposits. The main dolomitization model of the Asmari carbonate platform is seepage-reflux. Thus, the highest dolomite percentages occur near the sequence boundary when the sea level was low in the shoreface facies (inner ramp), the lowest percentage and dominant fabric destructive dolomite in the offshore facies, and fabric selective dolomite near the maximum flooding surface (MFS) due to the slow rate of dolomitization and low volume of dolomitizing fluids.