Control of climate, sea-level fluctuations and tectonics on the pervasive dolomitization and porosity evolution of the Oligo-Miocene Asmari Formation (Dezful Embayment, SW Iran)

The Asmari reservoir properties in various hydrocarbon fields across the Dezful Embayment (SW Iran) are modified by dolomitization. Petrographic and geochemical analyses of the dolomitized Asmari Formation units highlight their origin in the Bibi Hakimeh and Gachsaran oilfields. The petrographic studies revealed that dolomite in the Asmari Formation can be classified texturally (based on crystal sizes) into three main groups: (i) very fine crystalline (D1, 150 μm). Petrography, fluid inclusions, stable isotopes (δ13C, δ18O), radiogenic isotopes (86Sr/88Sr) and major and trace elemental analyses were used to interpret the origin and paragenetic sequence of each dolomite group. Pervasive dolomitization prevails in the upper part of the Asmari Formation where very fine and medium crystalline dolomites with partially and fully fabric-retentive and fabric destructive textures, replaced micrite and allochems of the platform-top facies, respectively. The parent fluids of the very fine dolomites originated from contemporaneous Oligo-Miocene seawaters produced in a sabkha environment. The fine to medium dolomites (D2) most likely shared the same seawater origin and relate to refluxing, sabkha-derived fluids. The medium to coarse crystalline dolomite has more depleted oxygen isotopes than the former, very fine to fine crystalline, dolomites suggesting a burial dolomitization origin. The estimated high salinities (≈19 to 20 wt% NaCl) together with compatibility of oxygen and strontium isotope data with evaporative seawater values, suggest Oligo-Miocene seawater was the parent fluid for precipitation of coarse and cement phase of the medium crystalline dolomites. This study reveals that reactivated basement faults with associated fractures have probably played an important role in convecting and heating evaporative seawater along their planes. This heated seawater provided the required fluid for recrystallization of coarser matrix and cement phases of medium (D2) and coarse (D3) dolomites at depth. Therefore, the interaction of climate, sea-level changes, and tectonics played a critical role in formation and development of pervasive dolomitization in the Asmari Formation. Additionally, dissolution (partly as a consequence of dolomitization) was one of the key processes during early diagenesis in the formation of different types of porosity such as biomolds, vugs and i