Sedimentological and diagenetic impacts on porosity systems and reservoir heterogeneities of the Oligo-Miocene mixed siliciclastic and carbonate Asmari reservoir in the Mansuri oilfield, SW Iran

The Asmari Formation is largely affected by some diagenetic processes such as dolomitization, fracturing, and dissolution in the Mansuri oilfield. These features have commonly caused the development of different types of porosities, resulting in various pore throats throughout the reservoir rock units. In this study, the porosity system and pore throat radius were considered as the main characteristics to separate the reservoir rocks into different units with distinct petrophysical attributes. Consequently, two petrophysical methods that have the direct link with pore throat and geometry including pore throat radius at 35% of non-wetting saturation using Winland equation (R35) and hydraulic flow units using flow zone indicator (FZI) were employed to define different rock types (RT) based on their pore system. The results of the current study show that the contribution of dolomitizing fluids to precipitation of dolomite crystals and dissolution of precursor limestone compositions resulted in prevailing various porosities and pore types. This study reveals that reservoir quality decreases from rock units dominated by dolomitization and dissolution overprints (e.g., RT-1) due to improvement in connectivity between pore networks, while some particular diagenetic features such as cementation (evaporite and calcite) occluded pore throats and then decline reservoir quality in their host rock units (e.g., RT-4 and RT-5). Afterward, distribution of RTs, throughout reservoir zones proposes that the middle part of the reservoir demonstrates higher reservoir quality compared to the upper and lower part of it. Occurrence of selective (e.g. intercrystalline and intergranular) and non-selective (fracturing and vuggy) porosities in middle parts promoted reservoir quality, while domination of isolated pores (e.g., moldic and intraparticle) in upper part decreases reservoir quality. •Dolomitization, dissolution, and cementation are the major diagenetic processes.•Pore systems have direct relationships with depositional and diagenetic characters.•Intercrystalline and intergranular played the key role in the enhancement of reservoir quality.•Siliciclastic and dolomite units show the highest reservoir quality.