Abstarct: Qale-Nar oil Field is located in the northern part of Dezful Embayment, about 30 km north of Andimeshk city. The Qale-Nar anticline, with east-west trend, is positioned in the simply folded belt of Zagros. The Oligocene-Miocene Asmari Formation is the main reservoir and producing formation in this field. The Asmari Formation has been investigated for determining the geneses of dolomites within formation and their effects on reservoir quality. In this project, petrographic studies with polarized light microscope, field emission scanning electron microscope (FE-SEM) and point elemental analysis (EDS), as well as, temperature and salinity analyses of fluid inclusions and rare earth elements analyses by induced coupled plasma spectroscopy Laser mass spectrometry (LA-ICP-MS) were used. Petrographic and geochemical studies have led to the identification of four types of dolomites in the Asmari Formation, including, dolomicrite, dolomicrosparite, dolosparite and pore and fracture filling dolomite cements. The petrographic and geochemical evidences have been revealed that the dolomicrites (type I dolomites) were formed in the Sabkha or high-evaporation intertidal environment, simultaneously with sedimentation. The petrographic evidences are included the fabric, shape and size of dolomite crystals, presence of lamination and algal filaments, fenestral fabric, evaporative mineral casts, anhydrite nodules and scattered quartz grains, and the geochemical proofs are composed of temperature, salinity and concentration of rare earth elements. Formation of dolomicrites occurred at the temperature of 27 to 35 degree Celsius, with the dolomitizing fluid’s salinity of 11.50 %wt NaCl. In dolomicrites, separation of diagrams of rare earth elements, normalized to the standard reference of seawater, compared to the diagrams of dolomicrosparites and dolosparites, indicates completely different conditions for the formation of dolomicrites than other dolomites. The similarity of the pattern of these diagrams with the representative limestones and the positive anomaly of cerium confirmed formation of dolomicrites from seawaters in reducing condition of Sabkha environment. The mechanism of formation of dolomicrosparites (type II dolomites) in Asmari Formation is baxsed on the shallow burial model (eodiagenesis), by replacement or recrystallization. Dolomicrosparites generated in the shallow burial depth at 45 to 55 degree Celsius. The average salinity of the pore-fluid was 16.69 %wt NaCl. With increasing burial depth, during the late diagenesis stages (mesodiagenesis), dolomite formed as pore and fracture filling cement (dolomite type IV) as well as overgrowth cement on the replaced dolomite. Detailed petrographic studies with FE-SEM showed that dolosparites (dolomite type III) formed within two diagenetic stages. Initially, in the shallower burial depth, dolomite formed by replacement, and then, in the deeper burial depth, overgrowth cements precipitated on the replaced dolomite. These cements formed at 65 to 80 degree Celsius. The high salinity of dolomitizing fluid (17.14 %wt NaCl) is associated to the closed to semi-closed diagenetic system in the deep burial depth. The similarity of the pattern of sea water normalized, rare earth element diagrams with the representative limestones, positive anomaly of cerium and non-positive anomaly of europium show that the main source of dolomitizing fluids of dolomicrosparites and dolosparites were from the connate water in the reducing condition of deep burial and it was not affected by hydrothermal waters. In the Asmari Formation, dolomitization, dissolution and fracturing are the main diagenetic factors increasing the reservoir quality. Since the hardness and brittleness of dolomites are higher than limestones, dolomicrites as primary sediments, have been affected by tectonic forces and thus fractures, which increase the permeability of reservoirs, are more abundant in this type of dolomite. In dolomicrosparite, the formation of dolomite by replacement of alochems or micritic matrix and also recrystallization of dolomicrites has increased the porosity and permeability. However, in this type of dolomite, due to presence of very fine grains (micrite and dolomicrite), there are less homogeneity and isotropy compare to dolosparites. Pervasive dolomitization and presence of coarse, euhedral dolomite crystals in dolosparite accompanied by sucrosic texture, have resulted in increasing porosity, permeability and isotropy of reservoir. Therefore, the highest reservoir quality is belonging to the dolosparites in the middle part of formation, which is the main producing reservoir zone.