In this research, a cross-section of the Gachsaran Formation in the Afrineh syncline (south of Lorestan) has been selected and sampled to understand the dolomitization model of this formation. This section, with a thickness of 380 meters, is located in 46 kilometers southwest of Khorram Abad and in the Zagros folded zone. The Gachsaran Formation in the studied area is situated on the Asmari carbonate Formation and is covered by the red clastics of the Aghajari Formation as a discontinuity. In this section, petrographic studies on 100 thin sections and geochemical studies on 17 samples show the significant diagenetic processes in the Gachsaran Formation, such as the extensive dissolution and dolomitization. In this section, four groups of dolomites were identified: microcrystalline dolomites (first type), medium crystal (second type), coarse crystal (third type), and very coarse-crystalline dolomites filling fractures (fourth type, Baroque). The dolomitization mechanism is considered to be the tidal environment for the first-type dolomites, the mixing zone of saline and fresh water for the second- and third-type dolomites, and the burial model for the fourth-type dolomites. The presence of extensive mold porosity, fresh water calcite cement equant, fresh water vadose environment cements, regular and bright sugar grain dolomites, as well as medium crystal dolomites (dolomicrosparites) during shallow burial are evidence of a mixed type dolomitization model in Gachsaran Formation deposits. .