The Lower Cretaceous carbonates succession -Shurijeh and Tirgan formations- in west Kopet-Dagh basin -NE-Iran- was examined to study the diagenetic modifications. Various diagenetic processes were controlled by the original facies characteristics, carbonate mineralogy, climatic condition, sea-level fluctuations and burial history.Four types of dolomite are identified including 1: microcrystalline matrix replacement dolomite -xenotopic, D1-, 2: fine to medium euhedral-to subhedral crystalline matrix replacement dolomite -D2e-, 3: fine to medium euhedral-to subhedral porphyrotopic crystalline matrix replacement dolomite -D2p-, and 4: fracture filling, euhedralto subhedral dolomite -D3-. All dolomite samples were analyzed for carbon and oxygen stable isotopes. The microcrystalline dolomites -xenotopic, D1- define a relatively wide range of δl8O values from 0.3 to -4.52‰V-PDB -pee dee belemnite-, and narrower range of δl3C values from 1.96 to 3.20‰V-PDB, which are slightly depleted compared with the original isotopic signatures for the Lower Cretaceous marine dolomites. The planar-e dolomites -fine crystals- in the matrix show δl8O and δl3C values ranging from -2.17 to -5.33‰ and 3.20 to 3.50‰ V-PDB respectively. Dolomitized orbitolinid and euhedral mosaic dolomite crystals with planar boundaries -D2e, medium crystals- show depleted δl8O and δl3C values ranging from -8.12 to -4.11‰ and 0.3 to 3.34‰ V-PDB respectively. Fine crystal dolomites -D2e- formed during early burial and could be considered as recrystallized forms of D1 dolomite. Fluids responsible for the formation of medium crystals dolomites -D2e, medium- and orbitolinid filled dolomite -D2e-, suffered higher temperature due to increasing the burial depth. Consequently, heavier δl8O values of finer dolomite crystals and elevated temperatures to lighter δl8O values indicate in higher burial depths that led to coarser euhedral crystals during dolomitization.

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