geochemical and prtrographical study of dolomite facies in the dalan and kangan gas reservoirs in the south of iran

in carbonate systems, dolomites often form the best reservoirs .the dolomitized part of dalan(upper permian) and kangan(lower triassic) formations makes up the essential part of gas reservoirs in the south of iran. the dalan and kangan formations were dolomitized to various degrees in the areas of south of iran and- persian gulf.in most cases, dolomitization enhanced porosity and permeability of the unit, leading to the formation of excellent hydrocarbon reservoirs. because dolomite is more resistant to compaction than limestone, dalan and kangan dolomite reservoirs remain porous and permeable to the depths more than 4 km in this area. the transition from dalan to kangan denotes a period of drastic changes in the deposition environment and the chemical milieu of the sedimentation basin. these changes are reflected by a marked positive excursion of carbon and oxygen isotopes from kangan to dalan formation. petrographic, chemical and isotopic studies of the dolomite in the kangan and dalan formations reveal that dolomitization was the result of several diagenetic events. the neomorphic alteration of these dolomites significantly modified their original chemical signatures. dolomite in the kangan and dalan forms in two stages by different mechanisms: in dalan formation dolomitization occurs in the early phase of syngenetic (penecontemporaneous) replacement of the pre-existing carbonate in an arid climate on a supratidal flat whereas   kasngan’s dolomite forms during deep burial