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Chaman-Bid Formation (Upper Bajocian-Lower Oxfordian) in Kopet-Dagh Basin is an important gas source rock in Khangiran area. This Formation is mainly composed of black to dark grey limestone and calcareous marl with thin interbeds of shale as well as fossiliferous and oncoidal limestones. The petrographic studies showed that the Chaman Bid Formation consists of nine carbonate and one siliciclastic lithofacies. Carbonate lithofacies include fossiliferous micrite/ mudstone–wackeston with floated skeletal debris in matrix, biomicrite/ wackestone with calcisphers and sponge spicules, laminated biomicrosparite / wackestone–packstone, oolitic and intraclastic biomicrite / wackestone, intraclastic biomicrite/wackestone, echinoderm biomicrite/wackestone–packestone, echinoderm intraclastic biosparite/ grainstone, biomicrudite/floatstone and oncoidal biomicrudite / floatstone–rudstone.
Based on field observation and petrographic studies, these lithofacies are related to three facies belt including outer shelf (Lithofacies A1-A6), shoal (Lithofacies B) and inner shelf (Lithofacies C1-C2).
Sea level change curve showed that the sediments of Chaman Bid Formation in the study area were deposited during nearly one complete transgressive cycle and several incomplet shallowing upward cycles.