Clay minerals have different effects such as reducing effective porosity and permeability on the petrophysical properties of hydrocarbon reservoirs. Also, the presence of clay in parts of the well causes instability of the well wall. For this reason, the study and understanding of clay is very important in petroleum studies. Clay minerals have many industrial uses due to their special characteristics. The role of this mineral in the engineering behavior of building materials and rock mechanics cannot be ignored. The type and abundance of this mineral is very important in the petroleum industry and causes many problems in this industry. Although water based drilling has been widely accepted in recent years due to its cheapness, ease and no damage to the environment, the effect of water on the swelling of clay minerals (clay hydration) causes many problems in drilling operations. Also, the migration of fine clay particles is another destructive effect of these minerals in the petroleum industry. Although this mineral has many problems in the petroleum industry, its presence plays a role in the processes of oil formation, its preservation, hydrocarbon composition, and it can be useful as a catalyst in various stages of oil refining. In this paper, importance of clay minerals in the petroleum industry has been discussed.