This paper provides the first empirical study of the market structure of the shale gas drilling industry in the United States. Modern shale gas drilling, which is a major revolution in the energy industry, was highly concentrated during its experimental stage, roughly from the early 1980s to the early 2000s, and has since become less concentrated, exhibiting a long tail of infrequent drillers. Nevertheless, even during the latter stage, the vast majority of shale gas wells have been drilled by a limited number of large independent oil and gas producers.
via The Market Structure of Shale Gas Drilling in the United States.