Shale gas reservoirs are characterized as organic-rich depositions with extremely low matrix permeability and clusters of mineral-filled ‘‘natural’’ fractures. Shale gas reservoirs are composed of fine-grained sedimentary rocks with permeability in the nano scale. Natural gas can be stored in shale reservoirs as free gas in nano pores and fractures, adsorbed gas on the surface of nano pores, and dissolved gas in the kerogen. Many geologic and reservoir properties such as depositional environment of shale, depth, structure, total organic content, and thermal maturity are used to provide an overview of the geologic characteristics of the major shale gas and shale oil formations. Also, these parameters can be used to select the shale gas basins and formations which are worth exploiting. An important factor among these parameters is the depositional environment of the shale (marine or non-marine). Marine-deposited shales tend to have lower clay content and tend to be high in brittle minerals such as quartz, feldspar, and carbonates. Brittle shales respond favorably to hydraulic stimulation. Shales deposited in non-marine settings (lacustrine, fluvial) tend to be higher in clay, more ductile, and less suitable for hydraulic stimulation.