The Goddard Shale (Chesterian, Serpukhovian) contains an exceptionally productive unconventional reservoir that was deposited in the southeastern Anadarko Basin. The Goddard Shale reservoir is unusual in that it is highly water reactive, containing an average of 18% swelling clay minerals. Understanding the mix of geologic factors that have facilitated the productivity of the Goddard Shale reservoir can potentially unlock other clay-rich reservoirs where realizing economic production has been problematic. The facies bounding the Goddard Shale reservoir contain physical and biogenic structures suggesting marine and shoreline environments. The reservoir is an organic-rich, argillaceous siltstone with porosity of approximately 10% and exceptional permeability of approximately 2.3 µd; it is an oil-prone facies that was deposited offshore, perhaps in an estuarine embayment in the southeastern part of the basin. The base of the reservoir marks a disconformity within the Goddard Shale, and the upper part of the reservoir is in complex facies relationship with upper Goddard and lower Springer Formation strata, which are interpreted as part of a tidally influenced shore-zone complex. Sedimentologic, paleontologic, and geochemical data indicate that the reservoir was deposited in mainly suboxic to anoxic environments. Clay mineralogy is the primary control on reservoir quality. Flocculation and settling of clay within a pycnocline, with a strong halocline, helped concentrate quartz and organic matter in the reservoir siltstone, and the deposits rich in ductile swelling clay confine the reservoir hydraulically, thereby facilitating effective well completions.