THE UPPER MISSISSIPPIAN GODDARD SHALE IN THE EASTERN ANADARKO BASIN: NEW INSIGHT INTO BLACK SHALE DEPOSITION IN THE APPALACHIAN-OUACHITA FORELAND

The Goddard Shale (Serpukhovian) is among the youngest parts of the Barnett-Fayetteville-Floyd foreland black shale succession, which extends from the Texas Promontory to the Alabama Promontory. The Goddard was deposited in the southeastern Anadarko and Ardmore Basins in Oklahoma, and this research focuses on the Purdy sub-basin of the Anadarko Basin, where the Goddard is a prolific shale oil reservoir. The Goddard Shale reservoir is unusual in that it is highly water-reactive, containing an average of 18% swelling clay minerals. Understanding the geologic factors that resulted in the Goddard Shale can inform the development of other Mississippian clay-rich reservoirs, which have giant untapped potential.

The Purdy sub-basin is at the southeastern end of the Anadarko Basin adjacent to the Carter-Knox reverse fault, which includes a basement-cored Wichitan anticline that separates the Anadarko Basin from the Ardmore Basin. The basin functioned as a miniature foreland basin during Goddard-Springer deposition, which facilitated accumulation of 150 ft of black shale, which is petrologically organic-rich siltstone. The Goddard overlies a disconformity and is in facies relationship with synorogenic Springer shale and sandstone (Chesterian-Bashkirian).

The facies bounding the Goddard Shale reservoir were deposited in shallow marine and shoreline environments. The reservoir siltstone has ~ 5% TOC, 10% porosity, and exceptional permeability of 2.3 µD; it is a highly oil-prone facies. Sedimentary structures indicate that the Goddard and bounding strata were deposited in a tidal embayment. 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.