Abstract

The Niobrara Chalk of late Cretaceous age is present throughout most of South Dakota, cropping out in a ring surrounding the Black Hills Uplift and along the Missouri and James Rivers in the southeastern part of the state, and subcrops over a wide area beneath glacial drift in all but the northeastern and southeastern corners of the state. It ranges in thickness from zero where it pinches out on older rocks beneath the drift and where absent in the Black Hills, to a maximum of about 300 feet (91 m) in southeastern Butte County. The formation consists of an upper chalky shale or marl unit ("Smoky Hill") and a lower more indurated limy shale and impure limestone ("Fort Hays"), and is recognized in cuttings as a white-speckled, light to dark gray or brownish-gray calcareous shale. The lower zone usually contains an abundance of prismatic calcite that represents thin coquina-like limestone layers made up of Ostrea and Inoceramus shells. The Niobrara in South Dakota conforms to the structural framework of the state, as controlled by the Precambrian basement rocks. The major structural parts of the region have been identified as the Black Hills Uplift, the Williston Basin, the Canadian Shield, the Sioux Ridge, the Forest City Basin, the Kennedy Basin and the Chadron Arch. Superimposed upon these regional structural features are numerous smaller structures suitable as traps for natural gas accumulations. Permeability of the formation is rather low, except in more limy zones which occur most frequently in the lower ("Fort Hays") part of the Niobrara.

The chalk lies at elevations ranging from about 1,500 feet (457 m) above sea level on the Sioux Ridge to about 5,000 feet (1,524 m) below sea level in the Williston Basin. The depth of burial is from several hundred feet in eastern South Dakota to about 3,000 feet (915 m) in parts of western South Dakota. No commercial production has yet been obtained from the Niobrara in South Dakota. However, numerous gas shows have been encountered in the Niobrara throughout the state over the years, indicating the possibility of widespread Niobrara gas accumulations.