Date of Award
Master of Science (MS)
The Poplar interval of interbedded evaporites and carbonates forms the uppermost part of the Mississippian Madison Forr.1ation in western North Dakota. A detailed mechanical well log study of the Poplar interval in North Dakota was conducted to interpret the Poplar's regional geology and to attempt to locate areas with potential for petroleum production. Limestone, dolomite, anhydrite and salt were differentiated using mechanical well logs.
The complex facies of the Madison are subdivided into para-time rock units (the Poplar interval is one) on the basis of extensive thin anhydrite beds which are considered time-parallel.
The base of the Poplar interval is a wide-spread anhydrite unit. The top of the Poplar on the basin flanks is a thin anhydrite, whereas in the basin interior a thick massive salt overlying the thin anhydrite is considered to be the top. Salt deposits formed during periodic increases in the rate of basin subsidence account for most of the increase in thickness (176 feet on the basin flanks, to 687 feet in the central basin area) of the Poplar interval.
The anhydrites and salts of the Poplar interval were deposited in a saline environment caused by the restriction of a regressing sea. Periodic transgressions caused near normal marine conditions that resulted in the deposition of carbonates. Some anhydrites that may have been deposited in sabkhas resulted in linear down dip trends of thicks and thins in gross anhydrite in the basin and a general increase in anhydrite from the basin center to the basin margins.
Except where influenced by the Nesson Anticline and other structures in Divide and Billings Counties the Poplar interval generally conforms to the Williston Basin. The Nesson Anticline and a small anticline named Divide High in Divide County were actively positive, although subdued in Poplar time. An anticline in Billings County appears to represent a post-Poplar event. Pre-Mesozoic erosion had little effect on the present day topography of the Poplar.
Poplar exploration should be carried out primarily as a search for anticlines similar to that in the East Poplar Field in Montana that produces oil from the Poplar interval. Stratigraphic traps should also be considered.
Cook, C. W., "A mechanical well log study of the popular interval of the Mississippian Madison Formation in North Dakota" (1974). Theses and Dissertations. 61.