Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

W.L. Moore


The Mississippian Midale subinterval is the basal unit of the Ratcliffe interval of the Madison Group. The Midale subinterval, an important petroleum producing unit in North Dakota, is a dolomitized limestone which is approximately 40 feet thick. A detailed study of 17 oil fields in north central North Dakota was conducted to determine the petroleum geology of the Midale subinterval. The study was done with the use of 563 mechanical well logs and core samples from 10 wells.

The anhydrite and dolomite units of the Rival subinterval, which underlie the Midale, and the Ratcliffe interval, which over lies the Midale, reflect the restricted conditions characteristic of a penesaline stage in a major evaporite cycle. The limestone unit of the Midale subinterval represents the unrestricted condi tions characteristic of a minor normal marine cycle within this otherwise penesaline stage.

The Midale limestones were dolomitized by magnesium-rich brines that formed under the restricted, penesaline environment that was present during early Ratcliffe time. This increased the porosity of the Midale and made it a good host rock for the accumulation of petroleum. During or after dolomitization, some of the Midale sediments were replaced or infilled by anhydrite which created the permeability barriers necessary for petroleum entrapment. The sulfates needed for the formation of anhydrite in the Midale were probably derived from brines forced from the overlying anhydrite unit by compaction. The overlying anhydrite unit upon compaction hardened to form a cap rock which served to prevent the vertical escape of petroleum. Gentle structural upwarps appear to have had a minor influence on the location of a few fields.

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