Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

F.D. Holland, Jr


About 250 thin sections of core samples from the Midale subinterval in the Bottineau-Renville Counties area of North Dakota (T161N-T164N and R78W-R86W) were studied petrographically. The Midale subinterval is the basal section of the Ratcliffe interval of the Madison Group of Mississippian age. In the area of study it is composed of interbedded fine grained limestones, pelletiferous and oolitic limestones, and primary anhydrite. Dolomitization of the limestones in varying stages is common and the dolomitized limestones are usually fine grained. Course crystals of secondary anhydrite are common as vug and fracture filling and as a replacement of the carbonates.

The Midale carbonates were deposited as part of a major evaporitic cycle which included the underlying evaporates of the Rival subinterval and the overlaying evaporites of the Ratcliffe interval. Dolomitization of the limestones took place shortly after deposition by the circulation of magnesium-rich brines during the deposition of the interbedded and overlying evaporites. Following dolomitization and lithification, secondary anhydrite filled vugs and fractures and replaced calcite and dolomite.

Porosity of the Midale carbonates is of the primary intergranular and intercrystalline types. Primary porosity was formed at the time of deposition of the carbonates and was reduced by compaction and by vug filling dolomite and anhydrite. Secondary intercrystalline porosity is not well developed because the dolomites are fine grained and still retain intercrystalline calcite and argillaceous or organic material.

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