Date of Award

1987

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

H.J. Fischer

Abstract

Rocks of the upper Mission Canyon and lower Charles Formations (Mississippian) in central Billings County, North Dakota consist of interbedded limestones, dolostones, and anhydrites which were deposited in a shallow epeiric sea. This study was limited to the upper Mission Canyon and lower Charles Formations of Treetop and Whiskey Joe fields, located along the Billings anticline in central Billings County, North Dakota. Close examination of ap~roximately 260 metres (850 feet) of upper Mission Canyon and lower Charles Formations core in the study area resulted in the separation of rocks into six lithotypes: 1) echinoderm wackestone, 2) dolomudstone, 3) neomorphic wackestone, 4) intraclast bioclast wackestone packstone, 5) laminated mudstone, and 6) anhydrite which represent sublittoral, littoral, and supralittoral depositional environments.

Numerous diagenetic processes were active in the upper Mission Canyon and lower Charles Formation rocks. Processes include micritization, compaction, dolomitization, cementation, replacement, fracturing, neomorphism, and pressure solution.

Pore types present within rocks of the upper Mission Canyon and lower Charles Formations include two primary pore types: interparticle and intraparticle, and four secondary pore types: intercrystalline, moldic, vuggy, and fracture.

Intercrystalline pores were examined in detail to determine the geometry of pores and pore throats. The three pore shapes found were polyhedral, tetrahedral, and interboundary-sheet. Polyhedral-shaped pores are the most common pore shape in the study area.

In the study area rocks of the upper Mission Canyon Formation were separated stratigraphically into lower, middle, and upper porosity zones. A comparison of porosity isopach maps of each porosity zone indicates that toward the north-northwest, porous zones tend to parallel the overlying anhydrite, and that the thickest accumulations of porous rock are developed progressively higher stratigraphically within each porosity zone. Progradation of littoral and supralittoral environments and early dolomitization may have been responsible for the dist ribution of porosity in the study area.

The diagenetic history of rocks of the upper Mission Canyon and lower Charles Formations represents a complex system of post-depositional processes. Pore fluids present during eogenetic diagenesis reflect near-surface water conditions and include hypersaline, marine and meteoric waters. Eogenetic diagenetic zones include hypersaline vadose and phreatic, sublittoral stagnant and agitated, meteoric, and meteoric and marine mixing diagenetic zones. Mesogenetic diagenesis resulted from a combination of local connate pore waters, fluids released during pressure solution, and an influx of fresher water.

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