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Sedimentology, facies architecture, and diagenesis of the Middle Three Forks Formation - North Dakota, U.S.A.




Droege, Lauren A., author
Egenhoff, Sven O., advisor
Ronayne, Michael, committee member
Rocca, Monique, committee member

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The upper Devonian age middle Three Forks Formation in the North Dakota portion of the Williston Basin consists of a mixed carbonate-siliciclastic succession deposited in the mid- to inner-ramp portions of a low-angle epeiric ramp. Based on lithological and sedimentological characteristics, ten facies are recognized in the succession that are grouped into three facies associations (FAs). Laminated siliciclastic mudstones with silty dolostone intercalations (FA1) record deposition in the most distal, overall low-energy mid-ramp environment, whereas massive to well laminated clay clast-bearing silty dolostones (FA2) reflect sedimentation in a relatively high-energy inner-ramp setting. Mud-rich conglomerates with dolomite and clay clasts (FA3) occur in both inner- to mid-ramp settings throughout the study area, however these deposit types generally increase in abundance toward the basin margins. Halite pseudomorphs coupled with an overall low diversity of ichnofauna indicate that basin salinities were elevated at times during middle Three Forks Formation deposition. Bedload transport processes are the dominant mechanism that led to deposition of FA1 and FA2 strata, evident in laminated siliciclastic mudstones as well as current and wave ripple-laminated silty dolostones, whereas mass gravity transport in which laminar flow processes dominated led to deposition of FA3 rocks. Recurring stacking of FA1 and FA2 strata reflect changes from low- to high-energy regimes and define up to six coarsening-upward parasequences throughout the study area, generally 1.0-5.5 m in thickness. In contrast, varying amounts of FA3 deposits throughout the succession define five distinct stratigraphic intervals, herein termed Intervals 0-4, which reflect fluctuating climate conditions from overall humid to arid settings throughout the course of middle Three Forks Formation deposition. Intervals 0, 2, and 4 are generally characterized by high amounts of FA3 deposits, while Intervals 1 and 3 contain very few, if any, FA3 intercalations into overall well-preserved FA1 and FA2 beds. The diagenetic history of the middle Three Forks Formation is complex and involves many different phases including: at least eight stages of dolomite, both replacement of a calcite pre-cursor and dolomite cements that precipitated directly from aqueous solutions; anhydrite precipitation and replacement of dolomite; silicification of dolomite rhombs resulting in diagenetic quartz; pyrite framboids, concretions, and disseminated euhedral pyrite crystals; micro-intercrystalline, intercrystalline, intracrystalline, moldic, and linear dissolution porosity types. Core descriptions, porosity estimation based on point-counting of forty-seven thin sections, isopach maps of potential "pay zones" throughout northwest North Dakota, as well as a complete lack of hydrocarbon residue in cores, suggest that reservoir quality of the middle Three Forks Formation is overall poor in this area, and few, if any, hydrocarbons have migrated to the middle Three Forks Formation from the overlying Bakken Formation.


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