Reservoir quality of the Upper Three Forks formation, Fort Berthold Indian Reservation, Williston Basin, North Dakota, U.S.A

This thesis uses a five well database from the Fort Berthold Indian Reservation within the Williston Basin of North Dakota to document the reservoir quality of the Late Devonian upper Three Forks Formation.;There are five facies observed in the upper Three Forks: dolomitic claystone, brecciated, tan dolostone, interbedded claystone and dolostone, and chaotic. Based upon sedimentary structures and depositional interpretations, three facies associations were identified: A) shallow shelf marine, B) supratidal flat, and C) storm dominated mixed flat. Shallow shelf marine deposits demonstrated structure less bedding with rare storm influence. Supratidal flat deposits display abundant syndepositional anhydrites which indicate arid conditions during deposition. Storm dominated mixed flat deposits display abundant episodic erosion surfaces, pinch and swell laminations, ball and pillow features, and hummock cross stratification. There is a depositional shift recognized in the upper Three Forks between facies association B and C. This depositional shift is controlled by intrabasinal allogenic processes in response to the Late Devonian transition of the Williston Basin. This upward stratigraphic allogenic shift is represented by a decrease in aridity in the facies of the upper Three Forks between facies association B and C. Also, high frequency vertical and lateral variability in facies indicates a response to local changes in eustacy and climate.;Ultraviolet light (UV) photographs demonstrate extremely localized and preferential hydrocarbon saturation throughout the facies of the upper Three Forks. The dolomitic claystone facies demonstrates poor UV fluorescence indicating lack of hydrocarbon saturation. The brecciated facies demonstrates highly localized UV fluorescence, with claystone matrix showing no hydrocarbon saturation and dolostone clasts showing moderate hydrocarbon saturation. There are dolostone clasts in the brecciated facies that show no UV fluorescence indicating diagenetic occlusion of pore space preventing hydrocarbon saturation. The tan dolostone facies demonstrates pervasive UV fluorescence with many locations of no UV fluorescence indicating diagenetic occlusion of pore space and no hydrocarbon saturation. The interbedded claystone and dolostone facies displays highly localized UV fluorescence, the claystone beds show no saturation while the dolostone beds display moderate to bright UV fluorescence. The chaotic facies also displays highly localized UV fluorescence, with claystones demonstrating a lack of saturation and dolostones showing moderate to intense saturation.;Pore types, sizes and shapes are variable throughout the facies of the upper Three Forks. The variability in the size, shape, and connectivity of these pores is a fundamental control on storage capacity and reservoir quality of the facies of the upper Three Forks. Inter-particle, intercrystalline, and slot microporosity are rare to moderate throughout facies of the upper Three Forks. Regional meteoric fluids and local mineralization are responsible for dissolution of both clay-rich and dolomite-rich intervals resulting in dissolution microporosity. This dissolution microporosity is the dominant storage capacity through the upper Three Forks. Microporosity of the upper Three Forks is connected by a common microfracture network found within the claystones of the interbedded claystone and dolostone and chaotic facies.;Diagenesis and cementation are complex throughout the facies of the upper Three Forks based upon mineralogy. The quartz and dolomite-rich brecciated and tan dolostone facies demonstrate coarse sucrosic dolomite framework with abundant rhombic overgrowth and laterally linked partial overgrowth cements. This pervasive cementation occludes pore space and prevents dissolution. The interbedded claystone and dolostone and chaotic facies lack pervasive dolomite cementation due to higher clay content. These facies display common secondary pyrite precipitation, and local dissolution. Facies with lithologic heterogeneity demonstrate a greater dissolution and secondary mineralization. Due to this, facies with higher frequency lithologic heterogeneity have the highest reservoir quality throughout the upper Three Forks.