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The Upper Devonian-Lower Mississippian Bakken Formation in the Williston Basin is an important source rock for oil production in North America. The Bakken Formation is comprised of three units: Upper and Lower Bakken black shales and Middle Bakken Member. Upper and Lower Bakken shales are high quality source rocks which source reservoirs in the Middle Bakken, Upper Three Forks and lower Lodgepole Formations. The Middle Bakken Member, consisting of predominantly gray, silty and sandy dolostone, is under investigation in this study. The goals of this study are to determine the regional distribution of lithofacies and depositional environments of the Middle Bakken Member and explain diagenetic sequence and reservoir quality parameters in the Williston Basin. The reservoir quality of the Middle Bakken Member is mainly influenced by mineralogical composition and cementation resulting in low porosity and permeability and linked to lithofacies distribution in the basin. Dolomitization is pervasive throughout the unit, and also occurs as dolomite cement. Moreover, cementation occurred including quartz overgrowths, K-felspar, clay cement and pyrite as both cement and nodules. Not only dolomitization but also pyrite cementation plays an important role in reducing pore space in the reservoir. The pore types that were identified are intergranular, intragranular, fracture and moldic pores. Secondary intragranular porosity generally resulted from dissolution of biogenic fragments and dissolution of other unstable minerals including feldspar and dolomite. In this study, five lithofacies and one sandy interval within lithofacies C were described throughout the North Dakota portion of the Williston basin. The sandy interval in Lithofacies C was interpreted as bars or channel fills, which differentiates this study from previous studies in terms of core description. N-S, W-E, NE-SW, NW-SE oriented cross-sections drawn via cores suggest that the lithofacies of the Middle Bakken Member pinch out towards the edges. However, the anticlines in the basin affect their thickness distributions. Sandy interval in Lithofacies C reaches its thickest succession in the center of the basin. Lithofacies C and D consist of up to 80% of dolomite although the other lithofacies consist of relatively lower dolomite (up to 65%). While well logs indicate 4-8% of porosity, point-counting results show up to 5% of porosity. The sequence of diagenetic events in the North Dakota portion of the Williston Basin is from youngest to oldest: micritization, mechanical and chemical compaction, calcite cementation, dolomitization, pyrite cementation, microcrystalline quartz cementation, syntaxial calcite overgrowth, quartz overgrowth, K-Feldspar overgrowth, dolomite dissolution, feldspar dissolution, dedolomitization, fracturing, anhydrite cementation and hydrocarbon migration.
An evaluation of the Three Forks Formation, to determine hydrocarbon potential, was performed in the Williston Basin of South Dakota using wells logs from the South Dakota Geological Survey. Basin analysis included identification of upper and lower boundaries, extent, lithologic description, stratigraphic correlation, and hydrocarbon potential. Thickness of Three Forks rocks ranges from 0 to 170 feet, and was thickest in northern Perkins and Corson counties and thinned toward the basin margins. Lithologically, the rocks consisted primarily of interbedded shale and dolomitic limestone. Stratigraphically, Three Forks rocks occurred between the underlying Birdbear Formation and overlying Englewood Formation. In areas where the Englewood was absent, it was overlain by the Lodgepole Formation. In North Dakota, eastern Montana, and Canada, the Three Forks underlies the Bakken Formation, serving as a reservoir for Bakken shale oil. No Bakken rocks were identified in well logs from South Dakota and the Three Forks and other Late Devonian formations have been underexplored. Well log analysis revealed three previously unidentified potential subsurface structures in the Williston Basin of South Dakota. In addition, data have supported proposed southeastern extensions of the Cedar Creek Anticline and Sheep Mountain Syncline. Black shale, indicating areas of restricted water circulation, have been identified in the Three Forks, in limited areas of estimated maturity, that suggest ideal conditions for the preservation of organic matter. These areas were correlated using gamma-ray spikes and estimated TOC values from 1.4 to 5.6 wt. %, at depths Three Forks rocks would be expected to contain mature hydrocarbons. Based on estimated TOC values and associated thicknesses of TOC-bearing intervals, limited potential exists for Three Forks source and reservoir rock in northwestern South Dakota, particularly in northern Perkins and eastern Harding counties.
The Late Devonian Three Forks Formation has increasingly become one of the productive plays in the Williston Basin, North Dakota. This growth has been attributed not only to new drilling and completion techniques but also to the availability of new geological information. Although production in the Three Forks Formation has increased, maximum recovery and overall hydrocarbon production is still limited. According to the North Dakota Industrial Commission, the oil recovery factor in the Three Forks Formation is 8.9±5.32%, so only a small percentage of the original oil in place is being produced. One of the most challenging aspects of reservoir quality in the Three Forks Formation has been characterizing the permeability, porosity and tortuosity which affect fluid flow and chemical transport in tight rocks. These parameters are macroscopic manifestation of the pore geometry and topology which are not easily accessible because of the dominant nanometer-scale pores. More information is needed with regards to the nanopore size distribution and how the pore connectivity is related to fracture networks. The mixed siltstone/sandstone/carbonate samples for all of five members of the Three Forks Formation were obtained from well Round Prairie 1-17H from Williams County (API No. 18257). For these tight rocks, we have studied the pore structure, edgeaccessible porosity, and wettability using the following complementary tests: mercury intrusion capillary pressure, tracer imbibition and tracer diffusion into fluid-saturated rock. The latter was done by pulling a vacuum on dry rock followed by the introduction of a tracer-free fluid. These tests use tracer-bearing fluids (API brine or n-decane) to examine the association of tracers with mineral or kerogen phases, followed by elemental analyses with laser ablation-ICP-MS to map out the presence and distribution of tracers inside the rock. The results from these innovative approaches indicate the limited accessibility and connectivity of nanopores in the samples. In addition, the Three Forks Formation seems to be mostly oil-wetting, with fast imbibition and diffusion for n-decane tracers. The interplay of wettability and connectivity could lead to the steep first-year production decline and low recovery factors because of the limited pore connectivity which inhibits the migration of hydrocarbon molecules in the rock matrix to the stimulated fracture network.