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Diagenesis and Fracture Development in the Bakken Formation, Williston Basin: Implications for Reservoir Quality in the Middle Member

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Diagenesis and Fracture Development in the Bakken Formation, Williston Basin: Implications for Reservoir Quality in the Middle Member FOLD Diagenesis and Fracture Development in the Bakken Formation, Williston Basin: Implications for Reservoir Quality in the Middle Member By Janet K. Pitman, Leigh C. Price, and Julie A. LeFever U.S. Geological Survey Professional Paper 1653 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey Charles G. Groat, Director First printing November 2001 For sale by U.S. Geological Survey, Information Services Box 25286, Denver Federal Center Denver, CO 80225 For more information on the USGS and its products Telephone: 1-888-ASK-USGS World Wide Web: http://www.usgs.gov/ Any use of trade, product, or fi rm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government III Contents Abstract .......................................................................................................................................................... 1 Introduction ................................................................................................................................................... 1 Analytical Methods ...................................................................................................................................... 1 Geologic and Structural Setting................................................................................................................. 2 Thickness and Age Relationships.............................................................................................................. 3 Lithostratigraphy........................................................................................................................................... 4 Depositional History ..................................................................................................................................... 5 Mineralogy and Diagenesis ........................................................................................................................ 7 Detrital Mineralogy.............................................................................................................................. 7 Diagenetic Alteration .......................................................................................................................... 7 Secondary Porosity ............................................................................................................................. 7 Fracture Occurrence........................................................................................................................... 8 Time-Temperature History and Burial Diagenesis ..................................................................................9 Reservoir Quality......................................................................................................................................... 13 Porosity................................................................................................................................................ 14 Permeability ........................................................................................................................................ 15 Formation of Fractures............................................................................................................................... 15 Conclusions ................................................................................................................................................. 16 Acknowledgments ...................................................................................................................................... 17 References Cited ........................................................................................................................................ 17 Figures 1. Index map showing approximate extent of the Williston Basin ........................................... 2 2. Map showing location of wells sampled for petrographic, geochemical, and reservoir quality analysis..................................................................................................... 2 3. Structure map on the top of the Bakken Formation................................................................ 4 4. Isopach map of the Bakken Formation ..................................................................................... 5 5. Lithologic column depicting lithofacies and sedimentologic characteristics of the middle member of the Bakken Formation ......................................... 6 6. Thin section photomicrographs of sandstone samples showing multiple generations of authigenic carbonate, Fe-calcite, non-Fe-dolomite, Fe-dolomite, anhedral dolomite cement, planar ferroan dolospar crystals, and euhedral dolospar crystals with ferroan dolomite rims................................................................................................................... 8 7. Thin section photomicrographs of sandstones illustrating occurrence and distribution of K-feldspar grains......................................................................................... 9 8. Graph showing relative peak heights of clay minerals in the <2-µm fraction of sandstones in the middle member of the Bakken Formation .......................... 10 IV 9. Thin section photomicrographs of sandstones depicting open (non-cemented), discontinuous fractures parallel to bedding ........................................... 11 10. Slabbed sandstone displaying reticulated fracture network on wet surface ............................................................................................................................ 12 11. Reconstructed burial and thermal history curves of the Bakken Formation showing relative timing of major diagenetic events in the deep part of the Williston Basin............................................................................................... 13 12. Chart depicting postdepositional events observed in sandstones and siltstones of the middle member of the Bakken Formation ......................................... 13 13. Plot of core porosity versus permeability in sandstones and siltstones of the middle member of the Bakken Formation ................................................. 15 14. Plot of mean sandstone porosity versus burial depth in the middle member of the Bakken Formation............................................................................... 15 15. Plot of mean sandstone permeability versus burial depth in the middle member of the Bakken Formation............................................................................... 16 16. Cross-plot showing stable isotope compositions of fracture-fill calcite cements in the middle member of the Bakken Formation...................................... 17 Tables 1. Locations of wells sampled for petrographic and geochemical analysis .......................... 3 2. Wells used in reservoir quality analysis ................................................................................. 14 3. Porosity and permeability of facies in the middle member of the Bakken Formation ........................................................................................................... 15 4. Stable isotope compositions of fracture-fill calcites in sandstones and siltstones of the Bakken Formation ................................................................................. 17 Diagenesis and Fracture Development in the Bakken Formation, Williston Basin: Implications for Reservoir Quality in the Middle Member By Janet K. Pitman,1 Leigh C. Price,1 and Julie A. LeFever2 Abstract static pressures that formed in response to increased fluid volumes in the source rocks during hydrocarbon generation. The middle member of the Bakken Formation is an attractive petroleum exploration target in the deeper part of the Williston Basin because it is favorably positioned with respect to source and seal units. Progressive rates of burial Introduction and minor uplift and erosion of this member led to a stable thermal regime and, consequently, minor variations in diagen­ The Upper Devonian–Lower Mississippian Bakken For­ esis across much of the basin. The simple diagenetic history mation in the Williston Basin, North Dakota (fig. 1), is recorded in sandstones and siltstones in the middle member a closed, low-permeability petroleum system that generated can, in part, be attributed to the closed, low-permeability approximately 200 to 400 billion barrels of oil in place. Most nature of the Bakken petroleum system during most of its of this generated oil was expelled into very fine grained sand- burial history. The dominant mineral cements consist of anhe­ stones and siltstones within the middle member, which is dral nonferroan planar dolospar, rhombic nonferroan dolospar bounded by organic-rich shales that are both sources and seals. grains with ferroan rims, and calcite commonly replaced by Source-rock thickness, thermal maturity, and total organic dolomite. Other authigenic phases include syntaxial quartz carbon (TOC) contents controlled the amount of oil generated and K-feldspar overgrowths, euhedral pyrite crystals, well- and expelled from the shales. ordered illite, and Fe-bearing chlorite. Bitumen is widespread The middle member of the Bakken is an oil reservoir in in sandstones and siltstones that are adjacent to thermally the thermally mature part of the basin and has been extensively mature shales. cored; thus, it offers the opportunity to examine
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