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dc.contributor.authorWentz, Raelene
dc.date.accessioned2014-11-04T23:13:17Z
dc.date.available2014-11-04T23:13:17Z
dc.date.issued2014-08
dc.identifier.urihttp://hdl.handle.net/11122/4656
dc.descriptionThesis (M.S.) University of Alaska Fairbanks, 2014.
dc.description.abstractUmiat oil field in the southeast part of the National Petroleum Reserve-Alaska is a shallow, thrust-related anticline in the northern foothills of the Brooks Range and was one of the earliest discovered oil fields on the North Slope of Alaska. Despite significant reserves of light oil, Umiat has remained undeveloped because the reservoirs are located at shallow depths within the permafrost. Recent development of horizontal drilling techniques could provide access to this shallow reservoir with a minimal surface footprint, and has caused industry to take a second look at Umiat. Fracture networks are valuable in petroleum systems because they can enhance both porosity and permeability in a reservoir and they act as migration pathways from source rocks to reservoir. At Umiat, natural fractures, if open, could enhance reservoir permeability or, if filled with cement or ice, could impede fluid flow. In order to determine the potential of fractures at Umiat, I examined core from older Umiat wells and surveyed fractures at four exposed anticlines similar to Umiat anticline. Three fracture sets were observed in the surface anticlines: an early north-south set of calcite-filled regional extension fractures that predate folding and are interpreted as due to elevated pore pressures during burial and under north-south compression; east-west oriented, unfilled hinge-parallel extension fractures that formed during folding due to outer arc tangential longitudinal strain in fold hinges; and a set of unfilled, vertical conjugate shear fractures oriented perpendicular to fold hinges that is interpreted as having developed on the fold limbs. Several natural fractures were identified in unoriented core from Umiat wells. These natural fractures dip steeply with respect to bedding and are calcite cemented and/or open. Lack of orientation data precludes assigning these fractures directly to a fracture set observed in surface exposures, but the presence of, calcite cement suggest that these fractures belong to the early, north-south oriented calcite-filled fracture set seen in nearby surface exposures. These observations suggest that production in horizontal legs could vary depending on the azimuth of the borehole. North-south, calcite-filled fractures could serve as permeability baffles and reduce flow in north-south oriented legs. Alternatively, horizontal legs that encounter the open hinge-parallel fractures or hinge perpendicular conjugate set could experience early water breakthrough or loss of circulation.en_US
dc.description.tableofcontentsChapter 1: Introduction -- Chapter 2: Fractures in Foreland Basin Systems -- 2.1 Introduction -- 2.2 Fracture formation and types of fractures -- 2.3 Mechanical stratigraphy and fractures -- 2.4 Fractures in flat-lying rocks -- 2.5 Fold-related fractures -- 2.6 Fracture evolution in foreland fold-and-thrust belts and fluid flow -- Chapter 3: Regional Geology -- 3.1 Geologic setting -- 3.2 Stratigraphy -- 3.2.1. Colville basin stratigraphy -- 3.2.2 Brookian Sequence, Torok Formation -- 3.2.3 Brookian Sequence, Nanushuk Formation -- 3.2.4. Brookian Sequence, Seabee Formation -- 3.2.5 Brookian Sequence, Tuluvak Formation -- 3.3 General structural style of the Brooks Range foothills -- 3.4 Previous fracture studies in northern Alaska -- 3.5 Age of deformation of the central Brooks Range foothills in the southern Colville basin -- 3.6 Umiat anticline and oil field -- 3.7 Geology of the study area -- Chapter 4: Methods -- 4.1 Introduction -- 4.2 Identification of fractures in cores -- 4.3 Surface fracture mapping -- Chapter 5: Observations -- 5.1 Fracture distribution and characteristics in Umiat core -- 5.2 Fracture character and distribution in the field -- 5.2.1 Structural data -- 5.2.2 Colville incision observations -- 5.2.3 Fossil Creek anticline: south limb observations -- 5.2.4 Big Bend south anticline observations -- 5.2.5 Big Bend north anticline observations -- Chapter 6: Statistical Analysis of Fracture Spacing Data -- 6.1 Statistical analysis of fracture spacing data -- 6.2 Fracture sets observed in field -- 6.3 Predicting fracture spacing in the subsurface -- Chapter 7: Discussion -- 7.1 Summary of observations and preliminary interpretation -- 7.2 Relations between fractures and structural/stratigraphic position -- Chapter 8: Fracture Model for the Umiat Anticline -- 8.1 Model introduction -- 8.2 Primary elements of all Umiat fracture models -- 8.3 Model 1: All fractures are associated with regional stresses -- 8.3.1 Model 1: Implications for fracture distribution at Umiat -- 8.3.2 Model 1 deficiency -- 8.4 Model 2: All fractures are associated with local fold-related stresses -- 8.4.1 Model 2: Implications for fracture distribution at Umiat -- 8.4.2 Model 2 deficiency -- 8.5 Model 3: Composite fracture model -- 8.5.1 Model 3: Implications for fracture distribution at Umiat -- 8.5.2 Model 3 deficiency -- Chapter 9: Discussion -- Chapter 10: Conclusions -- 10.1 Future work -- Chapter 11: References -- Appendix A: Fracture Transects Data Tables from Each Field Location -- Appendix B: Results of Statistical Tests Performed on Fracture Spacing.en_US
dc.language.isoen_USen_US
dc.titleFracture characteristics and distribution in exposed cretaceous rocks near the Umiat anticline, North Slope of Alaskaen_US
dc.typeThesis
dc.type.degreems
dc.identifier.departmentDepartment of Geology and Geophysics
dc.contributor.chairHanks, Catherine
dc.contributor.committeeMcCarthy, Paul
dc.contributor.committeeWallace, Wesley
refterms.dateFOA2020-02-18T01:36:31Z


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