The NE-trending Nenana basin is a Cenozoic-aged basin located in central Alaska between the Denali and Tintina fault systems. The narrow, deep basin is a current exploration target for oil and gas resources in Interior Alaska. Natural fractures were analyzed to further understand larger structural features such as faults and folds related to the structural evolution of the Nenana basin and surrounding areas. Fracture sets were measured and described on the margin of the basin at four field locations: the Fairbanks area, along the Parks Highway between Fairbanks and Nenana, and in outcrop around the Nenana and Healy areas. In addition to measuring fracture sets in outcrop and collecting oriented samples, statistical and thin section analyses were used to further analyze fracture characteristics. Calcite twin thermometry and apatite fission track analysis were used to constrain the timing and thermal evolution of the field area. Based on the orientations of observed map-scale faults, folds, and fracture sets, I divided the four field locations into two structural domains. Domain I is characterized by NE-striking faults and associated active seismicity while Domain II is dominated by E-W striking folds and faults related to the late Cenozoic development of the Northern Foothills fold-and-thrust belt. I interpret that fracture sets in Domain I are related to the evolution of high angle faulting between the Nenana basin and the Fairbanks area during Cenozoic time. In Domain II, I interpret fracture sets are related to the evolution of the fold-and-thrust belt north of the Alaska Range. By combining fracture characteristics and apatite fission track analyses I provide constraints for the timing and shear sense of larger structural features related to the opening history of the Nenana basin. Furthermore, I propose that the evolution of the Nenana basin took place in three distinct tectonic phases during the Cenozoic. The three phases represent the transition from a pure extensional setting in the Late Paleocene to oblique-extensional faulting from the Late Miocene to present day.

Umiat 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.

Naturally occurring fractures can play a key role in the evolution and producibility of a hydrocarbon accumulation. Understanding the evolution of fractures in the Brooks Range/Colville basin system of northern Alaska is critical to developing a better working model of the hydrocarbon potential of the region. This study addressed this problem by collecting detailed and regional data on fracture distribution and character, structural geometry, temperature, the timing of deformation along the Brooks Range rangefront and adjacent parts of the Colville basin, and the in situ stress distribution within the Colville basin. This new and existing data then were used to develop a model of how fractures evolved in northern Alaska, both spatially and temporally. The results of the study indicate that fractures formed episodically throughout the evolution of northern Alaska, due to a variety of mechanisms. Four distinct fracture sets were observed. The earliest fractures formed in deep parts of the Colville basin and in the underlying Ellesmerian sequence rocks as these rocks experienced compression associated with the growing Brooks Range fold-and-thrust belt. The orientation of these deep basin fractures was controlled by the maximum in situ horizontal stress in the basin at the time of their formation, which was perpendicular to the active Brooks Range thrust front. This orientation stayed consistently NS-striking for most of the early history of the Brooks Range and Colville basin, but changed to NW-striking with the development of the northeastern Brooks Range during the early Tertiary. Subsequent incorporation of these rocks into the fold-and-thrust belt resulted in overprinting of these deep basin fractures by fractures caused by thrusting and related folding. The youngest fractures developed as rocks were uplifted and exposed. While this general order of fracturing remains consistent across the Brooks Range and adjacent Colville basin, the absolute age at any one location varies. Fracturing started in the southwest deep in the stratigraphic section during the Late Jurassic and Early Cretaceous, moving northeastward and upsection as the Colville basin filled from the west. Active fracturing is occurring today in the northeastern parts of the Colville basin, north of the northeastern Brooks thrust front. Across northern Alaska, the early deep basin fractures were probably synchronous with hydrocarbon generation. Initially, these early fractures would have been good migration pathways, but would have been destroyed where subsequently overridden by the advancing Brooks Range fold-and-thrust belt. However, at these locations younger fracture sets related to folding and thrusting could have enhanced reservoir permeability and/or served as vertical migration pathways to overlying structural traps.

Fracture patterns can provide insight into the strain history and stress evolution of deformed strata. In southern Alaska’s Cook Inlet forearc basin, hydrocarbon traps are typically fault-cored anticlines, where fractures likely aid in the migration of hydrocarbons from lower Jurassic marine strata into Cenozoic non-marine deposits. Consequently, understanding the distribution and orientation of fracture sets with respect to these structures is necessary to improving the understanding of one of Alaska’s largest petroleum provinces. Furthermore, recent refinements in understanding southern Alaska’s Dynamic Cenozoic tectonic evolution allow us to interpret fractures in a regional tectonic context. Despite the important role fractures likely play in the Cook Inlet petroleum system, limited work exists linking fractures to regional tectonic events and structures. The objective of chapter one is to characterize from field and remote sensing observations the orientations, distributions, and relative ages of several regionally prominent fracture sets. Field observations focus on the area of the western Cook Inlet near Augustine Volcano, north to Tuxedni Bay. Remote sensing observations expand the study area from the Alaska Peninsula in the south to Mount Spurr in the north. I identified four fracture sets—with common orientations, opening modes, and relative ages—within the sedimentary sequence that spans early Jurassic to Miocene time in the Cook Inlet forearc basin. Within the field area, these sets fall into two structural domains: 1) the Iniskin Peninsula, site of an anticline–syncline pair and reverse slip on the SW-striking Bruin Bay fault; and 2) north of Chinitna Bay, where the Bruin Bay fault strikes ~N–S and preserves primarily sinistral displacement. Chapter two is aimed at quantifying the fracture intensity of the four regional fracture sets defined in Chapter 1, which are pervasive in deformed forearc basin strata of Jurassic age in the Iniskin–Tuxedni region of the lower Cook Inlet, Alaska. I document how fracture intensity changes between the four regionally identified fracture sets of chapter one. Analysis of fracture intensity indicates that changes in fracture intensity are guided by the opening of other fractures and grain size. I also measured fractures at the thin-section scale, via back-scattered electron microscopy, to test the feasibility of using micro fracture analysis to estimate macro fracture abundance. I conclude by discussing how natural fractures could enhance sub-surface permeability for the lower Cook Inlet hydrocarbon province; and serve as migration pathways in the lower and upper Cook Inlet petroleum systems.

"Fractures in detachment folded Mississippian-Pennsylvania Lisburne Group carbonates provide insight into the distribution and character of natural fractures as a function of folding and lithology. Data from five detachment folds suggest that hinges show a higher fracture density than limbs. This study also suggests that the amount of shortening does not play a significant role in determining fracture density or uniformity of fracture orientation. A mechanical classification based on lithologic homogeneity reflects natural fracture distribution as a function of lithology more accurately than conventional lithologic classifications. Two main fracture sets were observed, a N-S set, perpendicular to fold axes, and an E-W set, parallel to fold axes. Statistical analyses suggest that E-W fracturing occurred before and during folding and that N-S fracturing occurred both before and after folding"--Leaf iii.

"Fractures form in foreland basin rocks during their progressive incorporation into fold-and-thrust deformation and subsequent uplift. This study investigates the relationship between fracture distribution and the evolution of the fold-and-thrust belt. This study identifies four fracture sets in pre-orogenic carbonates of the Brooks Range northward into foreland basin. Fracture distribution, structural style, and apatite fission-track (AFT) data define four structural domains. Domain I consists of strongly deformed Mississippian through Triassic rocks. Fracture sets 1, 2 and 4 are present in domain I, reflecting Valanginian through early Tertiary deformation. Domains II-IV consist of clastic basin deposits hosting fracture sets 2-4. Domains II and IV share fracture set distributions (sets 3 and 4), AFT cooling ages (70-60 Ma) and deformational style of open ~symmetric detachment folds. Domain III includes fracture sets 2-4, AFT cooling ages of ~100 Ma, reflecting thermal immaturity and south-vergent structures consistent with back thrusting. Restoration of early Tertiary deformation is constrained by surface, seismic and thermal data. Reconstruction shows the importance of back thrusting within domain III during the early Tertiary, the northern extent of the orogenic wedge, and the relationship between fold-and-thrust deformation and the relative timing and distribution of fracture sets in the Brooks Range foothills"--Leaf iii.