Catherine L. Hanks

Lithologic and Structural Controls on Natural Fracture Distribution and Behavior Within the Lisburne Group, Northeastern Brooks Range and North Slope Subsurface, Alaska

The Carboniferous Lisburne Group of northern Alaska has been deformed into a variety of map-scale structures in both compressional and extensional structural settings, thus providing a series of natural experiments for observing the formation, distribution, and behavior of fractures in this thick carbonate unit. Two fracture sets dominate the Lisburne Group carbonates of the North Slope subsurface and the nearby northeastern Brooks Range fold and thrust belt. North-northwest-striking regional extension fractures probably formed in front of the northeastern Brooks Range fold and thrust belt. In the North Slope subsurface, this fracture set overprints east-northeast-striking fractures related to earlier extensional deformation; in contrast, in the fold and thrust belt, the north-northwest-striking fracture set is overprinted by younger east-northeast-striking fractures related to subsequent contractional deformation. Lithology is the primary control on the fracture density of both sets. In mildly deformed Lisburne ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received February 20, 1996; revised manuscript received November 25, 1996; final acceptance June 5, 1997. 2Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska 99775. 3Sandia National Laboratories, MS 0705, Albuquerque, New Mexico, 87185. 4Department of Petroleum & Natural Gas Engineering, New Mexico Institute of Mining & Technology, Socorro, New Mexico 87801, and Sandla National Laboratories, MS 0705, Albuquerque, New Mexico, 87185. This study was supported by a Department of Energy subcontract administered by Sandia National Laboratories. Additional support was provided by ARCO Alaska, BP Alaska, Chevron, Exxon, Mobil, and Japan National Oil Company. We would like to thank ARCO Alaska and BP Alaska for giving us permission to view selected Lisburne Group cores, W. Wallace for helpful discussions on deformational styles of detachment folds, A. J. Mansure for help in interpreting the interference tests, and W. Wallace, K. Biddle, W. Belfield, N. Hurley, and J. Kelley for helpful reviews of the manuscript.

Structural provinces of the northeastern Brooks Range, Arctic National Wildlife Refuge, Alaska

The dominant Cenozoic structures of the northeastern Brooks Range are anticlinoria with cores of sub-Mississippian rocks, reflecting a regional north-vergent duplex with a floor thrust in the sub-Mississippian sequence and a roof thrust in the Mississippian Kayak Shale. The number of horses forming each anticlinorium and the structural style of the overlying Mississippian and younger cover sequence varies regionally, providing a basis for dividing the northeastern Brooks Range into structural provinces. In the western province, each anticlinorium contains a single horse, and shortening above the Kayak Shale was accommodated mainly by detachment folds. To the north in the Sadlerochit Mountains, the Kayak Shale is depositionally discontinuous and rocks elsewhere separated b this detachment deformed together. In the eastern province, each anticlinorium contains multiple horses, and shortening above the Kayak Shale was accommodated largely by thrust duplication of Mississippian through Triassic rocks. In the narrow central province, the Devonian Okpilak batholith was detached from its roots, internally shortened along shear zones and by penetrative strain, and transported northward. Because the Kayak Shale is locally absent, the Mississippian and younger cover sequence deformed in part penetratively along with the batholith. East-northeast trends formed where sub-Mississippian rocks were not involved in deformation, and probably are normal to the direction of Cenozoic tectonic transport. East trends formed where sub-Mississippian rocks were involved in deformation, and probably reflect a pre-Mississippian structural grain. At any given location, east trends generally post-date east-northeast trends, reflecting a drop over time of the basal detachment into sub-Mississippian rocks. INTRODUCTlON Well-exposed structures in the northeastern Brooks Range fold and thrust belt (Figure l) may provide insights into the evolution of similar structures elsewhere in the world, as well as offering clues to the factors that control their geometry. In addition, the northeastern Brooks Range includes the nearest well- exposed analogs to structures that may underlie the Arctic coastal plain immediately to the north, the most promising area for onshore hydrocarbon exploration remaining in North America. The stratigraphy of the northeastern Brooks Range has had a significant influence on the geometry of structures formed during deformation, as is true in many other fold and thrust belts (Woodward and Rutherford, 1989). The interlayering of strata of differing thicknesses, lithologies, and structural competencies has resulted in a structural stratigraphy in which particular stratigraphic intervals display a specific structural style. Several different structural provinces can be defined in the northeastern Brooks Range based upon lateral variations in structural style (Figure 2). These lateral variations commonly correspond with lateral variations in stratigraphy. Recent discussions of the structural geometry and evolution of the northeastern Brooks Range have dealt mainly with the western part of the region (for example, Kelley and Foland, 1987; Leiggi, 1987; Oldow et al., 1987a). In this paper, we illustrate the variations in structural geometry that exist over a much larger region, and argue that lateral changes in stratigraphy influence the style of deformation. Our objective is to provide a regional overview of the structure of the northeastern Brooks Range, and to interpret the influence of variations in stratigraphy on the structural geometry of the fold and thrust belt. This overview and interpretation are based primarily on our own detailed geologic studies throughout the northeastern Brooks Range, complemented by studies by graduate s udents at the University of Alaska on specific structural problems. However, End_Page 1100------------------------------ we do not intend to do more than summarize the results of these studies here. Rather, we seek in this paper to provide a conceptual and testable regional structural interpretation that will serve as a framework for future, more detailed papers and for further detailed structural and stratigraphic studies.

Fracture paragenesis and microthermometry in Lisburne Group detachment folds: Implications for the thermal and structural evolution of the northeastern Brooks Range, Alaska

The distribution, character, and relative age of fractures in detachment folded Mississippian–Pennsylvanian Lisburne Group carbonates and overlying Permian–Triassic clastic rocks in the northeastern Brooks Range of northern Alaska provide important clues to the thermal and deformational sequence experienced by these rocks. Although paleothermal indices in the host rock limit the conditions of folding to temperatures equal to or less than 280C, field and petrographic relationships suggest that different fracture sets formed at different times during the deformational history of the rocks, providing a record of deformation under changing temperature and pressure conditions. These rocks probably initially entered the oil-generation window (80–140C) during the Early Cretaceous formation of the Colville basin via thrust loading by the Brooks Range to the south. Regional fractures formed during this time as a result of high pore pressures and low in-situ differential stresses. Shortening in these rocks related to the advancing northeastern Brooks Range fold and thrust belt began during the Late Cretaceous to early Tertiary. Early phases of detachment folding were via flexural slip, with associated fracturing. With continued shortening and growth of detachment folds, structural thickening resulted in deeper burial of the bottom part of the deforming wedge. Early fold-related fractures were subsequently overprinted by penetrative strain during peak folding at temperatures of approximately 280C. Continued shortening resulted in uplift and erosional unroofing at approximately 60 Ma. Late fold-related fractures formed at about 150C. Subsequent uplift of the thickened wedge through 60C occurred after about 25 Ma. Late pervasive extension fractures related to unroofing and/or regional stresses formed at relatively shallow depths and low temperatures, overprinting all the earlier fractures and penetrative structures.

Character, relative age and implications of fractures and other mesoscopic structures associated with detachment folds: an example from the Lisburne Group of the northeastern Brooks Range, Alaska

ABSTRACT Fractures and other mesoscopic structures formed at different times during the evolution of individual detachment folds in Lisburne Group carbonates of the northeastern Brooks Range. These structures provide clues to the mechanism of folding, the conditions under which folds evolved and the paragenesis of fractures in the fold-and-thrust belt as a whole. The earliest fractures strike NNW and probably represent orogen-normal extension fractures that developed in the foreland basin in advance of the fold-and-thrust belt. These rocks and fractures were later incorporated into the thrust belt, where they were thrust-faulted and folded. Later fractures, strained markers and dissolution cleavage developed during detachment folding as a result of flexural slip and homogeneous flattening. Fracturing associated with flexural slip occurred early in the development of folds. These early fractures were commonly overprinted or destroyed by ductile strain as later homogeneous flattening accommodated additional shortening. This penetrative strain was in turn overprinted by late extension fractures that formed during flexural slip in the waning phases of folding or after folding due to unroofing of the orogenic wedge. Early fracturing, overprinting by ductile structures and subsequent later fracturing in detachment-folded Lisburne Group emphasizes the importance of understanding the unique character and history of each fold-and-thrust belt in a successful hydrocarbon exploration effort. In particular, the mechanical stratigraphy and conditions of deformation play an important role in the type of fold that develops, the fold mechanisms that are active and the subsequent distribution and character of fractures and other mesoscopic structures. 1 Current address: National Park Service, Denali National Park, Alaska USA End_Page 121------------------------

Cenozoic thrust emplacement of a Devonian batholith, northeastern Brooks Range: Involvement of crystalline rocks in a foreland fold-and-thrust belt

Involvement of crystalline rocks in thrusting near the foreland basin of a fold-and-thrust belt is relatively uncommon. In the northeastern Brooks Range, the Devonian Okpilak batholith was thrust northward and structurally elevated above adjacent foreland basin deposits during Cenozoic fold-and-thrust deformation. The batholith may have acted initially as a regional structural buttress, but a drop in the basal detachment surface to greater depth south of the batholith resulted in northward transport of the batholith. Shortening within the batholith was accommodated by (1) the development of discrete thrust slices bounded by ductile shear zones, (2) simple shear and development of penetrative mesoscopic and microscopic fabrics throughout the batholith, or both. The Mississippian Kayak Shale, a regional detachment horizon at the base of the overlying cover sequence, is depositionally thin or absent adjacent to the batholith. Thus, most of the cover sequence remained structurally coupled to the batholith during thrusting and was shortened by the development of penetrative structures.

A Multi-Participant, Multi-Criteria Analysis of Energy Supply Sources for Fairbanks, Alaska

The selection of a future energy source for Fairbanks, Alaska, is a multi-criteria, multi-decision maker (MCMDM) problem as it involves a range of stakeholders who must consider economic, sociopolitical, and environmental criteria in deciding the best project. The primary motivation for the new energy project is to provide an additional affordable heating and electric source to local residents. Proposed projects range from liquid-natural gas pipelines to hydropower and differ greatly in development costs, environmental impacts, and political support. Stakeholder interests vary from local and state government officials, to local and international business developers, and to residents and environmentalists. Traditionally, water and resource MCMDM problems have been simplified to analyze a single decision maker (DM) for multiple criteria; this work defines model criteria at different levels based on input from stakeholder representatives through a collaborative process. Model inputs can be ordinal when cardinal information is unavailable, thereby increasing the models flexibility for a wide range of source data. The model employs a range of social choice, fall back bargaining, and MCDM to solve the problem. Uncertainty in the model is characterized by a Monte Carlo analysis, which measures sensitivity of the solution from the range of inputs provided by stakeholder data. Given the economic and social components included in this MCMDM analysis, characterizing the uncertainty associated with each outcome is crucial for policy interpretation. This work provides a new application for MCMDM problems combining a range of social choice and game theoretic methods with a rigorous sensitivity analysis to inform decision makers about the most feasible and stable alternatives.

The Cenozoic structural evolution of a fold-and-thrust belt, northeastern Brooks Range, Alaska

A Cenozoic fold-and-thrust belt in the eastern structural province of the northeastern Brooks Range exposes polydeformed lowgrade metasedimentary and metavolcanic rocks of the pre-Mississippian basement and its sedimentary cover immediately adjacent to much younger foredeep deposits. Analysis of mesoscopic and map-scale structures in the range-front region suggests that at least one pre-Mississippian deformational event was recorded in the basement sequence by north-vergent fold-and-thrust structures and associated penetrative structures. Most of later Cenozoic shortening of the pre-Mississippian rocks was accommodated by thrust duplication, with little development of penetrative mesoscopic structures. Although separated from the underlying basement rocks by a major regional decollement horizon, Cenozoic deformation in the overlying Mississippian through Lower Cretaceous cover sequence also was primarily by thrust duplication. Although local and regional structural trends within the cover sequence suggest that Cenozoic deformation was north-northwest directed, east-west Cenozoic structural trends within the pre-Mississippian rocks may reflect an inherited pre-Mississippian structural grain and/or pre-Mississippian-age structures reactivated during Cenozoic deformation. A regional balanced cross section of the eastern structural province was constructed by integrating the detailed structural data from the range-front region with subsurface data from the foredeep basin to the north and reconnaissance surface data from the interior of the range. This balanced cross section indicates that Cenozoic shortening across the region was 101 km (63 mi) over an undeformed length of 220 km (137 mi), or 46%

An integrated model of the structural evolution of the central Brooks Range foothills, Alaska, using structural geometry, fracture distribution, geochronology, and microthermometry

Episodic deformation, triangle zone development, and related back thrusting in the central Brooks Range foothills are major factors in the distribution of fractures and the thermal history of rocks involved in the deformation. Structural reconstructions suggest that the rocks forming the Endicott Mountains allochthon, the youngest and northernmost part of the orogen during its first phase, were emplaced during the Early Cretaceous (Valanginian) at temperatures approximately 150C. Fractures associated with that deformation are filled with synkinematic calcite cement, indicating that they formed in the presence of fluids. After a period of quiescence during the Late Cretaceous, renewed deformation involved the shortening of the existing orogenic wedge and the development of a triangle zone and overlying back thrust in adjacent mid- to Late Cretaceous rocks of the foreland basin. This later deformational event and subsequent uplift resulted in two sets of uncemented barren fractures that formed in all parts of the fold and thrust belt. Restriction of cement-filled fractures to the older and structurally deeper parts of the orogen implies that the youngest and most obvious fractures visible at the surface developed at shallow depths and temperatures and thus may not have been an important factor in petroleum migration.

Introducing the Geosciences to Alaska Natives via the Rural Alaska Honors Institute (RAHI)

The Rural Alaska Honors Institute (RAHI) is an intensive, six-week residential high school-to-college bridging program aimed at preparing talented rural Alaska youth for the social and academic challenges of college. Since its inception in 1983, RAHI has demonstrated that it is an effective means of encouraging Alaska Native students to attend college and finish a post-secondary degree. Since 2003, a four credit, college-level, field-intensive, introductory geoscience course has been part of the RAHI curriculum. While it is difficult to evaluate what effect this specific course is having on the long term goal of recruiting more minority geoscientists, short term indicators suggest that the course is very effective in increasing the visibility of geology as a desirable career option amongst college-bound Alaska Native youth.

Sedimentology, stratigraphy, and reservoir properties of an unconventional, shallow, frozen petroleum reservoir in the Cretaceous Nanushuk Formation at Umiat field, North Slope, Alaska

Numerous oil and gas accumulations exist in the Brooks Range foothills of the National Petroleum Reserve in Alaska (NPRA). We use cores and well logs from 12 abandoned legacy wells at Umiat field, near the southeastern boundary of the NPRA, to characterize the sedimentology and stratigraphy of unconventional shallow frozen reservoirs in sandstones of the Cretaceous (Albian–Cenomanian) Nanushuk Formation. The Nanushuk Formation at Umiat has five facies associations: offshore and prodelta, lower shoreface, upper shoreface, delta front, and delta plain.Three stratigraphically distinct, regionally extensive Nanushuk Formation depositional systems at Umiat contain several potential petroleum reservoirs. The lower Nanushuk Formation, including a reservoir interval known informally as the lower Grandstand, primarily consists of marine mudstone and shoreface sandstones. The middle Nanushuk Formation is dominantly deltaic and contains a second major reservoir interval in the informal upper Grandstand sandstone. Both the upper Grandstand and lower Grandstand are regressive. The transgressive upper Nanushuk Formation contains an additional potential reservoir interval in shoreface sandstones of the informal Ninuluk interval. The primary reservoir intervals at Umiat field are upper shoreface and delta-front sandstones in the upper Grandstand and lower Grandstand, where increased sorting and decreased bioturbation in high-energy depositional environments affect overall permeability and permeability anisotropy.