The Carboniferous Lisburne Group is a major carbonate reservoir unit in northern Alaska. The Lisburne is detachment folded where it is exposed throughout the northeastern Brooks Range, but is relatively undeformed in areas of current production in the subsurface of the North Slope. The objectives of this study are to develop a better understanding of four major aspects of the Lisburne: (1) The geometry and kinematics of detachment folds and their truncation by thrust faults. (2) The influence of folding on fracture patterns. (3) The influence of deformation on fluid flow. (4) Lithostratigraphy and its influence on folding, faulting, fracturing, and reservoir characteristics. The Lisburne in the main axis of the Brooks Range is characteristically deformed into imbricate thrust sheets with asymmetrical hanging wall anticlines and footwall synclines. In contrast, the Lisburne in the northeastern Brooks Range is characterized by symmetrical detachment folds. The focus of our 2000 field studies was at the boundary between these structural styles in the vicinity of Porcupine Lake, in the Arctic National Wildlife Refuge. The northern edge of thrust-truncated folds in Lisburne is marked by a local range front that likely represents an eastward continuation of the central Brooks Range front. This is bounded to the north by a gently dipping panel of Lisburne with local asymmetrical folds. The leading edge of the flat panel is thrust over Permian to Cretaceous rocks in a synclinal depression. These younger rocks overlie symmetrically detachment-folded Lisburne, as is extensively exposed to the north. Six partial sections were measured in the Lisburne of the flat panel and local range front. The Lisburne here is about 700 m thick and is interpreted to consist primarily of the Wachsmuth and Alapah Limestones, with only a thin veneer of Wahoo Limestone. The Wachsmuth (200 m) is gradational between the underlying Missippian Kayak Shale and the overlying Mississippian Alapah, and increases in resistance upward. The Alapah consists of a lower resistant member (100 m) of alternating limestone and chert, a middle recessive member (100 m), and an upper resistant member (260 m) that is similar to Wahoo in the northeastern Brooks Range. The Wahoo is recessive and is thin (30 m) due either to non-deposition or erosion beneath the sub-Permian unconformity. The Lisburne of the area records two major episodes of transgression and shallowing-upward on a carbonate ramp. Thicknesses and facies vary along depositional strike. Asymmetrical folds, mostly truncated by thrust faults, were studied in and south of the local range front. Fold geometry was documented by surveys of four thrust-truncated folds and two folds not visibly cut by thrusts. A portion of the local range front was mapped to document changes in fold geometry along strike in three dimensions. The folds typically display a long, non-folded gently to moderately dipping backlimbs and steep to overturned forelimbs, commonly including parasitic anticline-syncline pairs. Thrusts commonly cut through the anticlinal forelimb or the forward synclinal hinge. These folds probably originated as detachment folds based on their mechanical stratigraphy and the transition to detachment folds to the north. Their geometry indicates that they were asymmetrical prior to thrust truncation. This asymmetry may have favored accommodation of increasing shortening by thrust breakthrough rather than continued folding. Fracture patterns were documented in the gently dipping panel of Lisburne and the asymmetrical folds within it. Four sets of steeply dipping extension fractures were identified, with strikes to the (1) N, (2) E, (3) N to NW, and (4) NE. The relative timing of these fracture sets is complex and unclear. En echelon sets of fractures are common, and display normal or strike-slip sense. Mesoscopic and penetrative structures are locally well developed, and indicate bed-parallel shear within the flat panel and strain within folds. Three sets of normal faults are well developed in the area, and are unusual for the Brooks Range. One set is parallel to and another is transverse to the strike of the folds. A single major normal fault has an intermediate orientation. The normal faults cut across folds, but may have been active late during folding because fold geometry differs across faults and some folding apparently continued after normal faulting.

This Special Publication is a celebration of research into the Folding and Fracturing of Rocks to mark the 50th anniversary of the publication of the seminal textbook by J. G. Ramsay. Folding and Fracturing of Rocks summarised the key structural geology concepts of the time. Through his numerical and geometric focus John pioneered and provided solutions to understanding the processes leading to the folding and fracturing of rocks. His strong belief that numerical and geometric solutions, to understanding crustal processes, should be tested against field examples added weight and clarity to his work. The basic ideas and solutions presented in the text are as relevant now as they were 50 years ago, and this collection of papers celebrates John’s contribution to structural geology. The papers explore the lasting impact of John and his work, they present case studies and a modern understanding of the process documented in the Folding and Fracturing of Rocks.

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.