GEOLOGIC NOTE
AUTHORS
Fracture paragenesis and microthermometry in Lisburne Group detachment folds: Implications for the thermal and structural evolution of the northeastern Brooks Range, Alaska C. L. Hanks, T. M. Parris, and W. K. Wallace
ABSTRACT 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 280jC, 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 oilgeneration window (80 – 140jC) 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
Copyright #2006. The American Association of Petroleum Geologists. All rights reserved. Manuscript received November 24, 2004; provisional acceptance February 6, 2005; revised manuscript received August 1, 2005; final acceptance August 9, 2005. DOI:10.1306/08090504134
AAPG Bulletin, v. 90, no. 1 (January 2006), pp. 1– 20
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C. L. Hanks Geophysical Institute, University of Alaska, Fairbanks, Alaska 99775;
[email protected] After receiving her M.S. degree from the University of Washington, Catherine Hanks worked for ARCO Exploration in Anchorage, Alaska. She received her Ph.D. from the University of Alaska – Fairbanks in 1991 and joined the Geophysical Institute as a research faculty in 1994. Her recent research has focused on fracturing in detachment folds and the implications for reservoir behavior. T. M. Parris Petro-Fluid Solutions, LLC, 236 Shady Lane, Lexington, Kentucky 40503 Marty is a research geologist at the Kentucky Geological Survey and is the founder of Petro-Fluid Solutions. Previously, he held positions at the U.S. Geological Survey (postdoctorate, 1999 – 2002) and ARCO’s research lab (1997 – 1999). Marty received degrees from the University of California – Santa Barbara (Ph.D.), Texas Christian University (M.S.) and Tennessee Tech University (B.S.). W. K. Wallace Department of Geology and Geophysics and Geophysical Institute, University of Alaska, Fairbanks, Alaska 99775 Wesley Wallace received his B.A. degree in geology from Rice University and his Ph.D. from the University of Washington. After several years working for ARCO Exploration, he joined the faculty at the University of Alaska– Fairbanks, where his research interests include the tectonic evolution of Alaska and the structure of mountain belts. His current research focuses on the geometry and kinematics of detachment folds and duplexes in the northeastern and central Brooks Range.
ACKNOWLEDGEMENTS This study benefited greatly from numerous conversations in the field and elsewhere with the many geologists that work or have worked in northern Alaska. Special thanks go to J. Lorenz and J. Jensen and to the graduate students that have provided much stimulating conversation, including A. Karpov, T. Bui, P. Atkinson, J. Brinton, J. R. Shackleton, M. Jademec, M. Hayes, A. Loveland, and A. Duncan. The manuscript’s clarity was greatly enhanced by the reviews by A. Lacazette, S. Laubach, and an anonymous reviewer. This work was funded by the U.S. Department of Energy Contract DE-AC26-98BC15102 and support from BP Alaska, Anadarko, Encana, and PetroCanada.
fold-related fractures were subsequently overprinted by penetrative strain during peak folding at temperatures of approximately 280jC. Continued shortening resulted in uplift and erosional unroofing at approximately 60 Ma. Late fold-related fractures formed at about 150jC. Subsequent uplift of the thickened wedge through 60jC 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.
history suggests that a significant regional fracture network was probably in place during initial hydrocarbon generation and provided hydrocarbon-migration pathways to other parts of the basin. However, hydrocarbon generation and migration most likely occurred prior to the development of major structural traps. Several generations of fractures synchronous with and postdating folding could have enhanced reservoirs in the Lisburne Group during Tertiary generation and migration.
BACKGROUND INTRODUCTION Geologic Setting Accumulation of oil and gas in stratigraphic or structural traps is strongly dependent on where and when the traps formed relative to generation and migration. Fractures are an important factor in providing potential migration pathways from source rocks to reservoir and in enhancing both porosity and permeability in that reservoir. However, fractures can form at various times and conditions during the evolution of a fold and thrust belt (e.g., Parris et al., 2003; Bergbauer and Pollard, 2004; Hanks et al., 2004). Integrating the timing of fracture development with the timing of oil and gas generation and migration can provide valuable information on the thermal evolution of the basin and the timing of structures in the basin. Such information would be valuable in understanding evolving petroleum systems in the basin. The North Slope of Alaska has recoverable reserves (including cumulative production) of 15 billion bbl of oil and 45 tcf of gas (ANWR Assessment Team, 1999). The area of production and most intense exploration on the North Slope is limited to the northern margin of a large foreland basin, the Colville basin. Initial exploration in the 1960s and 1970s established that significant gas and light-oil accumulations could exist in the central and southern Colville basin. Further exploration for these potential resources has been slow, however, because of a lack of understanding of the controls on the distribution and trapping of the oil and gas. In this study, the age, distribution, and character of fractures associated with the detachment-folded Lisburne Group in the northeastern Brooks Range are integrated with fluid-inclusion microthermometry and existing geochronologic data. The integrated data are used to construct a burial history for the Lisburne Group that illustrates key deformation events and variation in pressure and temperature through time. This burial 2
Geologic Note
The Brooks Range is the northernmost part of the Rocky Mountain fold and thrust belt (Figure 1). Most of the shortening in the fold and thrust belt occurred in the Late Jurassic to Early Cretaceous, when a wide, south-facing late Paleozoic to early Mesozoic passive continental margin collapsed in response to the collision of an intraoceanic arc (Mayfield et al., 1988; Moore et al., 1994). Structural loading of the old passive continental margin by northward-advancing thrust sheets resulted in the initiation of a foreland basin, the Colville basin. Shortly after the main phase of compressional collapse of the continental margin, rifting led to the formation of the oceanic Canada basin to the north (present geographic coordinates) in the Early Cretaceous (Grantz and May, 1983; Moore et al., 1994). Postcollisional contraction has occurred episodically throughout the Cenozoic to the present and has resulted in the northward advancement of fold and thrust deformation in the northeastern Brooks Range and locally across the Cretaceous rifted margin (Grantz et al., 1990; Hanks et al., 1994). The stratigraphy of the northeastern Brooks Range can be divided into three depositional sequences (Figure 2) (Reiser, 1970; Mull, 1982). Slightly metamorphosed and deformed Proterozoic to Devonian sedimentary and volcanic rocks (the Franklinian sequence) are depositional basement for northerly derived Mississippian to Lower Cretaceous passive-margin sedimentary rocks of the Ellesmerian sequence (Reiser, 1970; Reiser et al., 1980; Lane, 1991; Moore et al., 1994). Ellesmerian sequence rocks are overlain by Lower Cretaceous to Holocene clastic rocks of the Brookian sequence, which were derived from the Brooks Range fold and thrust belt to the south and deposited in the Colville basin.
Figure 1. Tectonic map of northern Alaska, showing distribution of major structural features. Cross section AA0 is shown in Figure 3; the $ marks the location of the study area shown in Figure 5. Modified from Wallace et al. (1997).
The regional and local structural style of the northeastern Brooks Range is strongly influenced by the mechanical properties of the different stratigraphic units (Figures 2, 3) (Wallace and Hanks, 1990; Wallace, 1993). The largest structures are regional anticlinoria cored by pre-Mississippian metasedimentary and metavolcanic rocks. The anticlinoria are interpreted to reflect horses (fault slices bounded on all sides by thrusts) in a regional duplex between a floor thrust at depth and a roof thrust in the Mississippian Kayak Shale (Wallace and Hanks, 1990). The overlying Ellesmerian and Brookian sequences are decoupled from the basement and deformed as the roof of a passive-roof duplex. The Carboniferous Lisburne Group is the most rigid member of this roof sequence and deformed predominantly into kilometer-scale symmetrical de-
tachment folds that are only sparsely cut by thrust faults. The detachment folds are second-order folds above the basement anticlinoria. Overlying Permian and Triassic clastic rocks are slightly decoupled from the Lisburne Group, resulting in third-order folds. The evolution of regional in-situ stress patterns through time is an important aspect of evaluating potential hydrocarbon-migration patterns. The presentday in-situ stress pattern associated with the northeastern Brooks Range fold and thrust belt suggests that the current maximum horizontal in-situ stress maintains a consistent orientation at depths below 4000 ft (1219 m) and is generally oriented north-northwest, perpendicular to the observed thrust front (Figure 4) (Hanks et al., 2000). This in-situ stress orientation is also perpendicular to the strike of early Tertiary folds Hanks et al.
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and related faults involving rocks above the regional roof thrust (Wallace and Hanks, 1990). This implies that the northeastern Brooks Range fold and thrust belt has had a consistent north-northwest regional transport direction since the early Tertiary. A variety of thermal data constrain the regional thermal history of the northeastern Brooks Range and adjacent foreland basin. All Paleozoic rocks exposed in this part of the Brooks Range are thermally overmature (Johnsson et al., 1993). Conodont samples from Lisburne Group carbonates in the northeastern Brooks Range yield conodont alteration indices (CAI) generally greater than 4, indicating exposure to temperatures in excess of 150jC (Rejebian et al., 1987; Watts et al., 1995; Atkinson, 2001). 40Ar/39Ar and fissiontrack data constrain the age of uplift and unroofing in the northeastern Brooks Range. Published 40Ar/39Ar ages suggest thrusting at approximately 60 Ma, and apatite fission-track data indicate periods of uplift and unroofing at approximately 60, 43, and 28 Ma (O’Sullivan et al., 1995, 1998; Peapples et al., 1997). Thermal and structural modeling integrating both outcrop and well data from the foreland basin (Cole et al., 1999; Rowan, 1999) suggests that the lower parts of the Paleozoic section first entered the oil-generation window in the latest Cretaceous and left the oil window during Eocene uplift and erosion. At some point during this interval, the Lisburne Group rocks in the exposed fold and thrust belt reached temperatures in excess of 250jC. Depending on the geothermal gradient used, this corresponds to a burial depth of 8.3– 10 km (5.1–6.2 mi). This study focuses on the structural style, fracture distribution, and microthermometry in detachmentfolded Lisburne Group and overlying Permian to Triassic Echooka Formation on the south limb of one of the regional, basement-cored anticlinoria of the northeastern Brooks Range fold and thrust belt (Figures 1, 3, 5). Detailed thermal and fracture history in these rocks can provide important constraints on the timing of deformation and fracture formation with respect to thermal maturation during the early history of the thrust belt. Origin of Multiple Generations of Fractures in Folded Rocks
Figure 2. Generalized stratigraphy of the northeastern Brooks Range, emphasizing the mechanical stratigraphy of structural packages. Only the Triassic and older rocks are exposed in the study area and in most of the range. Modified from Hanks et al. (2004). 4
Geologic Note
Many authors have inferred or assumed that most of the fractures associated with folded rocks in a fold and thrust belt are directly related to the formation of those folds, with the orientation of shear and extension fractures interpreted to reflect the orientation of stresses during folding (e.g., Stearns and Friedman,
Figure 3. Regional cross section AA0 through the northeastern Brooks Range illustrating the general structural style of the fold and thrust belt. The study area is outlined by the circle and is shown in greater detail in Figure 5. LCu = Lower Cretaceous unconformity. Modified from Wallace (1993).
Figure 4. Map of northeastern Alaska showing orientations of maximum horizontal in-situ stresses as interpreted from borehole breakout data. Wells with a single consistent breakout orientation are shown in large, dark lines; smaller gray lines indicate wells with a consistent bimodal breakout distribution. The $ marks the location of the study area shown in Figure 5. Modified from Hanks et al. (2000).
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Figure 5. (a) Detailed geologic map of a part of the study area showing the sample locations with respect to structures. (b) Cross section BB0 showing the sample locations. 6
Geologic Note
1972; Cosgrove, 2000; Fischer and Wilkerson, 2000). In these instances, fracture characteristics, such as height and spacing, are interpreted to be related to layer thickness, structural position, the amount of bed curvature, and/or to be caused by changing mechanical layer thickness during diagenesis (e.g., Jamison, 1997; FlorezNin˜o et al., 2005; Shackleton et al., 2005). However, pervasive fracturing is not limited to deformed rocks. Regional extension fractures are common in flat-lying rocks and are interpreted as forming either at depth under low differential stresses and high pore-fluid pressure (e.g., Engelder and Lacazette, 1990; Lorenz et al., 1991; Engelder and Fischer, 1996) or at near-surface conditions as a result of uplift and erosion and consequent removal of lithostatic load (e.g., Hancock and Engelder, 1989). Rocks now exposed at the surface in a foreland fold and thrust belt have undergone a complex history of changing pressure, temperature, and stress conditions that provided different opportunities for fracturing (Bergbauer and Pollard, 2004; Hanks et al., 2004). Early extension fracturing could occur in the foreland basin, where the rocks were flat lying and experienced low differential stresses and high pore pressures caused by dewatering. These fractures would be basinwide in scale and oriented parallel to the maximum stress direction, perpendicular to the active thrust front. Subsequent incorporation of these rocks into the advancing fold and thrust belt would result in new extension and shear fractures related to folding and faulting and/or reactivation of the early-formed fractures (e.g., Bergbauer and Pollard, 2004). Folding and thrust faulting would also result in overall structural thickening of the thrust belt. Structural thickening or continued development of the thrust wedge into the foreland basin would result in uplift and unroofing of the deformed rocks. If fluid pressures are high, fractures may form under the low differential stresses present in the upper part of the thrust wedge. Alternatively, fractures could form as a result of unroofing and reduction in lithostatic pressure. Previous work suggests that such a progression in fracturing is recorded in detachment-folded Lisburne Group of the northeastern Brooks Range fold and thrust belt (Hanks et al., 2002, 2004). Different fracture sets in folded Lisburne Group carbonate rocks predate folding, are apparently related to folding, and postdate peak folding. Penetrative strain associated with peak shortening in the tightest folds provides an important relative time marker and establishes which fractures pre- and postdate folding.
CONSTRAINTS ON THE CONDITIONS OF DEFORMATION Up to this point, field observations and analysis of detachment folds and the mesoscopic structures associated with them have provided a relative sequence of deformational events (e.g., Homza and Wallace, 1995; Jamison, 1997; Hanks et al., 2004). However, this relative sequence of events does not establish when the folds and fractures formed with respect to hydrocarbon generation and migration. This can be achieved by combining the qualitative structural information with quantitative constraints on the thermal and unroofing history of the rocks. Structural Constraints Four generations of fractures associated with detachment-folded Lisburne carbonates were identified as pre-, early, late, and postfolding based on crosscutting and/or abutting relationships and the relationship to penetrative strain that developed during peak folding (Figure 6; Table 1) (Hanks et al., 2004). For a more detailed description of the fracture characteristics and their relationship to associated detachment folds, please see Hanks et al. (2004). The first fracture set (set 1, Figure 6; Table 1) has been interpreted to be a regional, prefolding set of apparent extension (mode I) fractures that formed in the undeformed foreland basin ahead of the advancing thrust belt (Hanks et al., 1997). In this interpretation, the fractures formed at depth and at high pore pressures, parallel to maximum in-situ horizontal stress. Fractures in this set are best preserved in relatively
undeformed sections of Lisburne and are uncommonly identified in folded sections. The second set of fractures (set 2, Figure 6; Table 1) consists predominantly of apparent mode I extension fractures oriented parallel and perpendicular to the fold axes. Rare bed-parallel fractures and shear mode II and/ or III fractures oblique to the fold axes are included in this set. All fractures in set 2 are commonly filled with calcite. Set 2 fractures are interpreted to have formed during the early phases of detachment folding, probably as a consequence of flexural-slip folding and/ or outer-arc extension in the hinge region. Alternatively, these fractures could have formed either synchronously or immediately after set 1 fractures as crossjoints and then could have been reactivated by folding (e.g., Bai et al., 2002; Bergbauer and Pollard, 2004). Regardless, these fractures predate ductile structures that facilitated thickening in the hinges and thinning in the limbs during peak folding. Penetrative strain postdates fracture sets 1 and 2 and predates fracture sets 3 and 4 (Figure 6; Table 1) (Hanks et al., 2004). In many Lisburne detachment folds, penetrative strain by ductile and semiductile processes occurs primarily in the fold hinges, where it accommodates bed thickening. In the tightest detachment folds, both the hinges and limbs have undergone ductile strain, with significant thinning in the limbs and thickening in the hinges (e.g., Atkinson and Wallace, 2003). Where this ductile strain occurs, mesoscopicscale structures include stretched and elongated chert nodules, deformed crinoids and peloids, sheared stylolites, and dissolution cleavage. Fracture sets 1 and 2 are generally not preserved in these areas; where they are preserved, the fractures from sets 1 and 2 are highly
Figure 6. Schematic diagram of a fold illustrating the observed geometry of the four fracture sets and the penetrative strain with respect to the fold and bedding.
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Table 1. Characteristics, Orientation, and Relative Age of Fracture Sets Observed in the Detachment Folded Lisburne Group in the Study Area
deformed and transposed (e.g., Figure 7). Fracture set 3 consists of apparent mode I extension fractures that strike parallel to the fold axes and shear mode II and III fractures that are oblique to the fold axes. Both fracture types dip perpendicular to bedding and cut ductile structures that formed during peak folding (Figure 6; Table 1). These fractures are narrow and only locally filled with calcite. Statistical analysis of fracture spacing in set 3 suggests that these fractures are not closely related to folding (please see Bui et al., 2003, for a more detailed discussion of the statistical analysis). This fracture set is interpreted to have formed late during folding or after folding, possibly as a result of the decrease in overburden caused by uplift and erosion and subsequent release of stored elastic strain. Alternatively, set 3 fractures could have formed as cross-joints to set 4 fractures (e.g., Bai et al., 2002). Fracture set 4 consists of pervasive open, unfilled apparent mode I extension fractures that strike northnorthwest perpendicular to fold axes (Figure 6; Table 1). 8
Geologic Note
Figure 7. Photomicrograph of sample 4 showing two sets of fractures. Sheared and deformed calcite cement in fracture set 2 is crosscut by undeformed calcite cement of fractures of set 3. Note the highly flattened host rock. Modified from Hanks et al. (2004).
These fractures are vertically extensive and cut across bedding. Statistical analysis suggests that this fracture set does not have a close relationship to folding (see Bui et al., 2003, for a more detailed discussion of the statistical analysis). Although superficially similar to fracture set 1, fracture set 4 is undeformed and not filled with calcite cement. Fracture set 4 can be interpreted as similar in origin to fracture set 1 (i.e., related to in-situ horizontal stresses in the subsurface) or, alternatively, related to overburden removal during uplift and erosion. Reactivation of or control by fracture set 1 may have been a factor in the formation of many set 4 fractures. Petrography and Fluid-Inclusion Microthermometry Methods Standard transmitted light petrography was used to characterize the mineralogy and texture of fracture cements and to map fluid inclusions in the fracture cements. Cement textures were used with field observations to characterize the timing of cementation relative to fracture opening ( Table 2). Understanding the relative timing of the two processes is necessary for determining if fluid-inclusion measurements represent conditions before, during, or after fracture opening. Synkinematic cement forms contemporaneous with fracture opening, whereas postkinematic cement postdates fracture opening (Laubach, 2003). When a fracture undergoes multiple periods of opening and cementation, the earliest cement can be postkinematic with respect to the initial opening, but the same cement is prekinematic with respect to later opening events. In this analysis, we attempt to reference cement formation to the initial period of fracture opening. Heating and freezing measurements of fluid inclusions trapped in fracture cements can provide information about the temperature, pressure, and fluidcomposition conditions of cement formation (e.g., Kisch and Van Den Kerkhof, 1991; Parris et al., 2003). Petrographic analysis of fracture cements revealed the presence of primary and pseudosecondary inclusions, which are trapped during primary crystal growth, and secondary inclusions trapped along healed microcracks after crystal growth. Trapping of pseudosecondary and secondary inclusions occurs when microcracks are recemented because of local mass transfer (healing) or transport over distances exceeding the grain scale (sealing) (Smith and Evans, 1984; Laubach, 1989). Inclusions for which an origin cannot be confidently ascribed are termed ‘‘indeterminate.’’ In addition, three types of inclusions were recognized and mapped based on their
room temperature phase relations. Two-phase inclusions contain aqueous liquid and vapor bubble. Single-phase inclusions with no recognizable bubble consist predominantly of an aqueous liquid or a nonaqueous gas phase. All samples were analyzed for oil inclusions using ultraviolet light and fluorescence light microscopy as outlined by Burruss (1991). No oil inclusions were observed. Heating and freezing measurements were done with a U.S. Geological Survey gas-flow stage. The stage was calibrated using an ice bath (0jC) and synthetic fluid inclusions (SYNFLINC, Inc.). Using the cycling technique described by Goldstein and Reynolds (1994), homogenization and final ice melting temperatures in aqueous two-phase inclusions were measured within 2 – 5jC and 0.2jC intervals, respectively. Heating of aqueous two-phase inclusions to the temperature at which the vapor bubble disappears yields the homogenization temperature. For primary inclusions, homogenization temperatures represent the minimum temperature of cement growth. Aqueous inclusion salinity was determined by measuring the temperature of final ice melting in the inclusion. An equation developed by Bodnar (1992) was used to determine salinity from the final ice melting temperature. Gas-rich single-phase and many aqueous two-phase inclusions were cooled to 170jC. The aqueous inclusions showed only freezing of the aqueous fluid (i.e., formation of ice). The gas-rich inclusions, in contrast, commonly developed a vapor bubble and liquid. During subsequent slow warming, the bubble disappeared (i.e., homogenized to liquid) at temperatures close to the critical temperature of pure CH4 ( 82.1jC), thus indicating the presence of CH4-rich gas in the inclusions (Van Den Kerkhof, 1990). Homogenization temperatures of the gas-rich inclusions were measured within a 0.5jC interval. Observations and Measurements Fluid-inclusion measurements were obtained from four cemented fractures from four different structural positions (Figure 5). These samples were chosen because the relative ages of the fracture sets were well established at these locations. Fluid-inclusion data were obtained primarily from fracture sets 2 and 3 (Table 2). Fracture set 1 was also cemented but was commonly overprinted and/or reactivated by set 4 and, thus, difficult to identify reliably in the field. Samples 2–4 were collected from the Mississippian Alapah Limestone. Sample 3, from the south limb of an isoclinal anticline, contains examples of fracture sets 2 and 3 (Figure 5; Table 1). Fracture set 2 is cemented Hanks et al.
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Table 2. Summary of Fluid-Inclusion Analysis
with deformed and extensively twinned calcite, which is postdated by quartz. Fracture set 3 is cemented with less deformed calcite (Table 2). Quartz cement in fracture set 2 sometimes shows a bent fiber habit, and the fibers have possible crack-seal fluid-inclusion trails, both of which suggest synkinematic crystal growth (Figure 8a). Synkinematic growth of quartz fibers into open-fracture porosity has been documented (e.g., Laubach et al., 2004), and it is likely that quartz in sample 3 grew into fracture porosity that formed during later deformation of the fracture. This interpretation is consistent with the synkinematic quartz texture and the deformed calcite in fracture set 2, both of which indicate deformation postdating the initial fracture opening. The crack-seal fluid-inclusion trails have pseudosecondary aqueous two-phase inclusions that yielded a mean homogenization temperature of 196 ± 8jC and 10
Geologic Note
a mean final ice melting temperature of 2.2 ± 0.3jC (Figure 8b; Table 2). Secondary aqueous inclusions along a healed microcrack, which postdates the crackseal inclusions, have a mean homogenization temperature of 225 ± 7jC and similar final ice melting temperatures (T mice = 2.2 ± 0.1jC). Thus, primary crystal growth and trapping of pseudosecondary inclusions and later crack healing and trapping of secondary inclusions occurred in the presence of a fairly dilute fluid (3.7 wt.% NaCl equivalent). Sample 4, collected on the south limb of the same isoclinal anticline as sample 3, contains calcite-cemented fractures assigned to sets 2 and 3 (Figure 5; Table 1). The set 2 fracture fill is extensively twinned and contains shear bands (Figure 7). Questionable primary inclusions in set 2 yield a homogenization temperature of 273 ± 4.4jC (Table 2). We view these results with
caution, however, because of deformation in fracture set 2. Fracture set 3 in sample 4 contains clear, undeformed calcite spar that is postkinematic. The calcite was relatively inclusion free; however, a single population of aqueous inclusions contained single-phase liquid and two-phase inclusions. The two-phase inclusions had somewhat variable liquid/vapor ratios. The assemblage suggests that the inclusions were trapped at low temperature (< 50jC) and subsequently thermally reequilibrated (Goldstein and Reynolds, 1994). Alternatively, the assemblage could have resulted from postentrapment necking. An attempt to do freezing analysis resulted in stretching and leakage of the inclusions. Sample 2 was collected from the south limb of a tight anticline at some distance from the hinge region (Figure 5). The sample contains one fracture that is cemented with relatively undeformed calcite, which is likely postkinematic. We interpret the cemented fracture to belong to either fracture set 1 or 2 (Table 1). Analysis of inclusion populations in close proximity to each other reveals a wide range of composition and thermometric characteristics. The oldest population consists of possible coeval aqueous two-phase and singlephase gas-rich inclusions of possible primary origin. The aqueous inclusions have a mean homogenization temperature of 223 ± 4jC (Table 2). If the coeval interpretation is correct, then the aqueous homogenization temperatures equal true trapping temperature (Roedder, 1984). On cooling to 170jC, the singlephase gas-rich inclusions homogenized as described in the Methods section, from 100 to 85jC (mean equals 93 ± 6jC). The coeval assemblage of inclusions is cut by younger secondary inclusions trapped along a healed microcrack. This younger assemblage consists only of gasrich inclusions. On cooling to 170jC, the secondary gas-rich inclusions showed the same phase behavior as the primary inclusions above (i.e., homogenization to liquid), but homogenization occurred over a broader temperature range of 117 to 78jC (mean equals 103 ± 17jC). The youngest population of inclusions consists of secondary aqueous two-phase inclusions trapped along a healed microcrack different from that containing the gas-rich inclusions. The aqueous inclusions have a mean homogenization temperature of 147 ± 6jC (Table 2). Final melting temperatures greater than 0jC (up to 1.2jC) indicate the presence of superheated ice, which precludes accurate determination of salinity (Goldstein and Reynolds, 1994).
Sample 1 is from a siltstone in the Echooka Formation preserved in a synclinal hinge (Figure 5). The sample is included because the Echooka Formation immediately overlies and shares the same structural style as the Lisburne Group, including the number, orientation, and relative age of fractures. However, fractures in sandstone and siltstone of the Echooka Formation tend to be cemented with quartz, and inclusions trapped in the quartz are less prone to postentrapment leakage as compared to inclusions in carbonate. At least two generations of quartz-cemented fractures are recognized in sample 1 (Table 2). The oldest is parallel to bedding and is interpreted to be part of fracture set 2 (Table 1). Quartz cement in fracture set 2 is sheared to the extent that the timing of cementation relative to fracture opening could not be determined. The younger fracture fill, interpreted to belong to set 3, is orthogonal to bedding and fracture set 2 and has vague fracture walls. Quartz cement in fracture set 3 is relatively undeformed, inclusion free as compared to quartz in the fracture set 2, and is likely postkinematic. Similar inclusion-free quartz cement also fills possible former pore spaces. Primary aqueous two-phase inclusions in quartz cement of fracture set 3 have a mean homogenization temperature of 155 ± 10jC (Table 2). Mean final ice melting temperature equals 3.8 ± 0.6jC, which is equivalent to a bulk salinity of 6.2 wt.% NaCl equivalent (Bodnar, 1992). The above fluid-inclusion data are interpreted to represent conditions of cementation during cementation of set 3 fractures; however, deformation in sample 1 makes the interpretation equivocal. Analysis and Summary of Fluid-Inclusion Data The oldest inclusions analyzed are in cements of fracture set 2. Set 2 fractures and their cements are commonly extensively deformed. This deformation calls into question the accuracy of the high homogenization temperatures in sample 4. Temperature conditions during cementation of set 2 fractures are probably best represented by the homogenization temperatures from samples 2 and 3. In sample 2, homogenization temperatures of approximately 223jC from primary coeval aqueous and gas-rich inclusions likely represent true trapping temperatures. This coeval inclusion assemblage records a condition of heterogeneous trapping of immiscible aqueous liquid and nonaqueous gas. Moreover, the assemblage provides the opportunity to estimate the pressure of inclusion entrapment and, hence, fracture pore-fluid pressure (Roedder, 1984). Hanks et al.
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Geologic Note
As noted, the phase behavior (liquid-vapor homogenizing to liquid) and temperatures ( 85 to 100jC) of homogenization suggest that the primary gas-rich inclusions in sample 2 are mostly filled with CH4 (Van Den Kerkhof, 1990). Homogenization of the inclusions at temperatures below the critical temperature of CH4 ( 82.1jC) can be attributed to one of two factors or some combination. First, the inclusions could be CH4 mixed with gas(es) having colder critical temperatures (N2, C3H8). A reliable and nondestructive method of determining gas composition is through Raman spectroscopy (Burruss, 2003). Although Raman spectroscopy was not done in this study, conditions of entrapment and the geologic setting allow us to make some generalizations about gas composition. Significant quantities of C3H8 appear to be unlikely given the temperature of inclusion entrapment (223jC) and the tendency of Ellersmerian gases to be dry (Hunt, 1996; Burruss et al., 2003). Significant quantities of N2 also appear to be unlikely in a predominantly carbonate succession and in a basin in which N2 is a minor part of the generated natural gas (Dubessy and Ramboz, 1986; Burruss et al., 2003). An alternative possibility is that the variation in homogenization temperature could be caused by trapping of pure or nearly pure CH4 inclusions of different densities (Van Den Kerkhof and Thiery, 2001). Using this assumption, we were able to estimate inclusiontrapping pressures for sample 2. Isochores (i.e., lines of constant density) and pressure estimates were constructed using the NIST Chemistry WebBook (NIST, 2005). CH4 inclusions that homogenize at 100 and 85jC (i.e., the range of homogenization temperatures seen in sample 2) have densities of 0.30098 and 0.27746 g/cm3, respectively. The intersection of the isochores with the trapping temperature defines the trapping pressure in pressure-temperature space (e.g., Goldstein and Reynolds, 1994). For a trapping temperature of 223jC, CH4 inclusions homogenizing at 100 and 85jC have trapping pressures of 1560 and 854 bar, respectively (Figure 9a).
The range of CH4 homogenization temperatures thus corresponds to a considerable range of trapping pressures. One likely explanation for the variation is the reequilibration of the inclusions. Because of high internal pressure in the inclusions, there is a considerable driving force for reequilibration, especially in calcite, which is a soft mineral that cleaves and twins readily (Goldstein and Reynolds, 1994). Reequilibration will tend to decrease inclusion density that will lead to lower calculated trapping pressures (Figure 9a). For gas-rich inclusions in this study, reequilibration would produce warmer homogenization temperatures. For a temperature gradient of 25jC/km and surface temperature of 0jC, an entrapment temperature of 223jC corresponds to a depth of approximately 9 km (5.6 mi). Hydrostatic and lithostatic pressure at 9 km (5.6 mi) is approximately 900 and 2025 bar, respectively. Given the possibility of reequilibration, the higher estimated pressures from sample 2 are interpreted to be most representative of initial trapping pressures. Calculated trapping pressures thus record conditions close to or greater than hydrostatic pressure but less than lithostatic pressure (Figure 9b). However, we want to emphasize that the calculated pressures are estimates. If the variation in homogenization temperature is, in part, related to the presence of other gases in addition to CH4, then pressures would need to be recalculated using the appropriate isochores and phase diagrams. Further information on the conditions of cementation of set 2 fractures comes from sample 3. Bent quartz fibers in this sample contain crack-seal fluidinclusion trails that provide unequivocal evidence of synkinematic fracture cement growth (Figure 8). The quartz cement, however, postdates deformed calcite in set 2 fractures and accordingly developed at some unknown later time. The quartz fibers have long axes generally orthogonal to the walls of set 2 fractures. The orientation suggests that quartz growth occurred under stress conditions similar to those that initially
Figure 8. Photomicrographs from sample 3 and quartz cement that postdates deformed calcite of fracture set 2. (a) Quartz fiber trends from upper left to lower right and is surrounded by twinned calcite. The quartz contains many fluid-inclusion trails (see arrows) oriented at a high angle with respect to the fiber’s long axis. The fluid-inclusion trails are interpreted to be crack-seal textures resulting from repeated opening and cementing episodes, that is, synkinematic growth. (b) Higher magnification view of aqueous two-phase inclusions trapped along crack-seal trails. Inclusions trapped along the crack-seal trails are considered to be pseudosecondary (P), and therefore, thermometric analysis of these inclusions provides an indication of conditions during deformation and quartz fiber growth. In contrast, inclusions along trails in the upper center and upper right of the image that trend parallel to the fiber length are interpreted to be secondary (S). These inclusions, which are younger than the quartz fiber, provide information about conditions postdating fiber growth. Hanks et al.
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Figure 9. (a) Bivariate plot of homogenization temperatures (i.e., homogenization of liquid-vapor to liquid) versus calculated trapping pressures for primary gas-rich inclusions in sample 2 (see text for assumptions and methods). The inclusions, interpreted to be coeval, show a considerable range of calculated densities (parentheses, in g/cm3) and pressures. The range of values might be the result of reequilibration, wherein the inclusions leak because of high internal pressure relative to lower external confining pressure. This reequilibration will tend to lower inclusion density; therefore, inclusions with higher calculated density and pressure are interpreted to best represent the conditions of initial entrapment. (b) At a trapping temperature of 223jC (homogenization temperature of coeval aqueous inclusions), the calculated pressures define fracture pore-fluid pressures that are near or exceed hydrostatic for temperature gradients of 25jC/km (solid lines) to 30jC/km (dashed lines).
produced the set 2 mode I extension fractures. A different fiber orientation would appear to be predicted had the quartz growth in these set 2 fractures occurred during later development of set 3 fractures, which are oriented at a high angle with respect to set 2. Aqueous homogenization temperatures from the pseudosecondary inclusions trapped along the crack-seal trails provide a minimum temperature of cement growth (196jC). Thus, it is possible that quartz growth occurred at temperatures near or equal to those recorded by primary aqueous inclusions in sample 2. Later secondary aqueous inclusions have hotter homogenization temperatures (225jC), and the relative sequence of entrapment indicates increasing temperature. Collectively, the textures and fluid-inclusion data in sample 3 14
Geologic Note
suggest that quartz cementation and at least some part of set 2 fracture cementation occurred synchronous with deformation as temperatures increased, possibly during burial. Aqueous primary and secondary inclusions provide evidence that fracture sets 3 and 4 were cemented at cooler temperatures as compared to set 2. Aqueous homogenization temperatures from primary inclusions in set 3 fractures in sample 1 provide strong evidence that postkinematic cementation of set 3 fractures occurred at cooler temperatures (Table 2). In sample 2, the timing of secondary aqueous inclusions that crosscut set 2 calcite fracture cement is ambiguous, but homogenization temperatures that are close to those from primary inclusions in set 3 fractures in sample 1
suggest that the secondary inclusions were trapped at comparable temperatures. Finally, in set 3 fractures in sample 4, the population of aqueous single- and twophase inclusions in undeformed calcite suggests the possibility of even cooler temperatures (< 50jC).
PRESSURE-TEMPERATURE-TIME DEFORMATION PATH A burial-deformation history for the Lisburne Group in the northeastern Brooks Range can be constructed by integrating the relative ages of the observed fractures and fracture cements, with fluid-inclusion microthermometry and published geochronologic and geothermal data (Figure 10).
Pre-60 Ma Burial of the Lisburne Group during the late Paleozoic and early Mesozoic was predominantly related to subsidence along a south-facing (present coordinates) passive continental margin. Approximately 1000 – 1500 m (3300– 6150 ft) of late Paleozoic, Triassic, and Jurassic sediments were deposited on top of the Lisburne Group during this time. Initiation and growth of the main axis of the Brooks Range south of this location during the Late Jurassic–Early Cretaceous resulted in approximately 4000 –5500 m (13,100 – 18,000 ft) of southerly derived Cretaceous clastic sediments being deposited in the foreland basin in the future location of the northeastern Brooks Range (Bird, 1999; Cole et al., 1999). Burial of the Lisburne Group and the overlying sediments was accompanied by heating, compaction, and dewatering. Structural and thermal modeling by Cole et al. (1999) suggests that the Lisburne Group and overlying source rocks in the Shublik Formation were probably at a maximum depth of 4–6 km (2.4– 3.7 mi) at this time and either well within the oilgeneration window (assuming a 25jC/km geothermal gradient) or past it (assuming a 30jC/km geothermal gradient). The earliest fractures, set 1, probably formed at this time (Figure 10) in response to low differential stresses and high fluid pressures in the growing foreland basin. Set 1 fractures formed perpendicular to the thrust front to the south and would have been open at considerable depths, possibly forming good migration pathways for the hydrocarbons being generated.
60 Ma Regional Deformational Event Regional apatite and zircon fission-track data and 40 Ar/39Ar data suggest that major exhumation occurred in the northeastern Brooks Range at about 60 Ma (O’Sullivan et al., 1995; Peapples et al., 1997). This age can be interpreted to reflect the uplift that resulted from the development of the regional, preMississippian-cored, anticlinoria during the latest Cretaceous and early Tertiary. It is not clear how much deformation of the passive roof of this regional duplex (Lisburne Group and younger rocks) occurred at this time. Some detachment folding of the cover rocks likely began during this initial deformational episode. Set 2 fractures formed at this time in response to flexural slip folding and associated tangential longitudinal strain (Figure 10) (Hanks et al., 2004). Structural thickening probably allowed the Lisburne Group to remain buried at a significant depth at this time despite the removal of some overburden as a result of uplift and erosion. Both the fluid-inclusion data from set 2 fractures (200–225jC) and the conodont alteration indices (280jC; Johnsson et al., 1992) indicate that these rocks experienced fairly high temperatures. Folding was probably accompanied by penetrative deformation in fold cores. This increase in paleotemperature reflects continued burial of the rocks and/or an increase in the geothermal gradient. In addition, trapping pressures calculated from sample 2 suggest that the fracture pore fluids were overpressured (Figure 9b). Overpressure can develop in response to disequilibrium compaction, tectonic stress, hydrocarbon generation, cracking of oil to gas, or some combination of these factors (Barker, 1990; Swarbrick and Osborne, 1998). Set 2 fractures were cemented at temperatures that exceed those for the generation and preservation of oil and gas (80–190jC) in the area (Bird et al., 1999). Moreover, Lisburne rocks in the area have low organic matter content (0.1–0.2 wt.% organic carbon; Magoon et al., 1987). The combination of high temperature and low organic matter would appear to preclude overpressure development as a result of hydrocarbon generation in the Lisburne itself. The most likely cause of overpressure and high temperature in the Lisburne Group was burial in the lower part of the deformational wedge. Burial of the Lisburne Group was probably facilitated by the combination of structural thickening of the wedge as it advanced into the foreland, the deposition of Brookian clastic rocks in the upper part of the wedge, and the isostatic subsidence caused by the tectonic and sedimentary load. Temperatures Hanks et al.
15
16 Geologic Note Figure 10. Depth-temperature-relative time graph showing tectonic environments of fracture formation and possible burial and uplift paths of the Lisburne Group and Permian to Triassic Echooka Formation in the study area. Curves i, ii, and iii are discussed in the text. Schematic cross sections AE illustrate the proposed structural conditions during the development of different fracture sets, with fracturing occurring at the $. Dashed lines in each cross section represents the ground surface. Temperature constraints are provided by fluid-inclusion measurements from fracture fill (this article), conodont alteration indices (Rejebian et al., 1987; Watts et al., 1995; Atkinson, 2001), and vitrinite reflectance data (Bird, 1999; Cole et al., 1999). Temperature is converted to depth using a constant geothermal gradient of 25jC/km (inside right graph edge) and 30jC/km (outside right graph edge). The age of deformational events as plotted on the horizontal axis is relative and based on regional apatite fission-track data from various localities in the northeastern Brooks Range (O’Sullivan et al., 1995, 1998; Peapples et al., 1997).
may have peaked because of a combination of burial and thermal relaxation but were followed shortly by cooling caused by erosional unroofing.
Early Tertiary Deformation and Uplift Contractional deformation in the northeastern Brooks Range continued episodically into the Tertiary, with three different regional exhumation events recorded by apatite fission-track data (45, 35, and 25 Ma; e.g., O’Sullivan et al., 1995, 1998). Subsequent tightening of the folds and unroofing resulted in the late-folding to post-folding fractures of set 3. Fluid-inclusion data suggest that set 3 fractures formed and filled at temperatures significantly cooler than those experienced during peak folding (153 versus 280jC; Figure 10, path i). However, these deformational events may not have affected all areas or all parts of the stratigraphic column equally. Different burial and uplift paths are possible depending on the regional structural setting and/or the stratigraphic position of the sample (paths i, ii, and iii; Figure 10). Rocks low in the stratigraphic section and/or located in a major synclinorium (such as the locations sampled in this study) could undergo continual burial and temperature increase during these Cenozoic events (path i). Higher parts of the stratigraphic section in the same location may have followed a flatter burial curve and never attained the high temperatures seen in the Lisburne samples (path ii). Rocks located on the crests of anticlinoria may have only undergone uplift and unroofing with an overall reduction of temperatures during these deformation episodes (path iii). There are no constraints on the depth and temperature of formation of the north–south-striking, postfold fractures (set 4, Table 2). These fractures are uncemented and appear quite young. They could have formed at a 3 – 5-km (1.8 – 3-mi) depth in response to the low differential stresses in the upper part of the deformational wedge if pore-fluid pressures were sufficiently high. Alternatively, they could have formed very near the surface because of uplift and removal of overburden. The first hypothesis is preferred because borehole breakouts in wells near the thrust front suggest that in-situ stresses become highly variable near the surface (Hanks et al., 2000), implying that there would not be a strong preferred fracture orientation if these fractures formed very near the surface.
IMPLICATIONS FOR HYDROCARBON MIGRATION AND TRAPPING To accumulate hydrocarbons, trap formation must precede or coincide with oil generation and migration. This is a complex process in fold and thrust belts, where burial and generation of hydrocarbons in any one location generally precede deformation. Effective migration pathways out of the hydrocarbon kitchen are necessary to fill distant traps and reservoirs. This study provides important constraints on the timing of fracturing and fracture cementation with respect to folding and source rock maturation in the northeastern Brooks Range. The earliest fractures (set 1) might have formed during the Cretaceous, coincident with burial caused by sedimentation and basin subsidence and early hydrocarbon generation and migration (Figure 10a). These regional fractures developed parallel to maximum horizontal in-situ stresses and were oriented perpendicular to the thrust front. Although fracture formation preceded the development of structural traps in the study area, hydrocarbons generated in the basin might have migrated along these open fractures into updip stratigraphic traps to the north-northwest or migrated north to the Barrow arch. In the detachment-folded Lisburne Group, a second set of fractures (set 2) developed early during folding in the latest Cretaceous to early Tertiary (Figure 10b), at temperatures exceeding those for oil generation (80 – 140jC). Thus, fracture-influenced traps in these types of structures would have postdated local hydrocarbon generation and migration. However, this does not preclude hydrocarbons generated in less deformed or undeformed parts of the foreland basin from migrating into these traps. For example, hydrocarbons generated in areas of lower structural and thermal maturity to the north, west, or northeast could have migrated up structural dip to fill these structures. However, the temperatures at which set 2 fractures formed and were cemented were probably too high for significant hydrocarbons to be preserved. Peak folding and penetrative strain occurred at very high temperatures (Figure 10c), probably destroying most existing porosity and permeability, including much of the fracture network. Late-fold (set 3) and postfold (set 4) fractures developed at significantly lower temperatures (Figure 10d, e), but by this time, the local hydrocarbon-generation potential was probably exhausted. However, these deformed and fractured rocks were structurally elevated Hanks et al.
17
relative to significantly younger rocks in the foreland basin to the north, which could possibly provide a hydrocarbon source for these younger fractures.
CONCLUSIONS 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, plus thermal information from fluid inclusions in fracture cements, can be integrated with existing thermal and geochronologic data to constrain the thermal and deformational sequence of these rocks. The Lisburne Group in the study area probably entered the oilgeneration window (80 –140jC) during the Early Cretaceous formation of the Colville basin caused by thrust loading by the growing Brooks Range to the south. Regional fractures formed during this time as a result of high pore pressures, and low in-situ differential stresses possibly served as a migration pathway for hydrocarbons. These rocks were not significantly shortened until the latest Cretaceous to early Tertiary, with the first phases of formation of the northeastern Brooks Range. Early detachment folding was characterized by flexural slip, tangential longitudinal strain, and associated fracturing. With continued shortening during the early Tertiary, structural thickening resulted in continued burial and/or increasing temperatures. Early fold-related fractures were subsequently overprinted by penetrative strain during peak folding at temperatures of approximately 280jC. Late fold-related fractures formed at approximately 150jC during subsequent uplift and unroofing of the thickened wedge. Late and pervasive postfolding extension fractures formed at relatively shallow depths and low temperatures and are related to unroofing and/ or regional stresses in the orogenic wedge. These results indicate that hydrocarbon generation predated initial trap formation at this particular location. However, hydrocarbons generated elsewhere in the foreland basin could have migrated into these rocks. During the latest Cretaceous and Tertiary, these highly deformed rocks were structurally elevated with respect to less deformed and thermally mature sediments that were generating oil and/or gas in the foreland basin to the north, northwest, and west. Thus, similar traps in the subsurface could be viable plays if hydrocarbons successfully migrated into them from less structurally and thermally mature parts of the basin. 18
Geologic Note
Collecting and integrating structural, thermal, and geochronological data from various locations throughout the North Slope, the Brooks Range and the northeastern Brooks Range fold and thrust belt will provide additional important clues to the evolution of this particular petroleum system. In addition, exploration efforts should distinguish between pre- and peak-folding fractures, which would be cemented before any later hydrocarbon migration, and late-fold and postfold fractures, which would have a greater likelihood of remaining open.
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to 1982: U.S. Geological Survey Professional Paper 1399, p. 143 – 186. Moore, T. E., W. K. Wallace, K. J. Bird, S. M. Karl, C. G. Mull, and J. T. Dillon, 1994, Geology of northern Alaska, in G. Plafker and H. C. Berg, eds., The geology of Alaska: The Geology of North America, Geological Society of America, Boulder, Colorado, v. G1, p. 49 – 140. Mull, C. G., 1982, Tectonic evolution and structural style of the Brooks Range, Alaska: An illustrated summary, in R. B. Powers, ed., Geologic studies of the Cordilleran thrust belt: Denver, Colorado, Rocky Mountain Association of Geologists, v. 1, p. 1 – 45. NIST (National Institute of Standards Chemistry Webbook), 2005: http://webbook.nist.gov/chemistry/ (accessed June 17, 2005). O’Sullivan, P. B., C. L. Hanks, W. K. Wallace, and P. F. Green, 1995, Multiple episodes of Cenozoic denudation in the northeastern Brooks Range: Fission track data from the Okpilak batholith, Alaska: Canadian Journal of Earth Sciences, v. 32, no. 8, p. 1106 – 1118. O’Sullivan, P. B., W. K. Wallace, and J. M. Murphy, 1998, Fissiontrack evidence for apparent out-of-sequence Cenozoic deformation along the Philip Smith Mountain Front, northeastern Brooks Range, Alaska: Earth and Planetary Science Letters, v. 164, p. 435 – 449. Parris, T. M., R. C. Burruss, and P. B. O’Sullivan, 2003, Deformation and the timing of gas generation and migration in the eastern Brooks Range foothills, Arctic National Wildlife Refuge, Alaska: AAPG Bulletin, v. 87, p. 1823 – 1846. Peapples, P. R., W. K. Wallace, C. L. Hanks, P. B. O’Sullivan, and P. W. Layer, 1997, Style, controls, and timing of fold-andthrust deformation of the Jago stock, northeastern Brooks Range, Alaska: Canadian Journal of Earth Sciences, v. 34, p. 992 – 1007. Reiser, H. N., 1970, Northeastern Brooks Range — A surface expression of the Prudhoe Bay section, in W. L. Adkison and M. M. Brosge´, eds., Proceedings of the Geological Seminar on the North Slope of Alaska: AAPG Pacific Section, p. K1 – K13. Reiser, H. N., W. P. Brosge´, J. T. Dutro Jr., and R. L. Detterman, 1980, Geologic map of the Demarcation Point quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1133, scale 1:250,000, 1 sheet. Rejebian, V. A., A. G. Harris, and J. S. Huebner, 1987, Conodont color and textural alteration: An index to regional metamorphism, contact metamoprhism, and hydrothermal alteration: Geological Society of America Bulletin, v. 99, p. 471 – 479.
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Geologic Note
Roedder, E., 1984, Fluid inclusions: Mineralogical Society of America, Reviews in Mineralogy, v. 12, 646 p. Rowan, E., 1999, Timing and extent of oil generation, Canning River region, North Slope, Alaska, based on a reconstruction of burial and conductive heat flow history, in The oil and gas resource potential of the 1002 area, Arctic National Wildlife Refuge, Alaska: U.S. Geological Survey Open File Report 98-34, p. BE1 – BE59. Shackleton, J. R., M. L. Cooke, and A. J. Sussman, 2005, Evidence for temporally changing mechanical stratigraphy and effects on joint-network architecture: Geology, v. 33, no. 2, p. 101 – 104. Smith, D. L., and B. Evans, 1984, Diffusional crack healing in quartz: Journal of Geophysical Research, v. 89, p. 4125 – 4135. Stearns, D. W., and M. Friedman, 1972, Reservoirs in fractured rock, in R. E. King, ed., Stratigraphic oil and gas fields — Classification, exploration methods, and case histories: AAPG Memoir 16, p. 82 – 106. Swarbrick, R. E., and M. J. Osborne, 1998, Mechanisms that generate abnormal overpressures: An overview, in B. E. Law, G. F. Ulmishek, and V. I. Slavin, eds., Abnormal pressures in hydrocarbon environments: AAPG Memoir 70, p. 13 – 34. Van Den Kerkhof, A., 1990, Isochoric phase diagrams in the systems CO2 – CH4 and CO2 – N2: Application to fluid inclusions: Geochimica et Cosmochimica Acta, v. 54, p. 621 – 629. Van Den Kerkhof, A., and R. Thiery, 2001, Carbonic inclusions: Lithos, v. 55, p. 49 – 68. Wallace, W. K., 1993, Detachment folds and a passive-roof duplex: Examples from the northeastern Brooks Range, Alaska, in D. N. Solie and F. Tannian, eds., Short notes on Alaskan geology 1993: Alaska Division of Geological and Geophysical Surveys Geologic Report 113, p. 81 – 99. Wallace, W. K., and C. L. Hanks, 1990, Structural provinces of the northeastern Brooks Range, Arctic National Wildlife Refuge, Alaska: AAPG Bulletin, v. 74, no. 7, p. 1100 – 1118. Wallace, W. K., T. E. Moore, and G. Plafker, 1997, Multistory duplexes with forward dipping roofs, north central Brooks Range, Alaska: Journal of Geophysical Research, v. 102, no. B9 (special section on the U.S. Geological Survey Trans-Alaska Crustal Transect), p. 20,773 – 20,796. Watts, K. F., et al., 1995, Analysis of reservoir heterogeneities due to shallowing-upward cycles in carbonate rocks of the Pennsylvanian Wahoo Limestone of northeastern Alaska: U.S. Department of Energy Final Report for 1989 – 1992, Bartlesville Project Office, 433 p.
