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A U T H O R S

Zhuoheng Chen $ Geological Survey ofCanada, Natural Resources Canada, 3303-33rdStreet, NW Calgary, Alberta T2L 2A7, Canada;[email protected]

Zhuoheng Chen obtained his Ph.D. from theNorwegian University of Science and Technologyin 1993 and held a position as an associateprofessor at China University of Petroleum(Beijing) before joining the Geological Surveyof Canada in 1998. He has developed methodsfor resource assessment and exploration riskevaluation. His research interests include pe-troleum resource assessment (methods andapplications), petroleum systems, and basinanalysis.

Kirk G. Osadetz $ Geological Survey of

Canada, Natural Resources Canada, 3303-33rdStreet, NW Calgary, Alberta T2L 2A7, Canada;[email protected]

Kirk Osadetz graduated from the University ofToronto (B.S. degree, 1978; M.S. degree, 1983).He manages the ESS Gas Hydrates Fuel of theFuture Program and is the head of Energy &Environment Subdivision at the Geological Surveyof Canada, Calgary. He is active in petroleumresource evaluation and has research interestsin gas hydrates, tectonics, and thermochronol-ogy. He worked previously at Gulf Canada Re-sources Inc. and PetroCanada Resources Inc.in Calgary.

Dale R. Issler $ Geological Survey ofCanada, Natural Resources Canada, 3303-33rdStreet, NW Calgary, Alberta T2L 2A7, Canada;[email protected]

Dale Issler has an honors (co-op) B.S. degree inearth sciences (University of Waterloo) and aPh.D. in oceanography (Dalhousie University).He is a Geological Survey of Canada researchscientist involved in quantitative basin analysis(apatite fission track thermochronology, shalecompaction, petrophysics, well-log analysis,thermal modeling). Presently, he is leadingan industry-government project on petroleumsystems in the Beaufort-Mackenzie area.

Hydrocarbon migration detectedby regional temperature fieldvariations, Beaufort-Mackenzie

Basin, CanadaZhuoheng Chen, Kirk G. Osadetz, Dale R. Issler, andStephen E. Grasby

ABST R AC T

The regional Beaufort-Mackenzie Basin temperature fieldis characterized using data collected from drill-stem tests and

bottom-holetemperature logs. We recognizetwothermalanom-alies, each of which is associated with a specific geological set-ting. Elevated temperatures are observed in (1) the westernBeaufort Sea, where post-Eocene erosion removed Cenozoicstrata and folding is common in a contractional tectonic re-

gime, and (2) along fault zones where upward flow transportsheat by advection. Depressed temperatures are observed inEocene and post-Eocene rapidly subsiding depocenters, withoverpressure developed below 3000 m (9843 ft). Older strataalong the southeast rifted margin are characterized by a more

normal thermal regime. Evidence from anomalously high tem-peratures in both map and cross-sectional views suggests thatfault zones and major regional aquifers accommodate the up-ward expulsion of fluids from deep overpressured zones. Many

significant petroleum discoveries occur in areas where anom-alously high temperatures are observed, suggesting that petro-leum migration occurs along the same flow networks. Iden-tifying anomalies in the temperature field may therefore be a

useful exploration technique.

INTRODUCTION

The Upper CretaceousCenozoic Beaufort-Mackenzie Basin(BMB) succession of northern Canada represents a postriftbasin containing more than 14 km (46,000 ft) of deltaic and

AAPG Bulletin, v. 92, no. 12 (December 2008), pp. 16391653 1639

Copyright#2008. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received January 18, 2008; provisional acceptance April 8, 2008; revised manuscript

received July 22, 2008; final acceptance July 30, 2008.

DOI:10.1306/07300808011

December Search Index

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marine sediments (Dixon et al., 1992). Recent increased de-mands for natural gas and crude oil in North America and aroundthe world have sparked renewed interest in the search for pe-troleum resources in the BMB region (Figure 1) (Bergquistet al., 2003). This has motivated a reassessment of the regional

petroleum potential. As part of that work, we examine the

temperature field within the basin.Basin thermal history and geothermal structure are es-

sential elements of basin analysis as well as being important

factors in petroleum exploration (e.g., Barker, 1996; MakhousandGalushkin, 2005). Thermal structureis controlled primarilyby deep heat flux, thermal conductivity of the basin, and rele-vant shallow boundary conditions (i.e., surface temperature or

base of permafrost). However, transient geological events andprocesses, such as denudation and fluid flow, can significantlyalter the thermal structure.

Previous studies of the thermal history and temperature

regime in the BMB include thermal modeling of several off-shore wells(Issler andSnowdon, 1990; WhiteandIssler, 1995),thermal gradient andheat flow studies (Majorowicz andDietrich,1989; Majorowicz et al., 1996), and organic thermal maturitystudies (Goodarzi, 1996; Stasiuk et al., 2005). The near-surface

temperature field and paleosurface temperature history wereexamined as part of a permafrost study (Allen et al., 1988).Isomaturity contours were shown to crosscut lithostrati-graphic boundaries (Goodarzi, 1996), the likely result of

variations of heat flow and the history of burial and exhuma-

tion. Thermal modeling of potential source rocks in the off-shore region shows low organic maturity in the upper 5000 m(16,400 ft) of rapidly buried Cenozoic sediments and suggests

that the oil window may occur above the 100140jC tem-perature threshold typical of most sedimentary basins becauseof lack of time (Issler and Snowdon, 1990). A comparison ofmeasured temperature and vitrinite reflectance data suggeststhat erosion could have had a larger effect on observed ma-

turity patterns than do heat-flow variations (Majorowicz andDietrich, 1989).

An analysis of the spatial distribution of significant discov-eries in the BMB indicates the alignment of the discoveries

with certain geological features, particularly the regional faultzones. For example, exploration drilling in the Tarsiut-Amauligakfault zone (TAFZ) had more than 50% success, much greaterthan other areas (Dixon et al., 1994). The geological conditions

close to the fault zone are apparently more favorable than otherareas in this basin. To improve our understanding of geolog-ical controls on petroleum occurrence for a better assessmentof petroleum potential, a basin-scale hydrodynamic study was

Stephen E. Grasby $ Geological Surveyof Canada, Natural Resources Canada, 3303-33rd Street, NW Calgary, Alberta T2L 2A7,Canada; [email protected]

Stephen Grasby obtained his Ph.D. in aqueousgeochemistry from the University of Calgaryin 1997. Since then he has worked at the Geo-

logical Survey of Canada, Calgary. He has beenactive in the Groundwater Program and cur-rently leads the project on assessment of regionalaquifers. He has worked extensively on thebiogeochemistry of thermal and mineral springsin Canada.

A C K N O W L E D G E M E N T S

IHS Energy Ltd. generously provided all thedigital logs for this study. Z. Chen thanks J. Dixon,M. Li, and J. Dietrich of Geological Survey ofCanada (GSC), Calgary, for numerous discussionsthat helped with a better understanding of thebasin geology and petroleum geochemistry. A.Jessop provided a digital data set of the CanadianGeothermal Data Collection Project. The tem-perature and pressure data are from an industryconsortium project for the Beaufort-MackenzieBasin, which were compiled by Z. Hu. The au-thors are grateful for the comments and sug-gestions made by internal critical reviewerGlen Stockmal of the Geological Survey ofCanada; AAPG reviewers David A. Pivnik, JohnD. Bredehoeft, and Jeffrey A. Nunn; and AAPGeditor, Gretchen M. Gillis, which have signifi-cantly improved this article. ESS contribution#20080239.

1640 Regional Temperature Field and Its Geologic Significance


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Figure 1. (a) Study area location map showing the general structural features (fault pattern is based on Lane and Dietrich, 1996).ELFZ = Eskimo Lakes fault zone; TFZ = Taglu fault zone; TAFZ = Tarsiut-Amauligak fault zone; WBTF = west Beaufort thrust front;BMTL = Beaufort-Mackenzie tectonic lineament. The black dashed line indicates the location of panel b. Panel b is a geological crosssection (modified from Dixon et al., 1985). Fish R. = Fish River; Mackenzie B. = Mackenzie Bay; Pz = Paleozoic; uK = UpperCretaceous; lK-J = Lower CretaceousJurassic undifferentiated; SL = sea level. See panel a for the section location. The BMB is shownby the red box in the index map.

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conducted with an emphasis on basin fluid-flowpatterns and their association with petroleum ac-cumulations. This article focuses on the effects ofhydrodynamics on basin thermal structure and pre-sents our interpretations of the spatial variation of

the temperature field and discusses primary con-

trols on the temperature profile and the associatedimplications for petroleum occurrence. We recog-nize that the present temperature regime may not

represent paleothermal conditions associated withprior petroleum generation. However, tempera-ture fingerprints may provide indications for basintectonics and regional flow systems, which are key

to understanding the links between petroleumsources and accumulations.

GEOLOGICAL SETTING

The BMB region (Figure 1) exhibits a complex evo-lution starting with an open-marine setting throughmost of the Paleozoic, followed by a rift-drift system

in the Jurassic to Early Cretaceous (Dixon et al.,1994). The Upper Cretaceous Cenozoic upper riftsuccessions of the BMB (Figure 2) comprise foldedand faulted deltaic complexes that exceed 14 km

(8.6 mi) in thickness (Lane and Dietrich, 1995).The regional geological setting and petroleum playtypes are well documented by Young et al. (1976),Dixon et al.(1985,1994), Lane and Dietrich (1995),and Dixon (1996). Many aspects of the petroleum

geology have been discussedpreviously(e.g., Snowdon,1988; Enachescu, 1990; Issler and Snowdon, 1990;Issler, 1992; Dixon et al., 1994; Bloch and Issler,1996; Dixon, 1996; Snowdonetal., 2004; Chen etal.,

2007).Basin tectonics is well studied at the regional

scale (e.g., Lane and Dietrich, 1995; Embry, 1998;Lane,1998,2002). LaneandDietrich(1995)mapped

several fault zones in the BMB, four of which areparticularly important to this study (Figure 1). TheEskimo Lakes fault zone (ELFZ), striking north-eastsouthwest, marks the southeast margin of theJurassicEarly Cretaceous rift system. The Taglu

fault zone (TFZ), parallel to the ELFZ, controls theLate Cretaceous to middle Tertiary depocenters inthe Mackenzie delta. The TAFZ, running nearly

Figure 2. A simplified BMB stratigraphic column indicating themajor regional aquifers and zones with significant petroleumdiscoveries (modified from Lane and Dietrich, 1995). Where theTaglu and Aklak sequences are undifferentiated, the name Rein-deer supersequence is applied. CK. = creek; SEQ. = sequence.



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eastwest, defines the southern boundary of the

post-Miocenedepositional center. The westBeaufortthrust front (WBTF) in the western Beaufort Sea,parallel to the southwestern coast line, defines thecontractional deformation front of the thrust faultsin the basin. We recognize a composite tectonic ele-

ment, or zone, defined by a variety of geological and

geophysical anomalies and marked by the transi-tional zone from the predominantly contractionaltectonic regime in the west to theextension regime

in the listric fault zone in the central delta. This zoneis parallel to the front of the thrust belt in the south-west offshore area and perhaps represents a south-ern extension of the Beaufort fault zone in the

Canada Basin defined by Lane (2002). We refer tothis zone as the Beaufort-Mackenzie tectonic linea-ment (BMTL). The offshore extension of the BMTL

can be traced by seismically mapped fault segmentsand folds.

The four major fault zones and the BMTL de-

fine four tectonic domains in the basin (Figure 3).Domain I (D-I) is the basin margin, where sedimentswere deposited in the JurassicCretaceous rift orolder systems in the southern part of the basin and

where postdepositional erosion has been substan-tial. Domain II (D-II) is the core of the Mackenziedelta, where listric faults and rollover folds pre-vail. Domain III (D-III) is in the northern part ofthe study area, where basin fill is predominantly

super-Miocene and shale-cored anticlines or diapirsare common. Domain IV (D-IV) covers the south-western Beaufort Sea, where the structural archi-tecture of the Upper Cretaceousearly Tertiary suc-

cessions was a passive margin complicated by the

Figure 3. The domain divisions of the BMB. D-I = basin margin domain; D-II = listric fault domain; D-III = shale-cored anticlinedomain; D-IV = west Beaufort contractional complex domain. Locations of the temperature profiles in Figure 7 are indicated. The dots

indicate the location of wells, and the dashed lines are the various faults.

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overprint of an Eocene foreland basin, includingthrusting and folding with significant erosion.

DATA ANALYSIS AND RESULTS

Data Source

Up to 2006, 268 wells were drilled in the study area,and data from 251 wells are publicly available. This

study is based on a comprehensive data set con-sisting of drill-stem test (DST) and repeat forma-tion test (RFT) results (176 wells), bottom-holetemperature (BHT) data (238 wells), vitrinite re-

flectance measurements (92 wells), and digital welllogs (251 wells). Of the 1243 temperature datapoints used for this study, 589 are DST/RFT tem-perature measurements and 654 are estimates ofthe true temperature from BHT measurements

using a Horner plot correction (Dowdle and Cobb,1975). Issler et al. (in press) provide a discussion oftemperature data corrections and quality assess-ments. All the temperature data are used for de-

termining the regional average temperature-depthtrend (Figure 4a)because the exclusion of the poorerquality data only increases the correlation coeffi-cient of the linear regression by 1%. Shallow tem-

perature measurements were obtained from theCanadian Geothermal Data Collection project con-ducted from 1955 to 1980 (Judge et al., 1981).Because an ice-saturated porous mediumhas a much

higher acoustic travel velocity and formation resis-tivitythan a water-saturatedporousinterval,thebaseof permafrost is a well-defined boundary on geo-physical logs. The inferred base of the ice-bearingpermafrost zone, assumed to be 0jC, was used as

an additional temperature constraint (Issler et al.,

in press). The climate records from the last 75 yrat the Inuvik station (from Environment Canada,2003) show an annual mean temperature of9jC,

suggesting that the temperature between the in-ferred base of permafrost and ground surface shouldnot be greater than 0jC. The temperature data, aswell as the inferred base of ice-bearing permafrost,

were plotted against depth in each well prior to tem-perature mapping to ensure data quality; obviouslypoor-quality data were removed to avoid distorting

temperature trends. Well information (from theNational Energy Board well history reports) andformation boundaries (constrained by log and seis-

mic interpretation and biostratigraphy) were de-rived from the in-house well database at the Geo-logical Survey of Canada, Calgary.

Vertical Temperature Profiles

Temperature data show a general linear trend withdepth but with a large variation (Figure 4a). A lin-ear fit gives a geothermal gradient of 25.7jC/km(7.8jC/1000 ft), used here as the normal gradient

for the basin. Althougha high correlation coefficientof 0.94 is observed, this generalized linear modeldoes not help us understand the spatial variation ofthe temperature field or the observed temperature

variation as a function of depth.Across thestudy area, three typical temperature

profiles were recognized, each of which is associatedwith a specific geological domain. For example, an

elevated temperature profilecharacterized by higherthan average temperatures at a givendepth ($20jC;red squares in Figure 4b) in D-IV may reflect theimpact of contractional tectonics and post-Eoceneuplift. In contrast, in D-II, the lower than normal

temperatures (green circles in Figure 4b) mayreflect

the postrifthigh deposition rate. A geothermal profilecloser to average is found in most of D-I, and thegeothermalprofile inD-III is similar tothe one inD-II.

Temperature Field at Depth 1000 m (3300 ft)

At shallow depths (1000 m, 3300 ft), two areasshow anomalously high temperatures (T> 25jC).

One is in the western Beaufort Sea (D-IV) and theadjacent onshore area, andthe other along the rifted

margin (D-I) southeast of the ELFZ (Figure 5a).Theseelevatedtemperaturezones are associated with

thin permafrost (Figures 6a, 7). Low-temperatureanomalies (< 15jC, in Figure 5a) are located in theEoceneOligocene depocenters in D-II as well asin the super-Miocene depocenter of D-III, north to

the TAFZ. The remainder of the area shows mod-erate temperatures ranging from 15 to 25jC, re-stricted to belts between zones of high and lowtemperature (Figure 5a).



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At2000 m (6600 ft), the size ofthehigh-temperaturearea in D-IVshrinks, and it is mostly restricted tothe

region between the WBTF and BMTL (Figure 5b).This temperature anomaly coincides approximatelywith the subcrop belt of the Reindeer Formation(Dixon et al., 1994, their figures 65, 66). The anom-

alous area of high temperature in D-I remains aboutthe same as observed in the shallower depth in-terval, as described above. Within D-I, the high-

temperature zone in the northern Tuktoyaktuk Pen-

insula (Figure 1) is coincident with an earthquakeswarm that includes events of magnitude up to 4.36.5 on the Richter scale (NRCan Earth Sciences Sec-tor, Canadian Hazards Information Service, 2006).


At 3000 m (9843 ft), the high-temperature zonesbecome more restricted to regions near major faultzones, for example, the ELFZ, the southern part of

Figure 4. (a) Temperature profile from the complete data set, showing the fit to a linear model. (b) Comparison of the temperatureprofiles from D-II and D-IV. The average temperature in D-IV is elevated about 20jC relative to that in D-II. See panel c for thelocation of the wells used in panels b (marked by green circles and red squares) and a (all wells marked by different symbols). DST =drill-stem test; BHT = bottom-hole temperature.

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the TFZ (where east-weststriking faults prevail),and the TAFZ. The linear nature of the anomalouslyhigh temperatures is also indicated by numeroushigh-temperature anomalies on temperature crosssections (Figure 7a f) near faults. These linear high-

temperature anomalies are not apparentat shallower

depths because most of the listric normal faults areoverlain by Pliocene sediments. High-temperatureanomalies are also observed associated with shale-

cored diapirs and similarly interpreted structuralhighs. This is particularly the case in D-IV betweenthe WBTF and the BMTL, and in the west of D-III,where contractional tectonics are also significant.

The areas of anomalously low temperatures showlittle change in character or extent compared toshallow depth maps.

Isotemperature Maps

An alternative way to examine the temperature-depth relationship is to map the depth to a constanttemperature. These isotemperature maps also show

important geographical variations (Figure 6). Areaswith thick permafrost occur in D-II and D-III andare bounded by the BMTL in the southwest andby the ELFZ in the southeast (Figure 6a), and arecoincident with the region of low-temperature

anomalies.The depth to30jCrangesfromlessthan500m

(1640 ft) in the southwest BMB to greater than2200 m (7220 ft) in the north. The shallowest

depths to 30jC are in D-IV and the adjacent coastalarea, as well as in the D-I rifted margin, south to theELFZ (Figure 6b). The depth to 50jC (Figure 6c)shows a similar pattern to the temperature varia-

tion at 3000 m (9843 ft) (Figure 5c). The shallowestdepths appear in D-IV and are restricted between

the BMTL and WBTF, and along the ELFZ in thesoutheast, the southern part of the Taglu fault, andmost of the Tarsiut-Amauligak fault (Figure 6c).

Temperature Cross Sections

Six cross sections showing vertical temperature pro-files along with stratigraphic boundaries were con-

structed to illustrate the relationship of temperatureto major geological features (Figure 7). In general,

Figure 5. Temperature contour maps at different depths:(a) 1000 m (3280 ft), (b) 2000 m (6561 ft), and (c) 3000 m(9843 ft). The outlines of petroleum discoveries are overlain onthe temperature map (c) to illustrate the spatial relationshipbetween anomalous temperatures and petroleum accumula-tions. ELFZ = Eskimo Lakes fault zone; TFZ = Taglu fault zone;TAFZ = Tarsiut-Amauligak fault zone; WBTF = west Beaufortthrust front; BMTL = Beaufort-Mackenzie tectonic lineament.



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isotemperature lines cross formation boundaries,but temperature lows occur where thick Tertiarysuccessions are well preserved (Figure 7a e). Nearthe BMTL toward the west, temperatures are ele-vated (Figure 7ac).

Numerous high-temperature plumes are pre-

sent on thetemperaturesections, most of which areassociated with fault zones. Temperatures can varysignificantly over a very short distance, for exam-

ple, between wells Toapolok H-24 and Ya Ya P-53(wells 87 and 46 in Figure 7f ). High-temperatureplumes are also observed in isolated highs and inhigh-amplitude anticlines in domain II (e.g., well

70 of Figure 7b), on top of which small faults andfracture systems are commonly seen from seismicdata (e.g., Elsley and Graham, 2006).

Interestingly, deep anomalously high tempera-

tures are also coincident with surface lakes (forexample, Parsons D-20; well 127 of Figure 7c), sug-gesting that some lakes may be in contact with sub-surface faults or fracture zones that could serve asan outlet for upwelling deeper and warmer basinal

fluids. Although the temperature data in this studycannot prove the relationship between these two,the model from a recent study of pingolike struc-tures in the offshore Beaufort Sea area supports

our interpretation (Paull et al., 2007). This is also

supported by the presence of natural gas seeps, orkushkas, into some of these lakes, which are moreobvious in the autumn and spring when the agita-

tion of the water by the rising gas results in theanomalous freezing and thawing of the ice coveron the lake. Whether the thermal karst underlyingsome of these lakes (e.g., Parsons Lake) is causedby thermal boundary conditions related to unfrozen

lake water or caused by a warm water flux ascend-ing along the faults or fracture zones should be

investigated further.

DISCUSSION

The description above provides a general picture ofthecurrentBMB temperature field.Low-temperature

anomalies are associated with Tertiary depocen-ters,particularly the Eocene Oligocene and super-Miocene depocenters in D-II and D-III where a

Figure 6. Isotemperature maps: (a) 0jC, equivalent to thebase of permafrost zone; (b) 30jC; and (c) 60jC. All depths arein meters from ground level, in a region of low relief, much ofwhich is near sea level. ELFZ = Eskimo Lakes fault zone; TFZ =Taglu fault zone; TAFZ = Tarsiut-Amauligak fault zone; WBTF =west Beaufort thrust front; BMTL = Beaufort-Mackenzie tectoniclineament.

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Figure 7. Temperature cross sections. See Figure 3 for the locations of the cross sections. The numbers on top of individual crosssections are well index numbers. Stratigraphic contacts are shown by colored lines. Contours are generated using the Matlab1 linearextrapolation algorithm. WBTF = west Beaufort thrust front; BMTL = Beaufort-Mackenzie tectonic lineament; M. Bay = Mackenzie Bay;U. Cret. = Upper Cretaceous; L. Cret. = Lower Cretaceous; L. Cret.-J = Lower CretaceousJurassic undifferentiated; uK = UpperCretaceous; L. K = Lower Cretaceous; Mz = Mesozoic.



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Figure 7. Continued.

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record of rapid subsidence and thick super-Paleocenesedimentary successions is well preserved. The shapeand size of these low-temperature anomalies ap-pear to remain about the same at least in the first3000 m (9843 ft) from the surface, indicating the

persistent geological setting in a passive margin

during the past few million years. The anomalouslyhigh temperatures are approximately coincident inplan view with both fault zones and the areas in the

western Beaufort Sea where post-Miocene exhu-mation and denudation are significant because ofcontractionaltectonicsin a foreland basin.The shapeand size of the high-temperature anomalies vary

with depth. These anomalies show the largestarealextent at shallower depths. With increasing depth,they become more restricted and more closely as-sociated with active fault zones, regional structural

highs, and anticlines.This suggests that tectonics is a significant fac-

tor in controlling the BMB thermal structure. Themost obvious indicators are elevated formation tem-peratures in the western Beaufort Sea that are re-

lated to post-Miocene contractional tectonics, whichinvolved thrusting or folding and subsequent de-nudation. This was accompanied by the northwardand eastward migration of super-Miocene depo-

centers. Although linear fits to the data givesimilar

temperature gradients in the Tertiary successions ofD-II and D-IV, nonlinear fits reveal different char-acteristics of variation of temperature with depth

(fitted lines in Figure 4b). The temperature profiledisplays a convex shape in the D-IV area and a con-cave shape in D-II. Convex-shaped temperatureprofiles occur in regions commonly associated withadvective heat flow (Allen and Allen, 2005). The

absolute temperatures could be 20jC higher inD-IV compared to those in D-II (Figure 4b). The

effects of contractional tectonics have led to a basin-scale uplift in the western and southern parts of the

study area and created a general eastward dip. As aresult, the tilted thick regional deltaic aquifers ofthe Taglu and Aklak sequences became the con-duits for the discharge of warmer fluids, which orig-

inated in the deep overpressured D-II depocenterand migrated into the western Beaufort Sea. Sec-ond, contractional tectonics inverted normal faultsin the paleopassive margin of the west Beaufort Sea,

resulting in the formation of shale-cored anticlinesor diapirs. Vertical movement of deep fluids along

the associated fracture systems increased formationtemperatures locally. This temperature increase,caused by advective heat transport, is restricted tothe area to the east of the WBTF. To the west, thrust

faults of the WBTF, and older more compactedsediments, form a barrier against lateral fluid mi-gration (Figure 7a, b, d). This explains the differ-ences in the areal extent of heat anomalies at dif-ferent depths in D-IV.

Another type of high-temperature anomaly ap-pears as linear temperature trends in map view;these trends are spatially associated with major faultzones (Figures 5c, 6c). Whereas faults can be ef-

fective barriers to fluid movement across the fault

plane, associated fracture zones commonly formhigh-permeability conduits parallel to the fault(Caine et al., 1996). Cenozoic sedimentary rocks can

be connected with Mesozoic and Paleozoic stratavia high-angle normal faults along the southeastcoast of the rifted margin in D-I (Figure 7d, e).The linear high-temperature anomalies within theELFZ are likely evidence of deep, warm fluids mov-

ing from overpressured lower Tertiary and oldersuccessions in the Kugmallit Trough (Figure 1). Be-cause of the stratigraphic seals provided by the

Mackenzie Bay and Akpak sequences, the Kugmallitoverpressure system is developed in D-III whereasthe overpressure developed in D-II occurs in theTaglu sequence, which is sealed by the Richards se-quence (Chen et al., in press). The TAFZ and TFZ

form the south and southeast boundaries of thetwo overpressured depocenters, respectively. Thesefaults have direct connection with the overpres-sured centers (Chen et al., in press). The linear high-temperature anomalies along the TAFZ and TFZ

suggest that they have been the locus of verticallymigrating warm fluids (Figures 5c; 6c; 7b, d, c).This is also consistent with evidence from forma-tion water geochemistry (Grasby et al., in press).

Temperature depressions are geographically co-incident withEocene Oligoceneand super-Miocenedepocenters, which are themselves bounded by re-gional faults and tectonic lineaments (Figures 5, 6).

Although variable in age, the Eocene and super-Eocene depocenters in D-II and D-III exhibit



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overpressure below 3000 m (9843 ft) (Chen et al.,in press). Thefine-grained sedimentaryrocks of theRichards sequence in D-II and of the Akpak andMackenzieBay sequences in D-III serve as top sealsfor the overpressured fluids confined in the Taglu

and Kugmallit aquifers, preventing direct vertical

migration of the deep warm fluids (Chen et al., inpress;Grasby et al., in press). Thefine-grained shalesand mudstones in the depocenter have relatively

low thermal conductivity (Blackwell and Steele,1988). In addition, the aquifers below the aquitardsare more porous because of overpressure, thus re-ducing the thermal conductivity compared to the

normal pressured sediments at the same depth. Allthese factors affect heat flow and result in lowertemperatures in these depocenters.

The observed temperature variations have sig-

nificant implications for source rockmaturity in theBMB region. Petroleum system modeling suggestsmultiple phases of petroleum generation and mi-gration from potential source rocks of differentages, at various burial depths, in different parts of

the basin (Chen et al., 2007). In the western Beau-fort Sea, because of the higher formation tempera-tures, Paleocene strata appear to be the effectivesource rocks for oils because they are in the oil gen-

eration window, whereas Cretaceous and older

source rocks passed through the oil window, andtheir oil was expelled before the major phase oftrap formation in the late Eocene, but could be

sources for gas.In contrast, Cretaceous source rocksappear to be the major petroleum contributors inthe eastern deep-water part of the basin becauseof the low thermal maturity of Tertiarysource rocks,as a result of temperature suppression in the Ter-

tiary depocenters.Given that thermal anomalies in the basin are

related to fluid migration, one would expect thatpetroleum generated from these successions would

migrate along the same routes. This in turn suggeststhat petroleum accumulations should be preferen-tially associated with the temperature anomaliesthat indicate the migration fairways. Many signifi-

cant discoveries are, in fact, found in the major faultzones and associated with local high-temperatureanomalies (Figures 5c, 6c), supporting the linkbetween temperature anomalies and petroleum mi-

gration. Discoveries are also made on tops of shale-cored anticlines in areas of low background tem-

perature in D-II and D-III. Detailed examinationsof temperature variation show that many discover-ies are associated with local temperature highs ontemperature residual cross sections (e.g., Kopanoar

M-13, well 131, Figure7a; Koakoak O-22, well 163,Figure 7b; and Nerlerk M-98 well 151, Figure 7b),indicating therelation between deep fluid migrationand high-temperature anomalies.

CONCLUSIONS

The observed BMB temperature anomalies at aregional scale are the result of both tectonics and a

stratigraphic framework that controls the major ba-sin aquifer architecture. The discharge of fluids origi-natingin the basincenter follows patterns of tectonicuplift resulting in anomalously high temperatures

in the western Beaufort Sea. There, the dipping re-gional aquifers serve as the conduits for flow out ofthe basin center. Where fault zones can be effectivebarriers to lateral fluid movement across the fault

plane, the major BMB fault zones (ELFZ, TFZ, andTAFZ) appear to serve as the preferred pathwaysfor the vertical migration of formation water origi-

nating at depth, causing linear high-temperatureanomalies associated with the fault zones. Many

significant BMB petroleum discoveries are locatedin the areas of local anomalously high temperature,indicating that both formation waters and petro-leum followed similar migration pathways from

primary petroleum source kitchens located in theadjacent depocenters to structurally higher posi-tions, either along regional aquifers or faults. Thismay provide additional constraints on relative pe-

troleum prospectivity and petroleum occurrencein the BMB.

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