Present heat flow and paleo-geothermal regime in the Canadian Arctic margin: analysis of industrial thermal data and coalification gradients

TECTONOPHYSICS ELSEVIER

Tectonophysics 291 (1998) 141-159

Present heat flow and paleo-geothermal regime in the Canadian Arctic margin: analysis of industrial thermal data and coalification gradients Jacek A. Majorowicz a,,, Ashton R Embry b a

Northern Geothermal Consult, 105 Carlson Close, Edmonton, Alta. T6R 2J8, Canada b Geological Survey of Canada, 3303 33rd St, N~, Calgary, Alta. T2L 2E7 Canada

Received 30 April 1997; accepted 23 June 1997

Abstract Calculations of the present geothermal gradient and terrestrial heat flow were made on 156 deep wells of the Canadian Arctic Archipelago. Corrected bottom hole temperature (BHT) data and drill stem test (DST) temperatures were used to determine the thermal gradients for sites for which the quality of data was sufficient. Thermal gradients evaluated for depths below the base of permafrost for the onshore wells and below sea bottom for the offshore wells were combined with the estimates of effective thermal conductivity to approximate heat flow for these sites. The present geothermal gradient is in the 15-50 mK/m range (mean = 31 4-7 mK/m). Present heat flow is mainly in the 35-90 mW/m 2 range (mean = 53 4- 12 mW/m2). Maps of the present geothermal gradient and present heat flow have been constructed for the basin. The analysis of vitrinite reflectance profiles and the calculation of logarithmic coalification gradients for 101 boreholes in the Sverdrup Basin showed large variations related in many cases to regional variations of present terrestrial heat flow. Paleo-geothermal gradients estimated from these data are mostly in the range of 15-50 mK/m (mean = 28 5=9 mK/m) and paleo-heat flow is in the 40-90 mW/m 2 range (mean = 57 ± 18 mW/m 2) related to the time of maximum burial in the Early Tertiary. Mean values of the present heat flow and paleo-heat flow for the Sverdrup Basin are almost identical considering the uncertainties of the methods used (53 + 12 versus 57 4- 18 mW/m 2, respectively). Present geothermal gradients and paieo-geothermal gradients are also close when means are compared (31 4- 7 versus 28 4- 9 mK/m respectively). A zone of high present heat flow and a paleo-heat flow zone coincide in places with the northeastern-southwestern incipient rift landward of the Arctic margin first described by Balkwill and Fox (1982). Correlation between present heat flow and paleo-heat flow for the time of maximum burial in the earliest Tertiary suggests that the high heat flow zone has prevailed since that time. © 1998 Elsevier Science B.V. All rights reserved. Keywords: heat flow; thermal maturation; coalification; sedimentary basin; continental margin; arctic

1. Geologic setting and previous work The high Arctic area of North America contains a number of sedimentary basins, and knowledge of *Corresponding author. Fax: +1 403 438 9385; E-mail: [email protected]

the present and paleo-thermal regime of these basins can be gained from temperature studies of wells drilled for oil and gas (Majorowicz et al,, 1990, 1996) and analysis of the coal maturation parameters from cores and whole-rock cuttings (Majorowicz and Dietrich, 1989; Skibo et al., 1990; Gentzis and Goodarzi, 1993). Knowledge of the present thermal

0040-1951/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0040- 1 95 1 (98)00036-5

J.A. Majorowicz, A.t~ Embry / Tectonophysics 291 (1998) 141-159

142

field and its relationship to paleo-conditions is an important constraint in the modelling of thermal history of the basin and organic maturation history. In many of such models, regional variability of the crustal heat generation is neglected and constant value for the crust is assumed (Stephenson et al., 1987; Issler and Beaumont, 1989). While more precise temperature profiles are available for some 40 wells in the study area, these are confined largely to the 200-700 m thick permafrost section. In some cases, these profiles show the large thermal effect of Holocene emergence due to uplift (Taylor, 1991) and hence are less useful than the much deeper BHT and DST data for calculating terrestrial heat flow. Studies of deep, subpermafrost thermal regimes from data of deep exploration wells have been made for the Beaufort-Mackenzie Basin (Majorowicz et al., 1988, 1990, 1996; Majorowicz and Dietrich, 1989) and the Arctic Alaska Basin and the Colville Trough of northern Alaska (Lachenbruch et al., 1982; Deming et al., 1992) (Fig. 1). Heat flow

studies from various parts of the adjacent Arctic Ocean have also been published (Taylor et al., 1986; Langseth et al., 1988, 1990; Lowden et al., 1990). In the Canadian Arctic Archipelago there are two major sedimentary basins, the Franklinian and the Sverdrup (Trettin, 1989). The Franklinian Basin occurs over the central portion of the islands and consists of Cambrian to Devonian strata which were uplifted and deformed in the latest Devonian-earliest Carboniferous (Ellesmerian Orogeny). The Sverdrup Basin lies to the north (Fig. 2) and contains up to 13 km of Carboniferous to Early Tertiary strata (Balkwill, 1978). Sediments of the youngest, mainly Cretaceous-Tertiary age (ca. 140-3 Ma), were deposited during and after the development of the adjacent Arctic Ocean, called the Canada Basin (Fig. 1). It resulted from an episode of crustal rifting (starting in the Middle Jurassic), which created the present Arctic margin framework (Embry et al., 1988; Embry and Dixon, 1994). The highest rates of subsidence occurred in Early Cretaceous time (124-103 Ma) along the basin center and are not associated

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Fig. 1. Location map for Amerasia Basin, Queen Elizabeth Islands (Q.E.I) and adjacent areas. The study area is shown hachured. North pole lies to upper right.

J.A. Majorowicz, A.E Embry /Tectonophysics 291 (1998) 141-159

143

Ellef

Melville

I v

Bathurst

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200

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Fig. 2. Map of Sverdrup Basin, Arctic Canada with well control.

with significantly reduced rates of subsidence along the marginal areas (Sweeney, 1977). Thermal subsidence of the Sverdrup Basin rejuvenated episodically in Mesozoic time, with concurrent mafic intrusion, marine transgression, and sediment filling and was terminated by latest Cretaceous-Early Tertiary regional crustal compression (Eurekan Orogeny). Embry et al. (1988) showed that the present continental margin was formed by continental rifting near the northern edge of the present Sverdrup Basin, with subsequent seafloor spreading. The margin reflects the adjacent ocean basin opening, the nearby Alpha Ridge development and the subsequent Eurekan Orogeny, all superimposed on Franklinian basement rocks. Normal faults, linear magnetic anomalies, gabbro dykes, aligned evaporite domes, and modern earthquake epicentres define a broad tectonic belt in upper Paleozoic and Mesozoic rocks of the western Sverdrup Basin. According to Balkwill and Fox (1982), the belt strikes northeastward, from Melville Island, at the southern margin of the basin, toward the continental margin, at northern Ellef Ringnes

Island (Fig. 10). They interpreted this zone as a longlasting domain of crustal dilatation, an incipient rift in the northern margin of the craton. More detailed description of the tectono-stratigraphy can be found in Sweeney (1977); Balkwill (1978, 1983); Balkwill and Fox (1982); Lawver and Baggeroer (1983); Embry (1990, 1991) and Embry and Dixon (1994). Previous heat flow studies in the Canadian Arctic Islands using BHT and DST data from 31 deep wells concentrated on two profiles across the Sverdrup Basin (Jones et al., 1989, 1990). These studies showed large variations of heat flow along the N-S profile (38-73 mW/m2). This was in sharp contrast with the minor variations noted along the E-W profile (46 -4-5 mW/m 2, Jones et al., 1989). Precise well temperatures to 800 m below the seabed in Cape Allison C-47 well obtained by Taylor et al. (1989). The terrestrial heat flow calculated from these data is 39 mW/m2; uncertainty in thermal conductivity suggests it maybe somewhat higher, probably not exceeding 52 mW/m 2 (Taylor et al., 1989). Considering these results, a regional study of the thermal gradient and heat flow, employing all suitable

144

J.A. Majorowicz, A.F. Embry /Tectonophysics 291 (1998) 141-159

wells in the Sverdrup Basin and adjacent Franklinian Basin, is required to access the nature and origin of the heat flow patterns in the Arctic Archipelago. The use of industry temperature records from exploratory wells at depths of a thousand to several thousands of meters, although of lower quality than precise temperature logs, can give important additional information about the thermal regime of the basin. Industry-collected BHTs and DSTs taken during the drilling in the area were the primary sources of information (Geotech, 1983, 1984). Such a study is also of value for regional tectonic and petroleum assessment studies in these basins. The analysis of the present thermal regime of the Arctic Archipelago and specifically of the Sverdrup Basin would not be complete without the analysis of the past thermal regime during the time of maximum burial in the Early Tertiary. Such an opportunity exists because over a thousand vitrinite reflectance samples from over 100 wells were measured from polished core samples and whole-rock cuttings (Skibo et al., 1991; E Goodarzi of GeoTEMPERATURE -25 0

I

25 I

I

logical Survey of Canada and K.R. Stewart, Arctic Geochem Consult, Calgary, pers. commun., 1995). This paper describes (1) a regional study of the present thermal gradients and heat flow of the Canadian Arctic Archipelago, and (2) regional patterns of the logarithmic coal maturation, the paleo-geotherreal gradients and paleo-heat flow for the time of maximum burial in the Early Tertiary in the Sverdrup Basin.

2. Present-day geothermal gradients The geothermal temperature gradient varies with depth due to the variations of thermal conductivity, and regionally due to the variations of deep heat flow. Thermal conductivity variations are related to the mineral composition of the lithological profile and porosity. Despite the variations in the thermal gradient with depth related to the conductivity variations in a well, it is possible to approximate temperature records based mostly on corrected BHT data and DST data using a linear function with depth (Fig. 3).

(°C) -25

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25

75

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\ b Fig. 3. Example of temperature-depth data for two selected wells in the Arctic Archipelago. (a) For a low geothermal gradient location with a least square linear fit to temperature data, constrained by temperature at the base of the ice-bearing permafrost ( - I ° C , symbol P). Crosses are drill stem test (DST) temperatures, diamonds and plus-signs are corrected bottom hole temperatures (BHT). (b) For a high geothermal gradient location.

J.A. Majorowicz,A.E Embry/Tectonophysics 291 (1998) 141-159 The procedure to correct temperature records for the effects of drilling is described in Appendix A. Majorowicz et al. (1990) show that in many cases the temperature records at the upper subpermafrost depths are anomalously high, mostly due to fluid flow problems in the upper sections of the wells. As an example, the flow of fluid was reported for the Sutherland # 0-23 well (King Christian Island) in the well report. Therefore the linear function with depth which approximates the variations of temperature in the well profile has to be constrained by a fixed point at the base of ice-bearing permafrost (assumed to be -I°C) as shown in the examples (Fig. 3). Analysis of the corrected temperature data shows that such a constraint works well. The shallow temperatures at depths less than 1 km often deviate from this relationship usually due to recent changes in surface temperature (Taylor, 1991) and are treated as an anomalous shallow feature unrelated to the deep temperature field. Deep temperature gradients relate to the deep heat flow that is important to regional studies and mapping. Temperature gradients were determined for the petroleum exploration wells throughout the Arctic Archipelago for which there were sufficient data. Deep average geothermal gradient and error were determined for 156 locations (Table 1). A contour map of the geothermal gradients was constructed (Fig. 4). Considering occasional high errors in the values, a contour interval of 10 mK/m has been chosen. The map shows very large regional variations of the geothermal gradient (VT) from less than 20 mK/m to local anomalies greatly over 40 mK/m. Locally, values higher than 50 mK/m are observed in the western part of the Banks Island Franklinian Basin). Because the average thermal gradient is a function of the effective thermal conductivity and a heat source, the interpretation of the thermal gradient contour map is ambiguous. An estimate of the regional thermal conductivity is therefore needed to calculate the terrestrial heat flow that is representative of the fundamental geothermal regime.

3. Thermal conductivity evaluation Thermal conductivity variations with depth are regionally related to changes in lithology and porosity and can be evaluated from average rock conductivi-

145

ties and net rock evaluations as a first approximation (Taylor, 1991; Majorowicz and Jessop, 1993). The method described in Majorowicz and Jessop (1993) was used. It is assumed that each rock type (i) occupies a discrete layer of thickness (l/) based on its fractional occurrence such that: l(n) = )--~ li

(1)

where the summation is over net rock type present and l(n) is the thickness of the nth sample interval. An effective thermal conductivity for that interval is calculated according to the equation: geff(n) ~

li/~-~(li/Ki)

(2)

where Ki is the conductivity of the ith rock type. The conductivities assigned to the rock types in Jones et al. (1990) were used in this study. The effective thermal conductivities for the 31 wells calculated in Jones et al. (1989, 1990) were combined with an additional 27 wells for which net rock analysis was calculated from well logs. The conductivity varies in the 1.6 to 2.4 W/mK range (mean = 2.0 W/mK). The error of Keff is 0.2 W/mK in the majority of cases though the range is between 0.17 and 0.37 W/InK. The contour map of the effective thermal conductivity variations in the Arctic Archipelago area is shown in Fig. 5. A contour interval of 0.2 W/mK (the error of the Kef~estimate) was chosen. The K~ff contour map shows variations up to 20% of the average thermal conductivity (2.0 W/mK). Generally conductivity is lower in the northern part of the area (central Sverdrup Basin) and higher to the south along the Sverdrup Basin margin and in the Franklinian Basin. The range in conductivity is much smaller than the regional variations of thermal gradient. Thus it would appear that the regional variations of thermal conductivity are not sufficient to explain regional variations of the thermal gradient and hence the heat flow varies significantly in the Arctic Archipelago.

4. Present-day heat flow The heat flow is a product of the thermal gradient and thermal conductivity: Q = K*ff(VT)

(3)

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Table 1 Present and paleo-geothermal parameters of the Canadian Arctic margin Lat. Long. Q 8Q K 8K G 8G pal. Q 8 pal. Q pal. G 8 pal. G (degr. m i n s ) (degr. m i n s ) ( m W / m 21 ( m W / m 21 (W/mK) ( W / m K ) (mK/m) (mK/m) ( m W / m 21 ( m W / m 21 (inK/m) (mK/m) 72 23 26.0 72 3344.0 72 41 23.0 72 45 18.0 72 47 42.0 72 54 09.0 73 05 13.0 73 2500.0 73 31 42.1 73 3600.0 73 36 08.1 73 40 52.1 73 51 29.0 74 07 11.0 74 15 27.5 74 39 02.0 74 4104.0 74 41 47.7 744415.9 74 45 06.0 75 02 20.3 750421.0 750940.0 75 11 52.4 75 26 06.1 75 3200.0 75 32 03.0 75 33 31.0 75 3809.1 75 4100.0 75 49 51.9 75 50 00.0 75 52 17.2 75 53 52.9 75 58 21.0 76 08 35.6 76 08 58.0 760937.0 76 10 12.5 76 10 54.5 76 13 19.1 76 1344.0 76 17 04.7 76 18 29.4 76 18 36.6 76 2000.0 76 20 30.0 76 20 33.2 76 20 34.3 76 20 37.5 76 21 00.8 76 21 01.1

121 50 49.0 122 42 12.0 96 49 34.0 117 11 13.0 120 45 49.0 124 33 29.0 123 23 45.0 120 0500.0 115 52 25.0 123 O0 00.0 117 26 59.0 90 36 45.0 98 56 50.0 120 49 59.0 123 53 49.0 113 22 59.8 944438.0 113 25 41.0 110 55 57.0 110 30 37.0 108 05 23.2 91 48 20.0 94 43 14.0 98 35 42.0 1100049.0 111 58 58.0 108 20 27.0 98 43 00.0 118 48 15.6 105 3500.0 108 31 50.0 113 36 00.0 106 24 37.0 118 07 39.0 109 29 38.0 121 48 36.0 121 48 36.0 1040443.0 112 58 55.0 10421 04.0 108 35 05.0 108 20 10.0 111 20 52.0 1030459.0 110 23 17.0 110 20 00.0 98 40 30.0 10400 35.0 101 35 03.0 108 58 29.9 103 49 19.0 115 33 310

58 38 53 56 51 65 119 56 38 66 44 35 42 37 74 53 123 57 64 49 38 77 60 66 46 88 38 88 34 42 74 64 51 43 50 66 65 46 42 44 49 70 63 48 63 67 68 42 34 65 32 84

14 10 13 14 13 16 30 14 10 16 11 9 11 9 18 13 31 14 16 12 10 19 15 17 12 22 10 22 9 11 19 16 13 11 13 17 16 12 10 11 12 18 16 12 16 17 17 10 9 16 8 21

1.6 1.6 2.7 1.8 1.7 1.5 1.5 1.7 1.9 t.6 1.9 2.3 2.3 1.7 1.5 1.8 2.7 1.9 2.2 2.3 1.8 2.6 1.9 2.2 2.1 2.0 2.0 2.2 1.7 2.1 2.0 1.9 1.9 1.8 2.0 1.7 1.7 2.2 1.9 2.2 1.9 1.9 1.9 2.1 1.7 2.0 2.1 2.2 1.9 2.0 1.9 2.1

0.2 11.2 0.4 0.3 0.3 (1.2 0.2 (1.3 0.3 0.2 0.3 0.3 0.4 0.3 0.2 0.3 0.4 0.3 0.3 0.4 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

36 24 20 31 30 42 79 33 20 41 23 15 18 22 49 29 45 30 29 21 21 30 31 30 22 44 19 40 20 20 37 34 27 24 25 39 38 21 22 20 26 37 33 23 37 33 33 19 18 32 17 40

4 2 2 3 3 4 8 3 2 4 2 2 2 2 5 3 5 3 3 2 2 3 3 3 2 4 2 4 2 2 4 3 3 2 3 4 4 2 2 2 3 4 3 2 4 3 3 2 2 3 2 4

29

8

16

2

48

16

28

4

51

16

27

4

54

16

27

4

47 29 48

13 8 13

23 15 23

3 2 3

J.A. Majorowicz, A.E Embry/Tectonophysics 291 (1998) 141-159

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Table 1 (continued) Lat. Long. Q ~Q K ~K G ~G pal. Q ~ pal. Q pal. G ~ pal. G (degr. mins) (degr. mins) (mW/m2) (mW/rn2) (W/mK) (W/mK) (mK/m) (mK/m) (mW/m2) (mW/m2) (mK/m) (inK/m) 76 21 05.7 76 21 09.4 76 21 15.9 76 21 16.7 76 21 21.0 76 21 27.1 76 21 50.7 76 21 52.5 76 21 54.0 76 23 08.2 76 23 08.8 76 23 09.3 76 23 14.5 76 23 43.2 76 24 00.0 76 25 00.0 76 25 15.4 76 25 24.0 76 25 38.4 76 25 58.0 76 26 26.8 76 27 00.0 76 27 05.1 76 27 45.0 76 28 36.5 76 30 31.0 76 31 00.0 76 33 00.0 76 36 32.2 76 40 12.0 76 42 00.0 76 42 43.0 76 42 45.3 76 44 35.7 76 48 00.0 76 56 04.0 76 56 04.6 76 58 26.6 77 03 16.0 77 08 00.0 77 12 00.0 77 12 24.0 77 11 37.5 77 14 00.0 77 15 09.3 77 15 27.0 77 15 27.0 77 17 27.0 77 21 01.6 77 21 14.0 77 25 01.0 77 25 06.0 77 29 13.0

104 00 53.0 110 13 46.0 104 18 44.0 110 52 00.0 104 01 14.0 103 58 15.0 103 58 11.9 110 50 44.0 104 06 56.0 109 54 21.0 108 16 03.0 104 17 10.0 114 17 44.7 113 11 24.0 110 32 00.0 107 49 00.0 108 35 38.0 108 28 43.0 115 18 22.0 86 26 06.0 103 01 03.0 108 55 00.0 108 55 42.7 111 19 14.0 108 58 58.0 117 19 48.0 103 41 00.0 108 43 00.0 104 02 14.0 116 43 45.4 105 57 00.0 113 43 21.0 109 46 19.0 108 52 36.0 108 45 00.0 109 08 07.0 109 08 07.0 118 45 36.0 110 21 10.0 104 34 00.0 106 53 00.0 106 53 26.0 118 14 14.0 105 06 00.0 106 38 13.0 86 18 07.0 86 18 07.8 116 55 10.0 105 26 57.0 90 51 25.0 106 23 35.0 99 38 11.0 110 27 05.0

33 61 36 63 35 31 42 48 37 53 55 35 56 76 57 49 51 51 50 42 46 42 56 99 46 63 34 67 38 55 49 66 52 59 51 60 71 72 73 62 72 70 63 77 65 38 51 54 51 41 70 46 61

8 15 9 16 9 8 10 12 9 13 14 9 14 19 14 12 13 13 13 11 12 10 14 25 11 16 8 17 9 14 12 17 13 15 13 15 18 18 18 16 18 18 17 19 16 9 13 14 13 10 17 12 15

2.2 1.9 2.1 1.9 2.2 2.2 2.3 1.9 2.2 1.9 1.9 2.2 2.1 1.9 1.9 1.9 1.9 1.9 2.1 2.1 2.1 1.9 2.0 1.9 1.9 1.9 2.1 1.8 2.1 2.0 1.9 2.0 1.8 1.8 1.7 1.7 1.7 1.8 1.8 2.0 1.8 1.8 1.9 1.8 1.7 2.1 2.1 2.0 1.8 2.3 1.7 1.8 1.8

0.3 0.3 0.3 0.3 0.3 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

15 32 17 33 16 14 18 25 17 28 29 16 26 40 30 26 27 27 24 20 22 22 28 52 24 33 16 37 18 28 26 33 29 33 30 35 42 40 41 31 40 39 33 43 38 18 24 27 29 18 41 26 34

2 3 2 3 2 1 2 3 2 3 3 2 3 4 3 3 3 3 2 2 2 2 3 5 2 3 2 4 2 3 3 3 3 3 3 4 4 4 4 3 4 4 3 4 4 2 2 3 3 2 4 3 3

59

19

31

5

74

23

39

6

55

16

29

4

48 59

15 19

25 31

4 5

39 72 48 76 51

12 23 15 23 16

18 38 25 40 27

3 6 4 6 4

48

13

23

3

48

15

25

4

137 48 44 23 47 38 57 48 56 59

42 15 13 8 15 12 17 15 16 19

72 25 23 14 26 18 29 25 28 33

11 4 3 2 4 3 4 4 4 5

63

21

37

6

56 122

18 38

31 68

5 10

61 81 51 41 60

19 26 16 12 19

34 45 27 23 35

5 7 4 3 5

62 49 71

19 15 21

31 27 31

5 4 5

41 72

12 23

23 40

3 6

J.A. Majorowicz, A.E Embry/Tectonophysics 291 (1998) 141-159

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Table 1 (continued) Lat. Long. Q 8Q K 8K G 8G pal. Q 8 pal. Q pal. G 8 pal. G (degr. min s) (degr. rain s) (mW/m 2) (mW/m 2) (W/InK) (W/inK) (mK/m) (inK/m) ( m W / m 2) ( m W / m 2) (inK/m) (inK/m) 77 29 47.0 77 32 00.0 77 33 16.7 77 36 29.0 77 37 11.0 77 42 53.1 77 45 47.0 77 45 54.0 77 46 05.0 77 47 08.9 77 49 00.1 77 49 13.8 77 49 30.8 77 52 00.0 77 52 13.4 77 59 19.0 77 59 40.0 78 04 00.0 78 05 00.0 78 06 00.0 78 07 23.3 78 07 48.9 79 09 24.0 78 09 59.0 78 10 37.0 78 11 59.3 78 15 01.0 78 17 00.0 78 1900.0 78 20 00.0 78 22 08.4 78 23 23.4 78 24 00.0 78 25 00.0 78 25 52.0 78 28 00.0 7841 32.6 78 44 57.0 79 16 40.0 79 17 34.2 79 21 00.0 79 24 00.0 79 31 00.0 79 37 00.0 79 51 00.0 79 52 37.0 79 53 35.0 79 59 22.3 80 02 00.0 80 16 00.0 80 45 00.0

94 38 58.0 103 56 00.0 109 09 56.0 99 31 08.0 100 22 24.0 102 08 38.8 97 45 26.5 101 02 19.0 100 17 20.0 101 26 50.0 114 17 24.0 104 57 18.0 100 18 05.0 102 25 00.0 102 26 48.0 111 21 45.0 114 33 51.0 99 34 00.0 101 07 00.0 99 46 00.0 103 10 33.0 103 15 04.6 104 57 23.0 101 49 43.0 99 54 13.0 99 58 22.0 102 32 25.0 89 45 00.0 96 1600.0 104 24 00.0 104 44 52.0 104 21 39.0 97 50 00.0 95 04 00.0 103 15 48.0 100 24 00.0 10036 18.0 102 41 58.0 105 16 36.0 103 43 37.0 85 01 00.0 85 44 00.0 87 01 00.0 84 43 00.0 84 23 00.0 94 57 10.0 83 47 00.0 84 04 10.3 98 55 00.0 84 07 00.0 83 07 00.0

58 56 72 42 44 65 59 52 45 61 55 87 86 50 52 59 51 50 47 46 51 57 73 41 46 65 58 50 46 44 75 71 66 53 68 49 50 69 56 58 41 68 51 50 54 54 56 41 44 43 44

15 14 18 10 11 16 15 13 11 15 14 22 22 13 13 !5 13 13 12 12 13 14 18 10 12 16 15 13 12 I1 19 19 17 13 17 12 13 17 14 14 10 17 13 13 14 14 14 10 II 11 11

2.5 1.7 1.7 1.9 2.2 1.7 1.9 1.7 2.4 1.7 2.2 1.7 2.0 1.8 1.8 1.9 2.2 1.8 1.5 1.9 1.7 1.7 1.9 1.6 2.0 1.9 1.7 2.1 2.1 1.7 1.7 1.7 1.5 2.1 t.7 1.8 1.8 1.9 1.9 1.8 1.8 1.9 1.7 2.1 2.11 2.0 2.(1 1.4 1.7 1.7 2. I

0.4 0.3 0.3 0.3 0.3 0.3 11.3 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.3 0.3 0.3 0.3 0.2 0.3 0.3 /).3 0.3 0.3 0.3 0.3 0.5 0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.3 0.3 0.3

23 33 42 22 20 38 31 31 19 36 25 51 43 28 29 31 23 28 31 24 30 33 38 26 23 34 34 24 22 26 44 42 44 25 39 27 28 37 29 32 23 36 30 24 27 27 28 30 26 25 21

2 3 4 2 2 4 3 3 2 4 3 5 4 3 3 3 2 3 3 2 3 3 4 3 2 3 3 2 2 3 4 4 4 3 4 3 3 4 3 3 2 4 3 2 3 3 3 3 3 3 2

34 63 38 63 49 55 52

11 21 12 17 15 16 18

20 37 20 29 29 29 31

3 6 3 4 4 4 5

53 77 63

18 21 21

31 35 37

5 5 6

45 36 74 56

14 11 23 16

25 20 39 25

4 3 6 4

41 38

11 12

27 20

4 3

61 56 32

19 16 9

35 29 20

5 4 3

70 34 40 21 46 39 39 53 61 44 25 34 69

22 11 12 7 15 12 16 14 17 14 8 11 20

37 20 19 10 27 23 23 35 29 25 14 19 37

6 3 3 2 4 3 3 5 4 4 2 3 5

79 32 57 56 46 66 46 58 32

26 11 18 18 13 20 13 17 9

44 18 30 33 22 33 23 29 23

7 3 5 5 3 5 3 4 3

44 53

15 16

26 25

4 4

J.A. Majorowicz, A.E Embry/Tectonophysics 291 (1998) 141-159

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LONGITUDE

Fig. 4. Averagegeothermalgradient contourmap for the Arctic Archipelago.Contours are in mK/rn; plus-signs indicate well locations. Shading indicates >40 mK/m. where V T is the average thermal gradient based on the least square fit to the thermal data in the depth interval for which Keff is known. Heat flow values were calculated for the 156 wells in the area using thermal conductivity computed for individual wells or taken from the contour map of Kern The heat flow values, thermal gradients and related thermal conductivity values are shown in Table 1. A contour map of heat flow is shown in Fig. 6. The contour interval of 20 m W / m 2 is most relevant in relation to the error of the heat flow estimate. A heat flow map helps to evaluate the deep thermal state of the crust and upper mantle and is relevant to the deep processes which shaped the tectonothermal structure of the Arctic basins. Our results show that this high heat flow is a part of a larger regional geothermal feature extending in a N E - S W direction from Ellef Ringnes Island towards

central and western Melville Island. Heat flow in that area exceeds 60 m W / m 2 and reaches 90 m W / m 2 in many wells. This thermal feature is close to perpendicular to the axis of the Sverdrup Basin and extends into the Franklinian Basin to the south. Heat flow values of 70 4- 10 m W / m 2 observed in the N E - S W direction (Fig. 6) are considered anomalous and approximately 204-10 m W / m 2 larger than expected for a passive margin basin that went through major subsidence in the Cretaceous-Early Tertiary, some 100 Ma ago (Langseth et al., 1990; Embry and Dixon, 1994). Such heat flow is typical of younger lithosphere (less than 50 Ma) and is comparable with the heat flow of the western segment of the Alpha Ridge and possibly the Makarov Basin (Fig. 1) (Taylor et al., 1986; Langseth et al., 1990) for which anomalously high magma at a 'hot spot' was considered as a source.

J.A. Majorowicz, A.F. Emb~, l Tectonophysics 291 (1998) 141-159

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LONGITUDE Fig. 5. Contour map of effective thermal conductivity in W/InK. Contour interval is 0.2 W/InK.

5. Paleo-thermal regimes derived from vitrinite Coalification data from whole-rock cuttings and cores for some 100 wells through the Sverdrup Basin were used in the analysis of the logarithmic coalification patterns and subsequent estimates of pale,geothermal gradients and flow for the time of maximum burial in the Tertiary period. Measurements of random reflectance for vitrinite and maximum reflectance for bitumens were done by Geological Survey of Canada and provided for this study by Dr. Goodarzi of Geological Survey of Canada in Calgary and K.R. Stewart of Arctic Geochemical Consultants in Calgary. The values of the average vitrinite reflectance data (R, %) versus depth for each well were plotted with depth and the values of d log R/dz were determined statistically. Careful selection of vitrinite reflectance data was used to represent prop-

erly in-situ conditions. Similar analysis was done for a few selected wells in the Sverdrup sasin and the Franklinian Basin by Skibo et al. (1991). The Sverdrup basin underwent rifling in the Carboniferous to Early Permian with thermal subsidence in the Carboniferous to Early Cretaceous, followed by renewed rifting in the Early Cretaceous and thermal subsidence in the Late Cretaceous (Balkwill, 1978; Stephenson et al., 1987). This was followed by uplift of the basin and deformation in the Early Tertiary (Eurekan Orogeny). The subsidence history of the basin shows that great burial depths/maximum temperatures were reached by sediments in the Early Tertiary, followed by uplift and erosion and lower temperatures (see also Skibo et al., 1990). Coalification depends very strongly on the maximum temperature reached during burial history according to the equation given by McKenzie (1981).

J.A, Majorowicz, A.E Embry /Tectonophysics 291 (1998) 141-159

151

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LONGITUDE Fig. 6. Contour map of heat flow in the Arctic Archipelago (20 mW/m contour interval).

The time temperature index TTI (Waples, 1980) is obtained by the integration of the element, 2 (r(t)-l°5)/l° over time dt, where T(t) is the temperature history. The statistical relationship established between TTI and Ro by Waples confirmed that the empirically determined factor of 2 in the above equation is adequate for most petroleum cases. Therefore the level of maturation of vitrinite is very sensitive to the maximum temperature reached and should correlate with the thermal gradient for that time. Uplift and erosion of approximately 1 km in an area with an average thermal gradient of 30 mK/m would mean that the level of maturation which could be reached during maximum burial is approximately 23 times more than for present depth since every 10°C temperature increase doubles the reaction rate of coalification for most typical cases. Therefore the logarithmic coalification gradient should indicate

the thermal gradient during maximum burial. In fact work in the area of Lougheed Island reported by Skibo et al. (1990) indicates that the degree of coalification often follows stratigraphic contacts so that it was probably acquired at the time of maximum burial. Therefore, the logarithmic coalification gradiem data can be used in the case of the Sverdrup Basin as an indicator of the relationship between present and paleo-geothermal conditions. The plot of the logarithmic coalification gradient calculated for the Sverdrup Basin wells versus the present geothermal gradients (Fig. 7) shows correlation (R = 0.6) which can be interpreted as an indicator of the link between the present thermal regime and thermal conditions during the time of maximum burial before the uplift and erosion took place. Calculation of the erosion from logarithmic coalification gradients and the near surface R values (typically

J.A. Majorowicz, A.F. Embry/Tectonophysics 291 (1998) 141-159

152 0.4

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in the 0.3% to 0.5% range) using extrapolation to an initial maturity level of 0.25% gave approximate thicknesses of eroded sediments in the 1 km to 2 km range (Fig. 8). The latter is significantly more than estimated from stratigraphic considerations (see Skibo et al., 1990 for discussions of the reasons for the difference). The logarithmic coal thermal maturation gradient values are mostly in the 0.1% to 0.25% range with the highest values in the areas of the highest present geothermal gradient and heat flow. In comparison, the logarithmic coalification gradient for the Beaufort-Mackenzie Basin is lower (typically close to 0.1; Majorowicz and Dietrich, 1989). The calculation of the paleo-geothermal gradient from the vitrinite reflectance data was based on the Middleton method (Middleton, 1982). The effective paleo-geothermal gradient for a typical burial history in the Sverdrup Basin can be approximated by the equation:

dT/dzp~aeo= 194.8d log %R/dz

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0.1747~69622"x^t

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Fig. 8. Statistical relationship between the thickness of eroded section (in km) and the geothermal gradient, correlation R = 0.8 estimated from extrapolation of the best fit log-linear coalification gradient through the missing section to the initial maturity level of 0.25%. Geothermal gradient in mK/m.

after fairly rapid sedimentation such that most of the 'thermal cooking' of organic matter takes place during the time of maximum burial (Early Tertiary) and is followed by uplift and erosion. Maximum burial was attained some 45 Ma B.P. in the western Sverdrup Basin, producing maximum coalification (see Skibo et al., 1990). The coalification gradient is effectively 'frozen' into the basin at the time of rapid erosion. The calculated paleo-geothermal gradient values are given in Table 1 together with an estimate of the error. Paleo-geothermal gradient values were contoured for the Sverdrup Basin with a contour interval of 10 mK/m, which in most cases is larger than the error of estimate. The contour map (Fig. 9) shows variations of the paleo-geothermal gradient from less than 20 m K / m to over 40 m K / m which is a similar range of values as for the present geothermal gradients; similar patterns are observed for both present geothermal gradients and paleo-geothermal gradients (compare Figs. 4 and 9, respectively). The largest values are in the zone of the northeasternsouthwestern trends of the extensional structural features landward of the Arctic margin (Fig. 10; Balkwill and Fox, 1982; Embry et al., 1988). Considering

J.A. Majorowicz, A.E Embry/Tectonophysics 291 (1998) 141-159

153

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uncertainties in interpretation of both vitrinite reflectance data and bottom hole temperature data, the mean values of the paleo-geothermal gradient and present geothermal gradient for the Sverdrup Basin are very close: 28 + 9 m K / m versus 31 4- 7 mK/m, respectively. Paleo-heat flow values were calculated from paleo-geothermal values and effective thermal conductivities and are shown in Table 1. The mean values of the paleo-heat flow and present heat flow are also very close (57 + 12 vs. 53 + 18 mW/m2). 6. Discussion 6.1. Geological controls

The opening of the adjacent Amerasia Basin was a major thermal event in the Arctic. Seafloor spreading and the opening of the Amerasia Basin by the

counterclockwise rotation of northern Alaska and adjacent northern Siberia from the Canadian Arctic margin occurred during the early Late Cretaceous and ceased near the Early Tertiary-Late Cretaceous boundary (Embry and Dixon, 1994). The timing of the onset of the breakup and the cessation of the processes are 135 Ma and 90 Ma, respectively. The heat generation of the old crust of the margin in addition to that of more recently deposited sediments contributes to the total heat flow measured today. Prediction of present heat flow from a plate cooling model gives approximately 45 ! 10 m W / m 2. The contribution of radiogenic elements from the continental crust is estimated to be 10 + 5 m W / m 2 for the old continental crust and 2.5 m W / m 2 for the sedimentary cover. The heat flow values observed for the large areas of the northern Canada continental margin in the Beaufort-Mackenzie area (40-60

J.A. Majorowicz, A.F. Embry/Tectonophysics 291 (1998) 141-159

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J.A. Majorowicz, A.E Embry / Tectonophysic s 291 (1998) 141-159

mW/m 2, Majorowicz et al., 1996) and the Arctic Archipelago area (40-60 mW/m, Fig. 6) are typical for an old continental margin. They are comparable to the heat flow of the northern Alaska plate (55 mW/m2; Lachenbruch et al., 1982). The final significant thermal events in the Arctic Archipelago are related to Cretaceous to earliest Tertiary extension events which were accompanied by mafic flows and intrusions. Three widespread episodes of faulting and volcanism occurred in midHauterivian, Aptian and latest Albian-Cenomanian (Embry and Osadetz, 1988). During these times tholeiitic basalts were extruded in the northeastern Sverdrup Basin and diabase dykes and sills were intruded over most of the basin. In latest Cretaceousearliest Tertiary, volcanics were extruded in a few localized areas (e.g., northernmost Ellesmere Island, southern Bathurst Island). Overall it would appear that the ages of these volcanic episodes are too old to have an impact on the present heat flow pattern. A thermal time constant of changes by conduction can be derived from the dimensionless time parameter defined as: C = st/d 2

(5)

where s is thermal diffusivity, t is time and d is depth (Jessop, 1986). The thermal disturbance at the 2 km depth has a time constant of approximately 100 ka and as much as approx 3000 ka for depth exceeding 10 km, for a typical value of s. Any thermal disturbance due to magmatic or volcanic effects would by now be close to minimum. The highs observed for the geothermal gradient and the heat flow (Figs. 4 and 6, respectively) correlate with an area of extensional structural features landward of the Arctic margin first mapped by Balkwill and Fox (1982) (Fig. 10). The correlation is not exact, but the match is relatively close considering that heat flow estimates are based on a lesser amount of data (limited to available well temperature data) than are the geological trends derived from well data, seismic, gravity and magnetic surveys). This zone of extensional structures was interpreted by Embry et al. (1988) to be analogous to similar structures on the Atlantic and Arctic margins formed by tiffing related to seafloor spreading and hot spot activity in the adjacent ocean (Forsyth et al., 1988). In the Piedmont and the Atlantic Coastal Plain of the eastern United

155

States, heat flow values higher than 60 mW/m 2 and thermal gradients higher than 30 mK/m are found locally (Blackwell et al., 1991). However, data are too sparse to make a comparison with the Arctic margin. The reason for the creation of the extensional features and related heat flow and paleo-heat flow anomalies are not known. However, the age and geometry of the structural elements in a broad tectonic belt in the upper Paleozoic and Mesozoic rocks indicate that the belt represents a long-lasting domain of crustal dilation, as an incipient rift in the northern margin of the craton (Balkwill and Fox, 1982). If the extensional features were a result of the spreading process landward of the continental margins similar to those observed on both sides of the Atlantic (Forsyth et al., 1988), then the age of breakup (onset in Early Cretaceous time and cessation near the Early-Late Cretaceous boundary; Embry and Dixon, 1994) and the thermal constant would be the constralnts. Any heat flow anomaly originating at that time would have dissipated long ago and would not have persisted through the Late Tertiary to the present. The present heat flow anomaly and the corresponding paleo-heat flow anomaly are likely related to a fixed crustal anomaly, mantle upflow or vertical upward fluid migration through faults. The lack of any significant magmatic activity in the recent history of the anomalous heat flow zone and the consistency of the magnitude of heat flow in time do not support the hypothesis of the mantle upflow. The high heat flow anomaly derived from the analysis of present thermal data (Fig. 6) and supported by the analysis of coalification data (Fig. 9) is an important thermal feature of the lithosphere of the Canadian Arctic margin that may have controlled the generation of hydrocarbons in the western part of the basin. The coincidence of the high heat flow and the area of faults is interesting though based on a limited amount of thermal data from wells. However, it is also observed that there are high heat flow areas outside the incipient rift zone as defined by Balkwill and Fox (1982), e.g., >60 mw/m 2 on northern Prince Patrick Island, western Banks Island and Cornwallis Island (Figs. 6 and 10). High heat flow in the rift zone is however most significant. The heat flow anomaly (Fig. 10) crosses from the Arctic Coastal Plain and Sverdrup Basin towards the Franklinian Basin. The anomaly is perpendicular to

156

J.A. Majorowicz, A.F. Embry /Tectonophysics 291 (1998) 141-159

the Ellesmerian Fold Belt on Melville Island and almost perpendicular to the Eurekan Fold Belt on Ellef Ringnes Island. The Late Cretaceous-Middle Tertiary Eurekan Orogeny reorganised the tectonic architecture of the central and eastern parts of the Sverdrup Basin. As a precursor, the eastern rim of the basin was uplifted in the Campanian and Maastrichtian, providing a clastic wedge that progressed to the Arctic continental margin (Balkwill and Fox, 1982). According to Balkwill and Fox (1982), it is uncertain whether or not there are Eurekan compressional folds in the western Arctic Archipelago where most high heat flow is observed. The age of the Eurekan Orogeny suggests that the present heat flow would not contain any significant effect of this event. However, any thermal events related to the Eurekan Orogeny would have influenced the Sverdrup Basin likely in the Late Cretaceous-Middle Tertiary.

6.2. Present heat flow versus paleo-heat flow The anomalous by high geothermal gradient geothermal gradient (>40 mK/m) and heat flow zone (>60 m W / m 2) correlates in the Sverdrup Basin with areas of high logarithmic coalification gradient, high paleo-geothermal gradient and paleo-heat flow. The statistical mean of the paleo-heat flow is slightly higher in comparison with the mean value of the present heat flow for the Sverdrup Basin (57 4- 12 m W / m 2 versus 53 4- 18 m W / m 2, respectively) but the difference is statistically insignificant and possibly arises from uncertainties in interpretations of thermal and coalification data. The above result, based on approximately 100 wells in the basin, is similar to the result of Skibo et al., 1990 based on only a few wells. In both cases, the conclusion is that heat flow has changed little since the time of maximum burial in the Early Tertiary in the western Sverdrup Basin. The equality of mean present and paleo-heat flow values shows that this thermal anomaly has been in existence since the Early Tertiary and no significant change has occurred in the area after uplift and erosion approximately 45 Ma ago. The 'anomalous' thermal conditions has not changed for at least since 45 Ma. A transient heat flow anomaly could not have been in place in the basin for such a long geological time with no change in magnitude (Eq. 5).

6.3. Hydrodynamic effect Heat flow values as high as 90 m W / m 2 were observed in the coastal areas of northern Alaska and interpreted by Deming et al. (1992) to be influenced by the hydrodynamic effect, i.e. gravity driven convection arising from the mountainous topography. This can be neglected in basins of the Arctic Archipelago due to a lack of topography. However, the upward flow of fluid and heat effect due to vertical permeability in the zone characterized by faults and fractures, is of unknown magnitude. The change in pressure due to the disappearance of the impermeable ice caps in the area (Taylor, 1991) could be important to the isostatic conditions and hydrodynamic stability of the area. These subjects provide the scope for considerable research and are not attempted in this paper. The possibility of deep overpressure zones is also a concern; however, deep zones where the amounts of trapped water exceed the natural pore space coincide with the zone of correspondingly low thermal conductivity (Jessop, 1990). Such a zone acts as an deep insulator and cannot explain channelling of heat into incipient rift as observed in the western Sverdrup Basin. The reactivation of the extensional features in the more recent history of the basin with the resultant vertical transport of fluids and gases adding a component of convective heat flow is also a possibility. The vertical movement of hydrocarbons and especially 'leaking' of natural gas in the basin was suggested by Balkwill and Fox (1982) and Waylett and Embry (1992). Many of the faults act as traps for hydrocarbons; however, some active ones in the central and western parts of the basin allow movement of hydrocarbons and water but especially gas. Whether such vertical movement of fluids and gas can contribute to the anomalous heat flow is not known since the velocities and continuity of the movements are poorly known. The near-equality of present and paleo-heat flow and of present and paleo-geothermal gradient would suggest that if the above mechanism was responsible for the anomalous heat flow zone it would have to be in place in the Early Tertiary in a manner similar to the present. This is quite unlikely considering the complicated history of vertical movements in the basin (Balkwill and Fox, 1982; Embry et al., 1988).

J.A. Majorowicz, A.E Embry / Tectonophysic s 291 (1998) 141-159

157

6.4. High heat flow and high heat generation

Acknowledgements

A static, high heat generating zone in the crystalline basement is most likely. It is well known that heat flow in sedimentary basins is controlled by crustal heat generation, which can vary by orders of magnitude in the basement (Majorowicz and Jessop, 1993). Lack of basement core samples and of measurements of radioactive isotope elements is therefore a reason for the difficulties in confirming the above hypothesis. The postulated high heat generation zone in the crystalline basement of the crust coincides with the later developed incipient rift zone as defined by Balkwill and Fox (1982). It may represent a long lasting basement feature enriched by radiogenic mineralisation containing uranium, thorium and potassium isotopes. Such 'hot rocks' are well known to cause high surface heat flow anomalies in Precambrian shield areas (for example, the Slave Province in Canada). The magnitude of heat flow correlates linearly with heat generation according to the heat flow versus heat generation relationship (Jessop, 1990).

The authors would like to thank K,R. Stewart of Arctic Geochemical Consultants in Calgary, Dr. E Goodarzi and Dr. D. Skibo (I.S.P.G., Calgary) for help with the coalification data used in this study. The helpful review by Alan Taylor is especially appreciated. Geological Survey of Canada contribution # 199802S.

7. Conclusions (1) The analysis of the first heat flow map of the Canadian Arctic Archipelago shows a large range of terrestrial heat flow, from 40 to 90 mW/m 2. (2) There is a good coincidence between the anomalous heat flow zone and high logarithmic coalification gradient in the western Sverdrup Basin. (3) The present heat flow and paleo-heat flow in the Tertiary in the western Sverdrup Basin are essentially equal. The anomalously high present heat flow and paleo-heat flow coincide also in the incipient rift zone as defined by Balkwill and Fox (1982). It is suggested that a static heat flow source has been in existence through the Phanerozoic history of the basin. (4) The large regional variability of present heat flow and paleo-heat flow in the Early Tertiary (4090 mW/m 2) shows that models of basin development on the continental margin have to take into account variable crustal heat sources.

Appendix A. Corrections of temperature records In an exploration well it is usual to record temperature at the bottom of the hole ('bottom-hole temperature', BHT); at the end of each time an electrical or other logging tool is ran. There usually are several runs during each cessation of drilling so that up to five or even six BHTs may be available for the purpose of estimating the true rock temperature at a particular depth. Since the procedure is repeated usually at several bottom depths reached in different stages of drilling of the well, there will be a series of temperature data with depth from which a thermal gradient can be determined. The process of drilling the well and especially the circulation of the mud filtrate in the well disturb the temperature in the surrounding rock. The walls of the well may be either cooled or warmed depending on the difference in temperature between the circulation mud and the natural rock temperature, duration of the drilling, and other factors. It is possible to make a correction for the drilling disturbance if a series of temperature measurements are taken at or within 20 m of the bottom hole, the so called bottom-hole temperature (BHT). The most widely used technique is the so-called Homer plot (Homer, 1951; Lachenbruch and Brewer, 1959; Fertl and Wichman, 1977; see also discussion of Drury, 1984). This technique is based on an empirical approach using the similarity between the diffusion and conduction equations. An extrapolation of the measured temperatures to the equilibrium temperature is achieved by plotting them against a logarithmic function of the time elapsed since the end of drilling (re) and the time of the duration of drill fluid circulation (to). The time function (t) is given by: t = In(tel(re + re))

(A1)

The function t becomes zero when te is very large, and thus the temperature extrapolated to the axis for which re~ (re + to) = 1 will give an equilibrium temperature at an infinite elapsed time (te). When the circulation time is missing from the wen-headers, BHT data from such wells can still be used. A statistical least squares line is used to extrapolate the temperature data to the equilibrium value and the slope of the fit can be determined. Unfortunately in half of all cases the circulation time tc was missing from the well-headers used in this study. In order that BHT data from such wells could still be used, a statistical analysis of the circulation times for the Beaufort-Mackenzie area and the Arctic Islands was made by Majorowicz et al. (1990). The data showed a large scatter, but most circulation times were under 8 h and only a very weak dependence of circulation time on drilling depth was observed. The average value of tc is 10.5

158

J.A. Majorowicz, A.E Embry /Tectonophysics 291 (1998) 141-159

h with a large standard deviation of 19 h in the BeaufortMackenzie area. However replotting the frequency histogram of (tc) with the logarithmic base produces a normal (or Gaussian) distribution, the median of which was found to be 4 h. This value was used for calculating the corrected temperature in cases where tc was not stated on the headers of well logs. The authors note that an increasing number of well logs in recent years do not record this information, perhaps limiting future use of BHT data for scientific research here and elsewhere. In summary multiple BHT data with adequate information on time since circulation has ceased (te) and circulation time (to) were corrected for cases with te significantly larger than tc using the method of the Homer plot. Where tc was missing from the log headers, it was assumed to be 4 h as discussed above, and the multiple BHT records with information on te and an assumed tc of 4 h were corrected similarly. In 20% of the wells only single BHT records exist but the te and tc is recorded on the header. To best utilise such data, an approximate correction was applied based on the calculation of the function t in Eq. A l from the known values of te and tc for each single temperature record combined with an average value of the slope of the Homer plot derived from the statistical depth relation obtained for the study area. A statistical analysis of the slopes of the Homer plots for the Arctic Archipelago data showed that the slope increases with depth as: slope = 62.4 - 0.048d + 0.000017d 2

(A2)

where d is the vertical depth in meters The above relationship was used to estimate the average slope of the Homer-type correction at any given depth d. Statistical analysis of the percentage corrections, i.e. the percentage difference between the BHT and equilibrium temperature, showed that it is less that 10% (with 67% probability) for elapsed time te exceeding twice the average circulation time tc (Majorowicz et al., 1990). Previously published calculations showed the average percentage correction to be 12% with a standard error of 6% (Majorowicz et al., 1988). This too provided constraints on the accuracy of the calculated temperatures. Drill-stem test temperature records where available were also combined with BHT data to add to the temperature versus depth information for individual wells. These tests record at a particular depth both temperature and pressure as a function of time of the test in an isolated section of the well and are used to evaluate conditions in reservoir horizons. In the case of significant inflow of the formation fluids, maximum recorded temperatures will give valuable information about the formation temperature which usually compares well with temperature data from other sources. Since such tests are usually performed for a variety of reservoir horizons at different depths they also can contribute to the determination of thermal gradients.

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