Tectonophysics, 194 (1991) 357-361

Elsevier Science Publishers B.V., Amsterdam

357

The thermal regime in the Jeanne d’Arc Basin, offshore eastern Canada

Anthio Correia a,b and F. Walter Jones b ’ Departmento de Fisica, Uniuersidade de .&ora, 7ooO koora, Portugal

’ Department of Physics, Unioersity of Alberta, Edmonton, Alta. T6G 251, Canada

(Received December 12, 1989; revised version accepted March 28, 1990)

ABSTRACT

Correia, A. and Jones F.W., 1991. The thermal regime in the Jeanne d’Arc Basin, offshore eastern Canada. In: V. Cermak and

J.H. Sass (Editors), Forward and Inverse Techniques in Geothermal Modelling. Tectonophysics, 194: 357-361.

Heat flow density values within the Jeanne d’Arc Basin from a previous study are compared with calculated heat flow

densities based on a two-dimensional conductive numerical model. Three profiles across the basin are considered, and it is

found that although water motion may affect the temperature distribution at shallow depths, it is unlikely that the thermal

field is perturbed by water motion at great depths. It is found that the temperature at 20 km depth lies between 420 and

440°C. and that the heat flow density from the upper crust beneath the basin is about 45 mW/m2.

Introduction

In a recent study, Correia et al. (1990) esti-

mated heat flow densities for the Jeanne d’Arc

Basin in offshore east coast Canada. The location

of the study area is shown in the insert of Fig. 1.

The geographical coordinates of the basin center

are about 47 o N and 48.5 ’ W, and the area of the

basin is approximately 16,000 km*. In the N-S

direction the length of the basin is about of 200

km, and it is about 100 km wide in the north and

narrows to a width of approximately 40 km in the

south. The basin is deep in the north, where the

sediments reach thicknesses of up to 20 km (Grant

et al., 1986; Keen et al., 1987; Enachescu, 1987)

but it becomes shallower toward the south. The

basin developed as a consequence of Mesozoic

extensional rift tectonics that led to the formation

of the North Atlantic Ocean, and is filled with

Mesozoic and Cenozoic sediments that overlie Tri-

assic red-bed and salt deposits.

The heat flow density pattern in the Jeanne

d’Arc Basin was determined by Correia et al.

(1990) from 600 bottom-hole temperature (BHT)

values obtained in 35 oil wells. A divided-bar

apparatus was used to measure thermal conductiv-

ities of rock samples (limestone, dolomite,

anhydrite, shale, claystone, sandstone, and silt-

stone) from four wells in the basin in either solid

disc form or by the cell method on cuttings (Sass

et al., 1971). For other rocks for which no samples

were available (marlstone, chert, coal, con-

glomerate, and salt) assumed thermal conductivity

values were used. The measured and assumed

thermal conductivities are presented in Correia et

al. (1990). The thermal conductivity values were

corrected for temperature dependence in the heat

flow density calculations.

The heat flow density data from Correia et al.

(1990) suggest that, for the upper 5 km of the

basin, consisting essentially of Tertiary and Upper

Cretaceous formations, the vertical component of

fluid flow is probably no greater than 1 mm/year.

Fluid flow in the basin is downward in the north

and upward in the south. Furthermore, consider-

ing the tectonic framework of the basin, with

sedimentary layers rising gently from north to

south, and the fact that the bottom of the ocean is

practically horizontal in the area, it was suggested

that fluids flow from the deep sediments in the

0040-1951/91/$03.50 0 1991 - Elsevier Science Publishers B.V.

358

46

Fig

o_

I

:. 1

A Fault (approximate) (ticks on down side)

49” 46”

46”

Heat flow density (HFD) values for the Jeanne d’Arc

Basin and locations of the profiles of the three model sections.

The insert shows the location of the study area in offshore

eastern Canada (modified from Correia et al., 1990). HFD are

in mW/m2.

north part of the basin to the south part following

the sedimentary layers, and that the driving mech-

anism for this fluid flow is probably dehydration

due to compaction of the deep sediments in the

north.

The present work is an extension of the previ-

ous work. A two-dimensional numerical model is

employed here to evaluate to what extent this fluid

flow is important in the redistribution of heat and

therefore acts to influence the temperature distri-

bution within the Jeanne d’Arc Basin, and to

estimate the heat flow density from the lower

crust.

Description of the model

Because of the elongated shape of the Jeanne

d’Arc Basin, a two-dimensional (2-D) model was

used. Since information concerning the possible

fluid flow pattern, the permeabilities of the geo-

A. CORREIA AND F.W. JONES

logical formations, and the hydraulic heads in the

wells drilled in the basin was not available, a

purely conductive model was chosen. If fluid flow

is important in the redistribution of heat within

the basin, then its effect should be seen when the

estimated heat flow density values are compared

with those calculated from the model. The numeri-

cal model uses an alternate direction implicit

method to solve the conservation of energy equa-

tion for heat conduction with heat production and

the algorithm is described in Jones and Ertman

(1985).

In order to model the Jeanne d’Arc Basin, it

was necessary to estimate the thermal conductivi-

ties of the geological formations and the heat flow

density from the lower crust. For this, the basin

was divided into five zones that correspond to the

Tertiary, Cretaceous, Jurassic (without salt), the

Jurassic salt layer, and the basement. Further-

more, the most abundant geological formations

encountered in the first three zones were divided

into three groups: (I) shale and claystone, (II)

sandstone and siltstone, and (III) limestone and

dolomite.

From published data (Grant et al., 1986; Keen

et al., 1987; Tankard and Welsink (1987); and

Enachescu, 1987), and lithologic logs from four

wells in the Jeanne d’Arc Basin, it was assumed

that, for the Tertiary, 80% of the geological forma-

tions belong to group I, 15% to group II, and 5%

to group III. For the Cretaceous it was assumed

that 50% of the geological formations belong to

group I, 35% to group II, and 15% to group III.

Finally, for the Jurassic it was assumed that 55%

of the geological formations belong to group I,

15% to group II, and 30% to group III.

Using the above assumptions, together with the

measured and assumed thermal conductivities for

the rock types encountered in the Jeanne d’Arc

Basin (Correia et al., 1990) the effective thermal

conductivities of the Tertiary, Cretaceous, and

Jurassic sediments were calculated by means of

the equation :

i Azi

(1)

THERMAL REGIME IN THE JEANNE D’ARC BASIN. OFFSHORE E CANADA 359

for a geological column of n layers, where AZ, is

the thickness of the rock layer of thermal conduc-

tivity K,. The values determined were 2.83 W m-’

K-‘, 3.02 W m-’ K-‘, and 2.77 W m-’ K-’ for

the Tertiary, Cretaceous, and Jurassic respectively.

For the basement, an average thermal conductiv-

ity of 2.75 W m-’ K-r was assumed (Weir and

Furlong, 1987), and for the salt layer an average

thermal conductivity of 5.7 W m-r K-r was used.

For the heat flow density from the lower crust, a

value of 45 mW/m2 was assumed.

Although it was not possible to determine heat

generation values for the geological formations of

the basin, heat production was included in the

model because of the great sediment thicknesses.

Assuming heat production values reported by

Keen and Lewis (1982) and Della Vedova and

Von Herzen (1987) for geological formations found

in similar geological environments, heat produc-

tion values for the sediments of the Jeanne d’Arc

Basin were calculated using the same approach as

that for the effective thermal conductivities. For

group I a heat production of 2.26 pW/m3 was

obtained, while for groups II and III heat produc-

tions of 1.73 rJ.W/m3 and 0.98 pW/m3, respec-

tively, were obtained. Using these heat production

values and the same percentages of rock types as

in the effective thermal conductivity calculations,

the heat production values obtained for the Ter-

tiary, Cretaceous, and Jurassic were 2.12 pW/m3,

1.88 pW/m3, and 1.8 pW/m3, respectively. A heat

production value of 1 pW/m3 was assumed for the

basement, and no heat production was assigned to

the salt layer.

To calculate the surface heat flow density within

the Jeanne d’Arc Basin, three profiles were consid-

ered (see Fig. 1). Profile 1 crosses the deepest part

of the basin, profile 2 crosses the middle of the

basin, and profile 3 crosses a shallow part of the

basin in the south. The models corresponding to

these three profiles were constructed from knowl-

edge of the basin structure (Grant et al., 1986;

Keen et al., 1987; Tankard and Welsink, 1987;

and Enachescu, 1987), but it must be emphasized

that they are approximate because the resolution

of geophysical methods and the complexity of the

Jeanne d’Arc Basin do not allow more detailed

models at this time. Figure 2 shows the model

cross sections corresponding to profiles 1, 2, and

3 of Fig. 1. The figures also show the calculated

surface heat flow densities as well as the heat flow

density values and errors from the bottom-hole

temperature data (Correia et al., 1990). It should

be noted that, as Correia et al., (1990) emphasized,

the error associated with the heat flow density

calculations for the wells in the basin are high

because of the relatively small number of thermal

conductivity measurements. A better control of

the thermal conductivities of the geological forma-

tions would substantially reduce the errors in the

heat flow density calculations.

Discussion and conclusions

If the models shown in Fig. 2 are reasonable,

the results of the heat flow density calculations for

them indicate that fluid flow may play an im-

portant role in the redistribution of heat in the

Jeanne d’Arc Basin. This conclusion is particularly

apparent from Fig. 2b, where a misfit between the

heat flow density values calculated from the bot-

tom-hole temperature data and the model is ob-

served even when the errors of the heat flow

density calculations are taken into account. For

Figs. 2a and 2c, a similar misfit is not obvious

and, within experimental error, the two sets of

results appear coincident. It is interesting to note

that in the area of the basin just north of profile 2

(Fig. 1) several oil reservoirs, including the giant

Hibernia oil field occur. If, as Roberts (1981)

claims, oil reservoirs coincide with regions with

relatively high fluid flow activity, this could ex-

plain, to a certain extent, the misfit observed in

profile 2. However, it is important to note that

while the models described here extend to depths

of 20 km, the heat flow densities calculated for the

basin employed data only from depths up to 5 km.

Nevertheless, it is probable that because of com-

paction, porosity and permeability decrease sub-

stantially with depth, and so the temperature dis-

tribution within the basin at greater depths than 5

km is not greatly affected by fluid flow. However,

at this stage, it is impossible to draw further

conclusions about the thermal regime of the Jeanne

d’Arc Basin at depths greater than 5 km, and more

data concerning the thermal conductivities and

360 A. CORREIA AND F.W. JONES

90

80

70

60

50

40 h

1

0 20 40 60 80 100 120 140

km 0 Tertiary Ez3 Cret%X?CJllS m Jurassic

N JlNaSSlC-Salt cl Basement

901

L 0

I I I

0 I I

20 40 I

60 80 I

100 120 1)

140

-_ 0 20 40 60 ii0 lb0 150 lb0

km 0 Tertiary

Ea Cretaceous m Jurassic

Ea Jurassic-Salt 0 Basement

,c) 0 20 40 60 80 100 120 140

-- 0 io 40 60 .80 lb0 li0 lh0

km 0 Tertiary ilam JW3SSlC EEzI Jurassic-Salt

Fig. 2. Profiles 1-3 of Fig. 1. The continuous line in the upper part of the figure represents the heat flow density calculated for the

model. Also shown are the HFD values and errors calculated for the wells in the basin on or near the profile. The dashed lines in the

lower part of the figure are temperature isolines in degrees Celsius. (a) Profile 1. (b) Profile 2. (c) Profile 3.

THERMAL REGIME IN THE JEANNE D’ARC BASIN, OFFSHORE E CANADA 361

heat productions of the sediments is necessary, as

well as information about how porosity and per-

meability vary with depth.

The temperature distributions as a function of

depth are also shown in Fig. 2. However, it must

be remembered that the temperature isolines rep-

resent the case for which no fluid flow exists, and

the porosity and permeability are small (Smith

and Chapman, 1983). This is not the case here, at

least for the upper 5 km of the basin.

Profiles 1 and 3 indicate that, for the corre-

sponding regions of the Jeanne d’Arc Basin heat

transfer is probably by conduction only. If this is

true, it means that the heat flow density from the

crust is about 45 mW/m2 as assumed in the

model. This value is close to the values reported

by Sclater et al. (1975), Lewis and Hyndman

(1976), and Wright et al. (1980). Furthermore, it is

seen that the temperature at 20 km lies between

420 o and 440 o C.

It is clear that the present knowledge of the

basin does not allow the models to be better

constrained, but the comparison of the models

with the heat flow density calculations indicates

results that are consistent with the conclusions of

Correia et al. (1990).

Acknowledgments

The authors thank M. Ertman for his help in

computing matters. One of the authors (AC) was

supported by the University of Evora, Portugal,

by the Junta National de Investiga@o Cientifica e

Tecnolbgica, Portugal, and by the University of

Alberta through a teacher assistantship. The work

was supported by the Imperial Oil Company of

Canada and the Natural Sciences and Engineering

Research Council of Canada.

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