Heat Flow at the margins of continents: example of the lower

Congo basin

Francis Lucazeau

Claire Perry

Frédéric Brigaud

Bruno Goutorbe

Question

Heat flow has been used for years to constrain subsidence and oil maturation models on divergent continental margins…

But do we really know how surface heat flow varies in space and time ?

– Very few measurements– Local variations– Geodynamic scale

0

1000

2000

3000

4000

Ocean Basin Oceanridge/rise

Continentalrise

Continentalshelf

Tectonic setting

Nu

mb

er o

f h

eat

flo

w d

ata

from Pollack et al, 92 database

Estimating heat flow on continental margins

Conventional measurements

water depths > ~1000 m Depth of BSR

water depths > ~500 m

not observed everywhere Oil exploration data (temperatures, well logs)

availability

non equilibrium temperatures

from White et al, 03

Location of the lower Congo basin margin

2000

2000

Africa

South America

Bouvet

Walvis

Etendeka

Paranà

0 1000 2000

Distance (km)

Lower Congo basin

from White et Mckenzie, 1989

from Contrucci et al, 2004

Possibility for comparison of different data sets

9600

9650

9700

9000

9050

9100

9150

9200

9250

9300

9350

9400

9450

9500

9550

500 550 600 650 700 750 800 850 900 950 1000

GABON

CONGO

RDC

ANGOLA

1076a

1077a

Structural aspects in the lower Congo basin

Water bottom

Pliocene base

Oligocene base

Pliocene to Actual

Miocene

Oligocene

Eoceneto Turonian

Cenomanian

Albian

Salt

StratigraphyDiscontinous BSR

Diapir

Tertiary turbiditic sequence

Time (ms TWT)

5000 m

Slope deposits

Recent heat flow measurements on extensional margins

Study Location WD range (m) Heat Flow (mWm-2 )

Louden et al, 97 Galicia margin 4000 - 6000 47 ± 3 (C and O)

Nagihara et al, 96 Gulf of Mexico 3000 - 3500 40 ± 47 (C and O)

Polyak et al, 96 Alboran sea 600 – 2400 69 ± 6 to 124 ± 8 (C)

Ruppel et al, 95 Carolina 2000 – 4000 49 ± 12 (C and O)

Foucher et al, 92 Gulf of Valencia 1000 – 2000 66 ± 4 to 78 ± 13 (C)

Louden et al, 91 Goban Spur ~ 4000 42 ± 4

Only few academic studies…

… but many studies sponsored by oil industry!

from TDI-Brooks website

Measurements of heat flow in the lower Congo basin

from Sultan et al, 2004

502 m 550 m 557 m

• Most of data unpublished• heat flow higher in the North, but no clear variation E-W

Recent Heat flow / BSR studies

Study Location Agreement Characteristic

Shankar et al, 04 Western India margin No comparison

Lüdman et al, 04 NW Black Sea ?

Martin et al, 04 Nankai prism calibration ODP

Lüdmann & Wong, 03 Sea of Okhotsk Poor

Bouriak et al., 03 Vøring / Storegga calibration conventional

Henrys et al, 03 Hikurangi, New Zeal. No comparison

Grevemeyer et al, 03 Central Chile calibration ODP

Vanneste et al, 03 Baïkal rift Poor Lower values

Nagihara et al, 02 Niger delta Mean Lower dispersion

Rao et al, 01 West India margin No comparison

Kaul et al, 00 Makran Poor Higher values

Golmshtok et al, 00 Baïkal smoother

Ganguly et al, 00 Cascadia margin Good

Shyu et al, 98 SW Taiwan ?

Townend, 97 New Zealand No comparison

Yamano et al, 82 Nankai prism

Estimation of temperature gradient from BSR

_( )BSR water bottomsurface

BSR

T TG

Z

Average velocity above

the BSR ( from seismic lines and boreholes):

1530 +/- 30 m/s

4

3

2

1

0

2 3 4 5 6 7

CongoAngolaZaiangoUT Congo

-5 0 5 10 15 20 25Temperature (°C)

5

10

15

20

25

30

Pre

ssu

re (

Mp

a)

10 % Nacl

0 % Nacl

Gas Hydrate stabilitydomain

Free gas Wa

ter

bo

tto

m (

km)

Temperature (°C)

Comparison of observed BSR depth and predicted depth

0.65 0.7 0.75 0.8 0.85 0.9

300

200

100

0

0 10 20 30 40 50

300

200

100

0

20 30 40 50 60 70

300

200

100

0

Dep

th (

km)

Thermal conductivity (Wm-1K-1) Temperature (°C) Heat flow (mWm-2)

ODP1076a

ODP1077a

BSR

BSR

Location of BSR and estimation of temperature gradient

1076a

1077a

BSR gradient versus surface measurements

Variation thermal conductivity with depth

0 0.2 0.4 0.60

0.4

0.8

1.2

1.6

Con

duct

ivity

(W

m-1

K-1)

Depth of BSR (km)

ODP1077a

ODP1076a

From surface heat flow (= q/G)

Estimation of thermal conductivity for the hydrate domain

0.6 0.7 0.8 0.9 1 1.1

Comparison heat flow estimated from BSR and measurements

Absence of heat flow anomalies related to fluids

Pockmark

Short length-scale anomalies related to conduction

Salt dome Canyon

Recent heat flow studies from oil exploration wells

Study Location Temperatures Conductivity Heat Flow (mWm-2)

He et al, 02 Yinggehai Basin Corrected BHT Measurements 79 ± 7 (6)

He and Middleton, 02 NW Australia ? ? 53 ± 4 (7)

Hu et al, 01 Bohai Basin Corrected BHT, DST Meas. outcrops 66 ± 15 (95)

Forster, 01 Northeast German Basin

Comparison BHT / logs

Lee and Deming, 99 Oklahoma Corrected BHT Measurements 36 ± 6 (9)

Gallardo and Blackwell, 99

Carter et al, 98

Anadarko basin Corrected BHT

profiles

Measurements

Cranganu et al, 98 Oklahoma Corrected BHT

profiles

Measurements (cuttings)

35 to 75

Majorowicz and Embry, 98 Canadian Arctic Corrected BHT, DST Estim., harm. 53 ± 12 (156)

Correia and Jones, 96 Jeanne d’Arc Corrected BHT Measur., harm.

Estimated

57 (35)

29 (35)

Problem of estimating thermal conductivity?

Procedure for estimating heat flow from oil exploration data

« Heat diffusion » equilibrium temp.

Horner equilibrium temp.

0 40 80 120 160 200

70

80

90

100

110

120 « Heat diffusion » equilibrium temp.

Horner equilibrium temp.

0 40 80 120 160 200

70

80

90

100

110

120

Correction for mud circulation

Tem

pera

ture

BH

T (

°C)

Elapsed time (h)

-30 -20 -10 0 10 20 30

0

0.1

0.2

-30 -20 -10 0 10 20 30

0

0.08

0.16

Homogenization with DST

Rel

ativ

e fr

eque

ncy

Norway Angola

Temperature difference (°C)

TERT

PALAEOU CRET

Litho-stratigraphic log Geophysical well logs

Mineral composition

Porosity

Estimation of conductivity

Inversion

Comparison of Heat Flow from oil data with other estimates

Conventional measurements

BSR

Global trend across the lower Congo basin margin

ocean deep offshore shelf

marine measurements Oil exploration wells

~42 mWm-2 ~52 mWm-2 ~70 mWm-2

measurements

BSR

Oil data

What is the heat flow value on the Archean basement?

100 km

-6shear wave velocity variations (%)

+6

from Ritsema & van Heijst, 00from Hartley et al, 96

-10

-20

-30

40

30

20

10

0

-25 -15 -5 5 15 25 35 45 55

100 km

80 km

60 km

40 km

30 km

20 km

10 km

5 km

0 km 84 estimates

Elastic plate thickness from gravity

No value on the Archean Congo craton, but 33 mWm-2 on the Tanzania craton (Nyblade, 97) + indirect evidence for low mantle heat flow

Interpretation of the trend on the lower Congo basin

Two possibilities:- local high mantle heat flow- local high crustal heat production (Pan African)

Evidences of higher heat flow on Mozambique margin

High heat flow

from Nyblade, 97

Heat Flow derived from global 3D seismic model

from Shapiro & Ritzwoller, 2004

Flexural rigidity of continental margins

Best fit model:Low rigidity on the marginhigher rigidity on the ocean

from Watts, 1988

Gravity modelling of theBaltimore Canyon trough

Laboratory experiments of convection with insulating lid

from Guillou and Jaupart, 1995

Heat flow discontinuityat the ocean continent boundary

Convection model geometry

-V +V

To Tc

µm=1019 Pa s µc=1022 Pa s

Tbase=3000 K

D~3

000

km

~ 300 km

Breaking of the Gondwana during Barremian-Neocomian

= 8D

= 6000 km

Temperature and velocity fields at 144 Ma (lower Cretaceous)

3000 km

1500 km

Temperature and velocity fields at 72 Ma (end of Cretaceous)

3000 km

1500 km

Temperature and velocity fields at 0 Ma (present time)

3000 km

1500 km

Comparison of T and velocity fields with and without continents

3000 km

1500 km1500 km

Surface heat flow at 0 Ma(present time)

Continent 0 Continent 1

Same experiencewith no continent

Temperature ratio at the base of continent vs heat production

-V +V

To Tc

~0.5-1.2 µWm-3 for the crust

Internal heating

Amplitude of heat flow at the continent margin

-V +V

To Tc

Ra= 5x106

Wavelength of heat flow at the continent margin

Concluding remarks about heat flow on continental margins

Possibility to combine different sets of data in the lower Congo basin :

– No bias observed between the different sets– Large variations at small wavelengths– Emerging trend across the margin

High mantle heat flow below old margins– Possibly related to perturbations in the convection field– Amplitude is a function of heat production in the continent– Wavelength does not depend on the continent properties

Need for a better knowledge of heat flow and processes on continental margin

– Importance of the slope and shelf domains– Still active oil exploration in the deep offshore comparison with

conventional probe measurements

Statistics on heat flow in the lower Congo basin

Type mean SD min max median skewness mean SD mean SD Number

Conventional 46.9 13.1 16.8 95.6 44.2 1.24 60.7 15.2 0.77 0.11 293

Short probe 46.9 13.2 16.8 95.6 44.0 1.23 60.6 15.2 0.78 0.11 282

Long probe 46.5 10.5 34.9 74.7 45.9 63.9 14.6 0.73 0.05 11

north 57.4 11.4 30.4 95.0 57.5 0.58 62.7 11.5 0.92 0.09 66

south 43.9 11.9 16.8 95.6 42.0 1.85 60.1 16.1 0.73 0.07 227

Oil 63.8 15.2 36.7 132.5 61.9 1.13 55.5 16.8 1.28 0.61 424

bathy > 200m 54.8 6.3 42.8 66.6 56.0 62.1 5.4 0.88 0.05 38

bathy < 200m 64.7 15.5 36.7 132.5 63.3 1.04 54.8 17.4 1.32 0.62 390

Oil (north) 65.8 16.7 40.4 132.5 64.6 0.94 55.0 17.9 1.35 0.66 301

Oil (south) 60.8 9.7 36.7 97.7 62.0 0.31 54.0 16.0 1.24 0.47 89

BSR1 52.2 9.5 8.7 138.1 52.7 1.51 61.8 11.0 0.84 993186

BSR1 (north) 53.7 7.5 8.7 138.1 54.5 0.91 61.6 8.3 0.87 711805

BSR1 (south) 48.4 12.5 26.5 137.9 45.5 2.44 62.2 15.8 0.78 281381

Gradient (mKm-1)

Heat flow (mWm-2)

Cond. (Wm-1K-1)