Will Gosnold and Richard LeFever

Heat Flow and Thermal Maturity in the Williston Basin

Outline• Heat Flow

– Earth’s Heat Flow – Steady-state Heat Sources and Transient

Disturbances– Subsurface Temperatures

• Thermal Maturity– Chemical Kinetics & Hydrocarbon Potential– Time–Temperature History

• New Perspective on Northern Hemisphere Heat Flow– A Hypothesis to Test– Implications for the Williston Basin

Global Heat Flow• Global average heat flow: 87 mW m-2

• Total surface heat flux: 44.2 x TW• 83% of present surface heat flow is due to

radioactive decay of U, Th, and K • Earth’s mantle is cooling at a rate of 36 °C Ga-1

• Average solar flux at TOA: 1365 W m-2

• Average solar flux at the surface: 400 W m-2

Heat flow within ocean basins correlates with age.

Heat flow on the continents is linearly related to radioactive heat production in the continental crust

HGU

Q

Q = Q0 + AD

•The Q vs. A relationship is recognized widely and allows definition of heat flow provinces with characteristic values of Q0and D• Globally, Q0 averages about 27 mW m-2

and ranges from 18 mW m-2 in the Sierra Nevada to 33 mW m-2 in the EUS•D averages about 10 km and ranges from 4.5 km in the WAS to 16 km in England and Wales•A does not vary significantly over time so heat flow on the continents can be considered constant.Roy, Blackwell, and Birch, 1968

Continental Heat Flow Steady-state Sources

• Mantle flux: averages 27 mW m-2

• Crustal radioactivity from U, Th, and K: accounts for about half of surface heat flow

• Heat flow correlates with tectonic age.• Tectonic events: 10s to 100s of mW m-2

Transient Signals in Continental Heat Flow

• Ground water flow: + or -• Ground cover change: + or -• Climate change: + or –• Characteristicthermal length1 y 10 y 100 y 1000 y 10,000 y

13 m 42 m 131 m 415 m 1,310 m

FotL α=

1 y 10 y 100 y 1000 y 10,000 y

9 m 28 m 87 m 275 m 868 m

α = 1x10-6 m2 s-1

α = 0.44 x10-6 m2 s-1

Fourier’s law of Heat conduction

Assuming we know heat flow, temperature at depth “z” may be calculated by

Γ= λq

∑=

=n

i i

iz

qzT1 λ

Subsurface Temperatures can be calculated if heat flow and thermal conductivity are known

Geothermal Map of North America, 2004

D. Blackwell and M. Richards, Eds.,

Ground water flow

Uplift and magma intrusion

Effect of radioactive heat production on temperature. Units are μW m-3

2.2

0.1

15.0

4.5

40 50 60 70

T vs. z vs. q

Thermal Maturity

• … is a measure of the degree of metamorphism of kerogen in a formation

• … gives a rough estimate of the maximum temperature a formation has reached

• Two fundamental requirements:– Understand chemical kinetics– Understand the thermal history

Chemical Kinetics• The basic premise of chemical kinetics

is that reaction rate depends on absolute temperature and the amount of reactants

• Arrhenius equation: Reaction rate (k) is a function of absolute temperature(T), frequency factor (A), and activation energy (E). R is the gas constant.

RTE

AeTk−

=)(

Hydrocarbon Potential• In general HC generation is too complex to

describe by precise equations so Hydrocarbon Potential (M0)is preferred.

( )mMmM0−=

∂∂=

∂∂− k

ttPotential (M) decreases as yield (m) increases and k is temperature dependent

Hydrocarbon Potential

• The task is to determine M0 and A for different kerogen types across a range of activation energies (E).

• This is normally done by pyrolysis using a constant heating rate.

Kinetic Parameters for some Type II Source Rocks

Source Rock E (kJ mol-1) A (s-1)

Bakken Shale1 226.1 4.750 x1013

Monterey Shale2 143.4 2,224 x107

Phosphoria Shale2 178.7 1.388 x1010

Alum Shale2 201.3 4.899 x1011

Woodford Shale2 218.3 1.792 x1013

1. Sweeney, Gosnold, Braun and, Burnham, 1992, A chemical kinetic model for hydrocarbon generation from the Bakken Formation, Williston Basin North Dakota, UCRL-ID-112038.

2. Hunt, Lewan, and Hennet, 1991, Modeling oil generation with TTI graphs based on the Arrhenius equation, AAPG Bull., 75(4).

depth t= +2 6 36512. .

Comparison of stratigraphic record with thermal subsidence model GHD1 (Stein and Stein, 1992)

Thermal conductivity profile for the Williston Basin

Temperature history is dependent on heat flow and thermal properties of the basin

Is our temperature history correct? Could we have incorrect values for heat flow?

Porosity varies with depth asΦ=Φ0e-cz c is a constant and z is depth

Thermal conductivity, K, varies with porosity and as a functionof the conductivity of the solid rock and water as K = Kr1-ΦKwΦ

If heat flow is constant, the temperature at depth is calculated as

T = T0 + ΣΓizi where Γi = q/Ki

The normal temperature vs. depth profile in a thick clastic sedimentary section has a convex curvature due to the increase in thermal conductivity with depth caused by compaction which reduces porosity.

Shell USA 22-24-27

Mondak Field McKenzie County

Curvature in the clastic section is opposite to expectations

Two possible explanations:1. Conductivity

decreases with depth2. Surface temperature

increased by 10 to 15 degrees C.

Empirical evidence for large magnitude postglacial warming

• T-z measurements in parts of Europe and North America show

a systematic increase in heat flow with depth.

0 20 40 60 80

100

120

140

160

01000

20003000

40005000

6000Depth (m

)

Heat flow

EuropeNorth America

Empirical evidence for large magnitude postglacial warming

• Heat flow in southern hemisphere shields averages approximately 61.4 mWm-2, but heat flow in northern

hemisphere shields averages 37 mWm-2.

• Brazil 64.8 ± ? mW m-2 (86)• Africa 52.3 ± ? mW m-2 (145)• Australia 68.1 ± ? mW m-2 (157)

• N. America 33.1 ± ? mW m-2 (315)• Fennoscandia and East European

Craton 35 - 40 mW m-2 (1,352)

Low heat flow in North America coincides with the center of the Pleistocene ice cap. (Blackwell and Richards, 2004).

Why have we not identified this signal before?

Conventional heat flow methodology does not sample the signal.

0

5

10

15

20

25

30

35

40

0 400 800 1200 1600Depth (m)

Deg

C

Steady-state T-z3 Deg T-z5 Deg T-z10 Deg T-z15 Deg T-z

The effect of postglacial warming on the thermal gradient is subtle.

•LSQ analyses of 200 m segments of a temperature log from the Williston basin all appear linear.

•The geothermal gradient increases systematically with depth.

•The surface intercept on the temperature scale decreases systematically with depth.

•Does the change in surface temperature show the amount of warming that has occurred at the surface?

•If so, the minimum warming has been at least 12 K.

•Three T-z profiles from the Williston basin exhibit an increase in heat flow with depth.

•The synthetic T-z shows an expected profile for constant heat flow with the effects of compaction on thermal conductivity.

•The 15 degree signal is a modeled curve for warming since 10 ka.

•The Glacx curve results from superposition of 3 glacial/interglacial cycles (90 ka/10) ka warming signal on the steady-state synthetic T-z. It appears to match closely the observed T-z profiles.

30%

40%

50%

60%

70%

80%

90%

100%

0 400 800 1200 1600 2000 2400 2800 3200Depth (m)

perc

ent Q 15 deg

10 deg5 deg3 deg

Percentage of q vs. depth as a result of warming

96 percent of heat flow determinations in North America were made in boreholes less than 2000 m deep.

Higher paleo heat flow would imply the area of mature Bakken is greater than thought.

Oil generation zone

%Ro with q = 40 mW m-2

%Ro with q = 60 mW m-2

How can the hypothesis be tested?

• Determine heat flow at multiple points in the borehole.– Core drill the clastic section for thermal

conductivity measurements.– Let the borehole recover thermal equilibrium

to obtain an accurate temperatures.

Optimum sites would be in periglacial regions in rocks having minimal variability in λ.

Summary

• Estimates of source rock thermal maturity require understanding of chemical kinetics and thermal history.

• Determining thermal history requires a complete understanding of:– Subsidence history– Thermal properties– Heat flow– Surface temperature history

Summary

• Evidence for large magnitude post-glacial warming in northern Europe, Asia, and North America is growing.

• Some northern hemisphere heat flow values may require revision because they were determined from boreholes too shallow for recognition of the gradient disturbance caused by a large post-glacial warming signal.

• The hypothesis requires further testing with data from deep boreholes in periglacial regions.

• If the hypothesis passes the test, thermal maturity estimates for oil source rocks in the Williston Basin require revision.