Publication Date:
2022-05-26
Description:
Heat flow estimates at two sites on the U.S. Atlantic continental margin
are presented. An estimate of the heat flowing from the basement also has
been obtained. About 4.8 km of sediments penetrated at the COST B-2 and 4.0
km at the COST B-3 were deposited since the Upper Jurassic. Well logs were
used to evaluate thermal gradients and sedimentation rates, whereas thermal
conductivities and radiogenic heat productions were measured on drill cuttings
samples.
A procedure to estimate in-situ thermal conductivity from drill cuttings
and well logs is described. A substantial set of samples, in the form of
drill cuttings, were sorted in four major lithologies: sandstones, siltstones,
shales and limestones. Laboratory measurements of density, porosity, thermal
conductivity, quartz (%), potassium (%), uranium (ppm) and thorium (ppm) were
performed on 128 reorganized and pulverized samples. A significant correlation
of the matrix thermal conductivity to quartz and potassium content was found.
In situ porosity and volume fraction of each lithology, determined mainly from
well logs, were used to calculate in situ mean thermal conductivity. Finally
the mean in situ vertical component of the thermal conductivity, as required
for heat flow values, has been estimated from a correction factor for the
anisotropy of each lithology. The in-situ temperature and anisotropy effects
substantially decrease estimates of thermal conductivity at depth.
Below the uppermost 1 km in both wells the best estimate of the thermal
gradient is 26.3°C km- 1 at COST B-2 and 26.1°C km- 1 at COST B-3, whereas
in situ mean thermal conductivities range between about 1.8 and 1.9 W m- 1
K- 1 (4.3-4.5 T.C.U.). The average heat flow is estimated as about 45 mwm- 2
(1.07 H.F.U.) at COST B-2 and 44 mWm- 2 (1.06 H.F.U.) at COST B-3, with an
uncertainty of about 20-25%. The mean radiogenic production in sediments at
the two sites has been estimated as 1.83 (COST B-2) and 1.44 (COST B-3)
10- 6Wm- 3
• With a 12-14 km thick sedimentary sequence a radioactive
contribution of 20-25 mWm- 2 can be expected.
The effects of sediment deposition, compaction, pore water advection and
radiogenic heat production have been combined in a numerical model (Hutchison,
1985) to estimate the undisturbed basement heat flux. Although the
sedimentation depresses the basement heat flux by 15-20%, this effect is more
than compensated by radioactive heat production in the sediments, so that the
surface flux is estimated to be higher than that from the basement. The
latter is calculated at about 33-39 mwm- 2 (0.8-0.9 H.F.U.), a relatively low
value. The overall uncertainity is about ± 20-25%, and other estimates on
continental margins with thick sediments (e.g. Reiter and Jessop, 1985)
probably have at least a similar uncertainty.
Keywords:
Earth temperature
;
Sedimentation and deposition
Repository Name:
Woods Hole Open Access Server
Type:
Technical Report
Format:
application/pdf
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