Page 156 - Physical Principles of Sedimentary Basin Analysis

138 Heat flow

6.10 Heat flow and salt domes
Salt structures are found in a large number of sedimentary basins, for instance the Gulf
of Mexico, the Persian Gulf and the North Sea (Hudec and Jackson, 2007). Large bodies
of salt are important reasons for non-vertical heat flow for at least two reasons – salt has
a heat conductivity that may be as much as six times as high as the heat conductivity of
saturated clay and shales, and the salt bodies often have complex geometries. Table 2.4
gives the heat conductivity of salt as ∼6W m −1 K −1 at surface conditions, while shale may
have a heat conductivity as low as ∼1W m −1 K −1 when the porosity is ∼40%. Although
the heat conductivity of salt decreases with increasing temperature from 6 W m −1 K −1 at
◦
◦
5 Cto4.1 W m −1 K −1 at 100 C according to the data in Table 2.4, it is still consider-
ably more than for shaly rocks. Salt begins as evaporites at the surface, which become
buried as a sheet-like structure. It behaves as a fluid and it can be viewed as a fluid under
lithostatic pressure. Complex salt structures may form as the salt is pressed up as domes
and diapirs by the weight of the overlaying rocks (Hudec and Jackson, 2007, Gemmer
et al., 2004). Figure 6.15a shows an example of a simple salt diapir that has its roots
in a sheet-like structure. We see that the diapir pushes the isotherms away. It becomes
hotter above the diapir and cooler below the diapir, than at similar depths away from
the diapir. It is possible to make a few general comments with regard to heat flow and
salt structures. Firstly, we notice that the salt does not bend the isotherms where the salt
is sheet-like. The isotherms become more separated because the salt has a higher heat
conductivity compared to the surrounding shale, but heat flow is vertical. A salt diapir
seems to affect its thermal environment a distance laterally that is roughly the same as
its height. The heat flow conditions at the base of the specific case of Figure 6.15aare
also affected by both the sandstone and the salt layer. The diapir alters the thermal con-
ditions vertically above and underneath a distance that is also roughly the height of the
diapir.
The hotter conditions above the diapir may lead to an increase in the surface heat flow, if
the diapir is not too deeply buried. Figure 6.15b shows the surface heat flux for the diapir in
Figure 6.15a. The surface heat flux illustrates the point that the diapir alters the isotherms
laterally a distance that is similar to its height. There is a clear peak in the surface heat flux
above the diapir, and the heat flux increases from 42 mW m −2 close to the left boundary
to the maximum 63 mW m −2 above the center of the diapir. It is possible to make a simple
assessment of this increase in the heat flow in terms of the average heat conductivities. This
estimate is based on the observation that the heat flow is nearly vertical at the center of the
diapir. The geotherm through the center of the diapir is compared with the geotherm close
to the left boundary in Figure 6.16. The two geotherms are at x-coordinates x 1 = 0.5km
and x 2 = 4.9 km, respectively, in Figure 6.15. We notice from the figure 6.15a that the
◦
isotherm for 74 C would have passed through the middle of the diapir nearly unbent.
The depth of this isotherm is where the geotherms in Figure 6.16 cross. This depth and
◦
the corresponding temperature are z c = 1.7 km and T c = 74 C, respectively. The heat
¯
flow along the two geotherms can be written as q i = λ i T c /z c , where λ i is the average heat
¯