Page 185 - The Petroleum System From Source to Trap

9. Overburden Rock, Temperature, and Heat Flow 177

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Figure 9.1 0 . Effect o f vertical groundwater movement on conductive heat flow.

must also increase. Heat flow is still depressed compared the bottom of the layer, and
to the initial state of no sedimentation, but the magnitude
of the depression is smaller. s = k lpwCwv (11)

Groundwater Flow where k is the bulk thermal conductivity of the
fluid-rock aggregate, Pw and Cw are the density and heat
Groundwater flow has the potential to be an effective capacity, respectively, of the fluid moving with Darcy
agent for redistributing heat in sedientary basins. The velocity v, and v is negative for downward flow.
heat capacity of water (-4200 J/kg K) is more than four Assuming typical values of k = 2.5 W /m K, Pw = 1000
times as high as the average matrix component of sedi­ kg/ m3, and Cw = 4200 J /kg K, the reduction of surface
mentary rocks ( -1000 J /kg K). Significant perturbations heat flow (and geothermal gradient) can be calculated as
to the background thermal regime can be obtained in the a function of groundwater velocity and depth of circula­
presence of Darcy velocities as low as a millimeter per tion (Figure 9.10). For downward percolation through
year, depending upon the depth of circulation (see later 1000 m at a Darcy velocity of 1 cm/yr, the conductive
discussion). surface heat flow (and geothermal gradient) is reduced
Vertical fluid movement is usually required to perturb by 41 % (0.59 on Figure 9.10). If the extent of fluid
the thermal regime. Little or no heat is transported by the movement reaches a depth of 5 krn, then the conductive
horizontal movement of groundwater because isotherms heat flow at the surface is reduced by 23% (0.77) even if
are almost always parallel to the ground surface. The the Darcy velocity is only 1 mm/yr. As a consequence of
extent to which heat flow (or the geothermal gradient) is the relatively low fluid velocities needed to appreciably
enhanced or reduced by upward or downward perturb the thermal regime, hydrologic disturbances are
movement of groundwater depends on the Darcy (volu­ possible even in basins composed primarily of aquitards
metric) velocity and depth of fluid circulation. Lachen­ (permeability of -l0-14-l0-17 m2). High-permeability
bruch and Sass (1977) have shown that under steady­ (> l0-14 m2) aquifers and conspicuous signs of under­
state conditions, ground flow (e.g., artesian wells) are not a prerequisite.
In areas of high relief and rugged topography, the
(10) presence of groundwater flow is nearly ubiquitous,
making it difficult to obtain accurate estimates of back­
where q(z ) is the conductive heat flow at the top of a ground thermal conditions in these locations.
1
layer of thickness Az, q(z2) is the conductive heat flow at Groundwater moves (1) in response to potential