Heat flow studies of the Exmouth Plateau, offshore North West Australia
Date
1990
Authors
Swift, Michael
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Abstract
The surface heat flow pattern for the Exmouth Plateau region
offshore North West Australia has been defined from 86 new, widely
spaced heat flow measurements using conventional marine heat flow
techniques. The average surface heat flow is 59 mW/m² , which is the
expected level for continental crust. The heat flow frequency has a
Gaussian (bell shaped) distribution about this mean within the range
of 19 mW/m² to 106 mW/m². Overall the areal heat flow variation
is rather smooth, and there is a gradual trend from high heat flow,
in the order of 100 mW/m², from a confined region in the south
east, to average heat flow values on the plateau. It is within this
background heat flow on the plateau that a single region of low heat
flow low is situated. The area of the heat flow low, which has levels
below 40 mW/m² , is about 4000 km² and the centre of the low is
about 230 km from the heat flow high. The confined heat flow high and
the heat flow low are the only major heat flow anomalies (relative to
the average heat flow) for the region. The nature of these anomalies,
how and why they have come into existence and the general
consequences to our understanding of heat transfer in sedimentary
basins is the main stimulus for this thesis. As surface heat flow patterns are an expression of subsurface
thermal processes; i.e. diffusion, convection and internal heat
generation, the surface heat flow pattern can be interpreted in order
to describe the dominant methods of heat transfer in the crust. The
presence of the heat flow low provides the key to interpreting the
regional pattern. As the heat flow level is less than base crustal
level, it indicates that natural (thermally driven) pore fluid
convection together with conduction are probably the main means of
heat transfer in the continental crust within the region. It is
reasonable to assume that the single heat flow low and high are
linked in some way. So a single convection cell, based on the absence of other anomalies, is postulated and thought to be largely confined
to the uppermost 10 kilometres of sediment where there is sufficient
permeability.
There are however two major problems that need to be addressed,
firstly the separation of the surface heat flow low and high is 230
kilometres, consequently the cell has an extremely low aspect ratio
of 0.04. Secondly the system appears to be operating in very low
energy conditions and so what is the cause of the convection?
Confirmation of the existence of a natural convection cell in the
sedimentary section together with the problems of pore fluid
convection in such a low energy environment and low aspect ratio
cells are addressed by numerical modelling. The mathematics for two
dimensional non-linear heat and mass transfer in heterogeneous and
anisotropic porous medium that is applicable to the geological
environment has been developed. The equations governing fluid motion
are coupled to the diffusive and convective heat equation, so
enabling an emphasis on free or natural convection rather than forced
convection. A numerical model, based on Alternate Direction Implicit
method, has been developed around the mathematical formulation to
simulate heat and mass transfer in a porous media. The results of the
modelling support the proposition that natural pore fluid convection
is a dominant process for heat transfer in the Exmouth Plateau
region.
Some independent evidence exists that supports the proposition of
natural pore fluid convection in the region, most notable is the
temperature and vitrinite reflectance data from the many oil
exploration wells drilled on and near Exmouth Plateau. A new thermal
geohistory analysis, based on numerical methods has been developed to
take full advantage of this data. The results from 13 offshore wells
confirm the overall heat flow pattern and lend support to convective flow. However the major support for the postulated pore
fluid convection pattern is derived largely from numerical modelling.
The heat flow data and the results from the numerical modelling
stimulated a study on the necessary and sufficient conditions for
natural pore fluid convection to occur in sedimentary basins. There
exist two essentially independent aspects important in the study: the
first is the driving force, which largely depends on the lateral
variation of rock thermal conductivity, and the second is the
modifying force, which depends on the permeability distribution and
the fluid viscosity. It is concluded that lateral variations in rock
thermal conductivity give rise to horizontal temperature gradients
that are the major driving force of natural convection. By the
intrinsic nature of sedimentary basins, they contain rocks of low
thermal conductivity and high permeability and are surrounded by or
contained within rocks that have a relatively higher thermal
conductivity and lower permeability, together with the temperature
dependency of fluid viscosity, natural convection is probably an
active process in most sedimentary environments.
Discussion on the consequences and application of these findings
is focused on hydrocarbon maturation, migration and trapping.
Convection usually gives rise to areas of anomalously high or low
temperature within the sedimentary section and this has direct
consequences to hydrocarbon maturation. Furthermore as the pore
fluids are moving, horizontal path lengths may be significant, so the
study also has application to hydrocarbon migration. As the fluid is
moving it will alter the hydrostatic pressure field to a hydrodynamic
pressure field. Hydrodynamic hydrocarbon traps may be generated as a
consequence of pore fluid flow.
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