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Chapter 29
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Comparative
Hydrodynamic and
Thermal Characteristics
of Sedimentary Basins and Geothermal Systems in
Sediment-Filled Rift
ValleysWolfgang Polster
H. L. Barnes
The Pennsylvania State
University
University Park, Pennsylvania,
U.S.A.
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ABSTRACT
Detailed geophysical, hydraulic,
and geochemical data were compiled from the literature for sedimentary
basins and for liquid-dominated geothermal fields of sediment-filled rift
valleys. The objective was to use the geo-thermal data as a guide to the
effects to be expected in sedimentary basins from the upflow along growth
faults and other high-permeability zones.
These geothermal reservoirs usually
lie 0.6-0.8 km below the surface. Representative conditions in such reservoirs
are temperatures of 120-370°C, thermal gradients of 15-80°C/km,
salinity of 3-27 g/l, pH of about 4.5-5.5, pressure from hydrostatic to
lithostatic, average porosity of 10-20%, and permeability of 0.1-600 md,
typically. Comparable zones in sedimentary basins with similar thermal,
geochemical, hydrodynamic, and lithological conditions are, for example:
(1) upflow zones along growth faults (e.g., Wilcox trend, northern Gulf
of Mexico Basin, up to 60°C/km) or piercement structures (e.g., Danish
Central Graben, North Sea Basin, up to 50°C/km), (2) upflow zones along
deep, but permeable strata of sedimentary basins (e.g., Alberta basin,
~40°C/km),
and (3) sediments near ancient rift zones (e.g., Gabon basin).
Average bulk permeability and porosity
are typically reduced by hydrothermal upflow in both environments. For
example, the bulk porosity at a depth of 0.6-2.5 km in unaltered sediments
is up to 10% higher than in an adjacent, moderate-temperature (120-200°C)
geothermal reservoir . Apparently, the upflow of hydrothermal fluids into
sedimentary strata can cause significant reduction of porosity and permeability
in a short time (<16,000 yr) even at moderate temperatures and geothermal
gradients (15-60°C/km). Representative bulk porosities of hydrothermally
altered sediments at temperatures exceeding 250°C are about 3-10%,
comparable to the porosity of basins commonly found at depth below about
4-5 km.
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