The Role of Fluids in Crustal Processes (1990) / Chapter Skim
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Overview and Recommendations
Overview and Recommendations
Pages 1-26
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From page 3... ...
INTRODUCTION Fluids play a vital role in virtually all crustal processes. Circulation of fluids has important effects on the transport of chemical constituents and heat, and is the principal control on the formation of hydrothermal ore deposits.
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From page 4... ...
Present day surface exposures indicate that, at all crustal levels, fluids have been present. Field, isotopic, fluid inclusion, and phase equilibria studies of rocks and mineralized fractures from shallow crustal levels, vein and pegmatitic masses from intermediate levels, and gneissic units from deeper crustal levels, document that fluids were present in significant volumes.
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From page 5... ...
Indicator Water table Deep wells Reservoir induced seismicity Crustal low velocity zones Crustal electrical conductivity zones Oxygen isotopes Metamorphism Crack healing and sealing Formation of hydrothermal ore deposits Crustal seismic attenuation zones Low stress on faults Silicic volcanism Fluid inclusions Depth Range 0 to 2 km to 12 km to 12km 7 to 12 km 10 to 12 km to 12 km to >20 km .
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From page 6... ...
However, even if the rocks were initially impermeable, the pore-fluid pressure generated by heat dispersed into the host rocks is high enough to create hydraulic fractures. Thus, the system can generate its own permeability by making fractures.
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From page 7... ...
Parameters Fluid velocity Hydraulic conductivity distribution Porosity Elastic constants Sources and sinks of fluid Mass or Chemical Fluid velocity distribution Composition of Transport (steady Chemical boundary conditions fluids and rocks flow and isothermal) Parameters Porosity Dispersivity Chemical Reactions Identify Kinetics Heat Transport Fluid velocity distribution Temperature (steady flow and Heat boundary conditions no change in fluid Parameters chemistry)
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From page 8... ...
Boundary Conditions Topography xxxx Heat Sources xxxxxx Tectonic Effects (rock mechanics) xx xx xx Sources and Sinks - xxx xxx Parameter Distribution Permeability Porous media xxxxxx xxx Fracture media xxxxx xxx Porosity xxxx xx Fluid Density x x Viscosity xx x Chemical Reactions Identification xxxxxx Kinetics xxxxx Dispersivity Mass xxx x Heat xx x Thermal Conductivity xxx x Elastic Constants xxx Plastic Behavior xxxxx Note: Also shown is the type of investigations needed relative to the field or laboratory.
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From page 9... ...
The extent to which porous media versus channelized flow occurs has yet to be determined. Mineral-Fluid Reactions The advection of chemical components by fluid flow and dispersive fluxes from one chemical environment to another causes chemical reactions between the minerals and fluids.
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From page 10... ...
To fully understand the role of fluids in crustal processes, it will be necessary to unravel the complex coupling between thermal, chemical, mechanical, and hydrological processes. Each of the processes, properties, and driving forces that are currently included in the basic theory is the consequence of existing experimental data, theoretical analysis, and computer simulation.
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From page 11... ...
Toth (1963) pointed out that the water table is the upper boundary for saturated groundwater flow, and that this boundary is usually closely approximated by the land surface.
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From page 12... ...
The high fluid pressures accompanying this topographically driven flow later facilitated the development of first-order structures in the Valley and Ridge Province. Subsequent joint sets, which are not correlated across the Allegheny Front, are likely to be a consequence of fluid pressure pulses developed during local tectonic compaction.
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From page 13... ...
There is, however, a wide range of direct and indirect observations that indicates fluid pressures can build up locally to approach the lithostatic pressure in a variety of geological environments. To further complicate the problem, laboratory experiments and geological evidence suggest that rocks in situ rapidly seal hydraulically.
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From page 14... ...
In addition, there may be a thermal generation of high pore pressure caused by high thermal gradients. In contrast to the Gulf Coast Basin, the Caspian Basin is subsiding at a rate of 1,100 m per million years and has a maximum thickness of accumulated sediments of 25 km.
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From page 15... ...
The extent to which similar systems can exist at intermediate and deep crustal levels has yet to be established. Since most hydrothermal systems occur in the vicinity of cooling magma bodies, will similar fluid flow systems develop when magmas are trapped deeper in the crust?
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From page 16... ...
Geothermal reservoir engineering today involves numerical simulation of the reservoir's performance, which requires that the coupled partial differential equations be solved for the boundary conditions of interest. Source of Fluids What are the sources of fluid in the crust?
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From page 17... ...
4. A greater understanding is needed of fluid flow through rocks of low permeability, for delineating the history of flow through sedimentary basins and for questions of the disposal of hazardous materials in the shallow crust.
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From page 18... ...
As Table 3 indicates, a large set of data must be assembled and evaluated quantitatively if one is to apply the theory rigorously. Commonly there is a lack of data to understand a wide range of fluid flow problems; some critical parameters must be evaluated by analogy to other areas where they are known.
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From page 19... ...
In these areas one can observe heat and mass transport properties directly along with concurrent chemical reactions. Much work has gone into modeling heat and fluid (including steam)
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From page 20... ...
This is especially true when one looks at rocks of low permeability, the "confining layers" for the hydrologist, the "cap rock" of reservoir engineering. One major phenomenon that occurs in transport of chemical constituents in a porous medium is hydrodynamic dispersion, a process of physical mixing of the fluid through the matrix.
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From page 21... ...
. Indirect methods suggest that many of the tectonically active areas may have pore pressures approaching lithostatic.
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From page 22... ...
There are, however, problems of interest that tax even the largest computers. The approach to mass and heat transport in the geosciences has adopted methods of nonequilibrium thermodynamics developed for chemical engineering; this approach, outlined in Table 2, involves writing a separate equation for fluid pressure and temperature and for each chemical constituent of interest.
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From page 23... ...
On the maintenance of anomalous fluid pressure, I Thick sedimentary sequences, Geological Society of America Bulletin 79, 1097-1106.
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