Solid-Earth Sciences and Society (1993) / Chapter Skim
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4 Resources of the Solid Earth
4 Resources of the Solid Earth
Pages 137-184
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From page 137... ...
The resources required for a complex civilization are more diverse than those needed by simpler organisms. Besides the obvious needs for primary materials- chemical and energy sources shared by all living beings, humans depend on physical and biological systems that process primary resources.
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From page 138... ...
As market values change, managers must decide whether a particular resource is economic. if it is economic, it is worth the effort to discover and extract deposits.
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From page 139... ...
Recovery of petroleum resources also can threaten the environment. Oil spills come to mind the Exxon Valdez spill and the of!
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From page 140... ...
This information supports the opinion that extensive fractures and large volumes of aqueous fluids permeate the deep crust. Researchers testing the behavior of crustal rock SOLID-EARTH SCIENCES AND SOCIETY ,//~ ~ \ \ i~./~ - ~ ~ , WE / i sopleth va I ues in cm~ kl lometers l FIGURE 4.1 Vein/joint abundance map of the Sierrita system, Arizona The total number of fractures per centimeter increase as the pluton (patterned area)
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From page 141... ...
While seismologists analyze the influence of fluids on fault susceptibility, research continues to reveal how aqueous fluids move in subduction zones and return to the surface through volcanic eruptions. Aqueous fluids also cycle through crustal material at ocean spreading centers, spewing from vents loaded with particulate matter as "black smokers." On continents such fluids bubble to the surface as geothermal springs and geysers.
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From page 142... ...
And only trace amounts, relative to the oceans' vast capacity, cycle through the atmosphere as vapor and through the crust as aqueous fluids. Water circulates among these reservoirs in a system known as the hydrologic cycle, and it abides SOLID-EARTH SCIENCES AND SOCIETY SPACE Be_ ~_q ~\ \ \ At,.
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From page 143... ...
. ~ ~ ~ ~ Sudace Waler _~ 143 Hater ~16~ ~tcr circulates tbrou~b the hydrologic cycle, dissolving claimants, sometimes carrying them over Scat d~tanccs, and cvcntu~Hy deposhing them into any.
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From page 144... ...
From New, 1984, Groundwater Contamination ter dissolves salts from soil, carries them to the ocean, and then evaporates, leaving the collected residue to accumulate. Moving through the atmosphere, water arrives again over the land surface to replenish the puddles, rivulets, lakes with fresh water.
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From page 145... ...
Water samples from these stations are chemically analyzed for the major anions and cations, while some are tested irregularly for traces of metals and organic materials. To assess the state of the nation's water quality, the Environmental Protection Agency (EPA)
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From page 146... ...
And if vast quantities of water are mined from a 100,000-year-old aquifer and run through an irrigation system, resulting in extensive evapotranspiration, the aquifer may not be replenished for another few hundred thousand years. Because the hydrologic cycle transfers water from one reservoir to another at various rates and because it does not always transfer to the location most convenient for the schemes of civilized minds, humans must use available water resources very carefully.
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From page 147... ...
In the conterminous United States, the 17 western states consume 84 percent of the country's fresh water, mostly for agriculture. The acreage irrigated in the 17 western states is shown in Figure 4.6.
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From page 148... ...
Arizona has also implemented a new law to eliminate groundwater mining by the year 2020. California has eliminated relative depletion in the Central Valley by increasing surface water supplies.
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From page 149... ...
The geoscientist s role is to understand and synthesize the multiple factors involved in the maintenance of adequate water resources so that policy makers-regionally, nationally, and internationally can reach informed decisions. The demands of that role represent a remarkable opportunity for newly integrated studies of the hydrologic cycle and particularly for the application of new observational, analytical, and modeling approaches to understanding the groundwater system.
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From page 150... ...
Lead is just one example of crustal element occurrence that can generate a bell-shaped distribution curve. Such distribution patterns indicate the challenge of mineral exploration, which is to discover the small amounts of concentrated materials constituting ore bodies.
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From page 151... ...
Thus, molybdenum and tin are examples of mineral resources in which suppliers with no rivals suddenly found themselves downgraded or displaced by more economically competitive sources. But new deposits are not the only reasons for change.
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From page 152... ...
~7 Understanding Mineral Deposits Ore deposits can be subdivided into two major classes, epigenetic and syngenetic. The former involve addition of minerals to existing rock; the latter are mineral concentrations that form when the host rock forms.
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From page 153... ...
The application of these four kinds of advances to problems in mineral exploration and exploitation has provided a number of deposit models, which represent application of new ideas to a wealth of new data. Crustal Processes and Ore Deposits Plate tectonic concepts have been applied to mineral exploration as associations between tectonic environment and certain types of mineral deposits have become clear.
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From page 154... ...
. Recent developments that have contributed to understanding this process include the discovery of brines rich with zinc and lead in deep petroleum exploration wells drilled throughout the region, recognition that metal-bearing brines have permeated carbonate rocks extensively through the midcontinent regions, and advanced modeling of fluid flow in compacted sedimentary basins.
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From page 155... ...
The role of organic compounds in the formation of ore deposits serves as an outstanding example of how studies of mineral deposits are enhanced by other sciences. Organic matter can mobilize or fix metals directly; it can also control the oxidation or hydrolysis state of the system.
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From page 156... ...
For example, the thermal maturation of organic matter can provide a guide to related ore-forming thermal events. In addition, living or fossil species provide clues to the environment prevailing at sites of ore deposition.
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From page 157... ...
Thus, nonconventional ore deposits continue to be found, and prospects continue for discovering additional unique mineral concentrations. Such ore bodies provide a stimulus for continually revising the conceptual models of mineral deposits.
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From page 158... ...
Mineral Exploration and Exploitation Mineral deposits display great diversity in material; grade; size; and style of localization, accessibility, and minability. These largely independent variables complicate the search for profitable mineral concentrations.
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From page 159... ...
Data plotted from the known Kuroko ore bodies show that the median size of a Kuroko ore body is 1.5 million tons, at an average grade of 1.3 percent copper (Figure 4.11~. The tabulation also predicts that few Kuroko ore bodies can be expected to exceed 18 million tons or to contain grades of more than 3.5 percent copper.
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From page 160... ...
An examination of rocks for evidence of a former thermal event may prove useful in locating buried mineralizing systems. In mineral exploration, genetic concepts have been augmented by empirical methods that observe what has been overlooked at the surface, seek clues about deposits that are only thinly veiled, and attempt to detect the presence of deeply buried deposits having no surface expression.
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From page 161... ...
At the close of the twentieth century subsurface and surface mining are providing the bulk of needed mineral resources, but researchers are looking for breakthroughs with in situ mining techniques in the future. In situ mining is analogous to the hydrothermal mineralization process but works in reverse.
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From page 162... ...
However, in most situations, they are not yet economically competitive with fossil fuels. Fossil fuels are our main energy source.
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From page 163... ...
Petroleum Resources Significant accumulations of oil and gas are geological anomalies, and they constitute only a minute fraction of the total material within the sedimentary basins where they are found. These two factors create the scientific and technical complexity of the geoscientist's role in petroleum recovery and of the industry itself.
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From page 164... ...
Origin of Petroleum Within Sedimentary Basins Significant petroleum resources result from series of events that fall into four categories: genesis of oil and gas from preserved organic debris within a source rock, primary and secondary migration of the generated hydrocarbons over distance, entry of the hydrocarbons into a properly prepared reservoir, and finally entrapment of the hydrocarbons (Figure 4.14~. All of these events occur within the confines of a specific basin.
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From page 165... ...
In this way further subsidence is induced; the fill in sedimentary basins may become as thick as 15 km. Petroleum source rocks are accumulations that are rich in organic matter.
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From page 166... ...
In the former circumstance, the first sediments deposited in the basin are likely to mature rapidly through the oil-generation phases, but in the latter circumstance an initially cold pile of sediments gradually warms, with oil maturation taking place later in the basin's evolution. Sedimentary basins formed at continental ruptures rifts where oceans have failed to open and Atlantic-type margins where oceans have successfully formed are likely to contain petroleum source rocks that attained maturation within a few tens of million of years of deposition.
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From page 167... ...
Oil and gas stop moving when they reach a capillary pressure equilibrium or when they are trapped by capillary seals that reduce or prevent further migration. The Alberta Basin, including its southward extension into Montana, provides an example of how fluid flow has evolved as a sedimentary basin developed (Figure 4.15~.
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From page 168... ...
Mathematical modeling of fluid flow in sedimentary basins represents an important complement to the study of diagenesis. The tradition of modeling began decades ago in groundwater research, when simple electrical conductivity analogs of water flow were successfully developed.
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From page 169... ...
Examples of these are oil shales that are exposed source rocks and tar sands that are fossil compounds altered by exposure to near-surface agents. Some shales contain exceptionally large amounts of kerogenic organic matter, and for more than a century small quantities of oil have been distilled from oil shales in various parts of the world.
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From page 170... ...
Some scientists have related these oscillatory variations to global sea-level change, but others suggest that vertical motions of the crust, variations in sediment supply, and climatic fluctuations are just as important. The importance of sequence stratigraphy to this controversy is that the formation of sequence boundaries requires changes of relative sea level, not merely of sediment input.
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From page 171... ...
By piecing together information from a grid of intersecting seismic profiles and incorporating data from wells and boreholes, stratigraphers can often interpret the three-dimensional layered geometry in the sedimentary basins. Recognition of regional unconformities is especially significant because rocks above an unconformity are everywhere younger than rocks below the unconformity.
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From page 172... ...
The teams are charged with characterizing the established fields and reservoirs and then using the information to manage further development drilling and the application of enhanced recovery techniques. In addition, such teams explore for untapped reserves that become economically viable because of their proximity to existing .
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From page 173... ...
Studies are developing more sophisticated models for future investigations. Detailed outcrop and subsurface studies of individual coal basins are similar to the basin analysis methods used in the petroleum industry; they aim at defining depositional environments and subsequent coalification processes for model building.
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From page 174... ...
Some of these processes are available commercially, while others are in various stages of laboratory or pilot plant testing. Some coal beds contain large quantities of methane gas, which can be produced directly.
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From page 175... ...
The effects of such elements from fluidized-bed combustion wastes on groundwater quality is not known. Other research should focus on the technical viability of disposing of scrubber sludge wastes in abandoned or existing underground mines, considering the potential effects on groundwater and surface subsidence.
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From page 176... ...
Anaerobic sulfate-reducing bacteria may have been important in precipitating uranium minerals at the reduced surface (Figure 416) The transport and deposition of uranium present yet another example of the importance of fluid flow through porous media in geologically significant environments Research related to uranium deposits is, in many ways, similar to the basin analysis that dominates research in the petroleum industry The nuclear power industry faces a continual challenge in seeking options to dispose of waste materials This costly need is another reason for the decline in the use of uranium as a means of electric power generation in this country Geoscientists are active in research related to this problem, since the currently preferred disposal option is to deposit radioactive waste materials beneath the surface The possible interaction of fluids with buried radioactive waste and the subsequent migration of those fluids through porous rocks to the biosphere must be projected over the long intervals during which hazardous material can remain radioactive.
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From page 177... ...
, geochemical, and geophysical methods are being developed and tested for this purpose. Other uses of geothermal energy have been developed.
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From page 178... ...
The comprehen The material carried out of drainage basins is deposited in the sedimentary basins of the world. It is from these sedimentary basins both active basins currently being filled and ancient basins that were filled long ago that the world's oil, gas, coal, and groundwater, together with many of its mineral deposits, including its uranium deposits, are produced.
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From page 179... ...
Sustain Sufficient Resources Water, Minerals, Fuels C Evolution of mineral deposits through time `>al petrology and quality · Most of the opportunities listed under III below relate to integral parts of biogeochemical cycles · Organic chemistry and the origin of petroleum · Kinetics of water-rock interaction · Analysis of drainage basins · Water quality and groundwater contamination · Modeling water flow and hydrocarbon migration · Source-transport-accumulation models · Numerical modeling of depositional environments · In situ mineral resource extraction · Crustal fluids · Sedimentary basin analysis · Surface and soil isotopic ages · Prediction of mineral resource occurrences · Concelaed ore bodies · Intermediate scale search for ore bodies · Exploration for new petroleum reserves · Advanced production and enhanced recovery methods · Coal availability and accessibility · Concealed geothermal fields _ D II. Global Geochemical and Biogeochemical Cycles III.
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From page 180... ...
Numerical Modeling of the Depositionat Environment of Ore Bodies Modern computers have recently made possible numerical modeling of the complex hydrologic, geological, and geochemical systems related to ore formation. For the first time, drawing on reasonably comprehensive thermodynamic/kinetic data bases for water-rock interactions and fluid flow models, it is possible to develop integrated models that reasonably reflect natural systems for flow through media with irregular fractures, accommodating conditions under which liquids boil and allowing fluid compositions to change with time.
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From page 181... ...
Energy Resources Exploration for New Petroleum Reserves In much of the world where the petroleum industry is less mature than in the United States, the challenge lies in exploration for new reserves in new oil fields, calling for an integrated approach to basin analysis that ranges from establishment of the tectonic environments in which basins have developed; through interpretation of depositional, structural, and thermal evolution; to an understanding of oil and gas generation within the basin, of fluid migra tion, and of related diagenetic change; and finally to definition of the types of traps in which oil and gas are held. Advanced Production and Enl?
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From page 182... ...
The availability of three-dimensional seismic data will contribute greatly to our understanding of the shallow crust and will call for complementary advanced well logging and well-to-well measure ments. Mineral explorationists use advanced geophysical research techniques, including audiomagnetotelluric methods, and remote sensing from space and aircraft by methods such as side-looking radar and high-wavelength-resolution infrared spectroscopy are beginning to be used.
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From page 183... ...
For example, hydrologists commonly need good digital topographic data. They also need geological maps of both surficial and bedrock distribution and may need data on water quality and quantity for both surface water and groundwater over an area of interest.
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From page 184... ...
NRC (1990~. A Review of the USGS National Water Quality Assessment Pilot Program, Water Science and Technology Board, National Research Council, National Academy Press, Washington, D.C., 153 pp.
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