Distribution and Origin of Organic Ligands in Subsurface Waters from Sedimentary Basins
Yousif K. Kharaka, Thomas H. Giordano, Paul D. Lundegard, 1997. "Distribution and Origin of Organic Ligands in Subsurface Waters from Sedimentary Basins", Ore Genesis and Exploration: The Roles of Organic Matter
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Since the widespread occurrence of low-molecular-weight organic ligands in subsurface waters of sedimentary basins began to be recognized about 20 years ago (Willey et al., 1975; Carothers and Kharaka, 1978), their origin, distribution, and significance has become an intensively studied field in geochemistry (Gautier et al., 1985, Gautier, 1986; Pittman and Lewan, 1994). Acetate and propionate (for the International Union of Pure and Applied Chemistry (IUPAC) names and chemical structure, see Appendix II of this volume) are generally the most abundant organic ligands reported in oil-field waters, although a variety of additional monocarboxylic, dicarboxylic, and other reactive organic species are present (Germanov and Mel'kanovitskaya, 1975; Willey et al., 1975; Carothers and Kharaka, 1978; Workman and Hanor, 1985; Hanor and Workman, 1986; Kharaka et al., 1986; Fisher, 1987; Means and Hubbard, 1987; Fisher and Boles, 1990; Lundegard and Kharaka, 1990, 1994; Lundegard and Trevena, 1990; MacGowan and Surdam, 1990a; Giordano and Kharaka, 1994). The highest concentrations (close to 10,000 mg/L) are present in formation waters obtained from relatively young (Tertiary-age) reservoir rocks at temperatures of approximately 80° to 140°C (Carothers and Kharaka, 1978; Lundegard and Kharaka, 1994).
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The assooatwn of organic matter with ore minerals, gangue, and host rock in many low-temperature ( 120C) o moderate-temperature (120-350C) ore deposits is a well-known phenomenon (Saxby, 1976; Leventhal, 1986; Parnell et al., 1993; Giordano, 1996; Gize, 1999) and was recognized early in the twentieth century (Siebenthal, 1915; Harder, 1919; Schneiderholm, 1923; Bastin, 1926; Fowler, 1933). The study of organic constituents in ores, particularly if coupled with studies of other ore components and conditions, can provide much information on both active and passive roles of organic matter before,during, and after ore genesis, and in some cases can leadto the development of valuable exploration techniques.By the 1950s, it was recognized that biological sequesteringof metals, sulfide production by sulfate-reducing bacteria,biological precipitation of metals, sorption of metals byorganic colloidal particles, modification of geochemicalenvironments by organic processes, and the mobilizationof metals by metal-organic complexes were all potentiallyimportant roles played by organic matter in the concentrationof metals to form metalliferous shales and certaintypes of ore deposits (Berger, 1950; Krauskopf, 1955). Bythe 1960s, it was recognized that dead organic matter(organic matter not in living organisms) may be a powerfulreducing agent for sulfate and thus may provide asource of sulfide for ore-forming systems (Barton, 1967;Skinner, 1967). Roedder (1967) reported the presence ofhydrocarbons and sulfate in fluid inclusions from oredeposits. This observation was cited by Barton (1967) asstrong evidence that organic matter was present at thetime of ore formation and that thermodynamic equilibrium(which predicts hydrogen sulfide and carbon dioxide)was not attained in the ore fluid because of sluggishkinetics at the low temperature of ore formation. Hoering(1967) summarized his pioneering work on organic matterassociated with gold and uranium in the Carbon LeaderFormation of the Witwatersrand district, South Mrica.Because it was relatively immature Precambrian organicmatter (rather than graphite), it was suitable for analysis ofsimple and complex chemical compounds and led the wayfor future studies of organic matter in ore deposits andPrecambrian rocks (Leventhal et al., 1975).It was not until the 1970s and early 1980s that majorefforts on a worldwide scale were initiated to study theroles of organic matter in ore genesis (Breger, 1974; Leventhalet al. 1975; Connan and Orgeval, 1976; Saxby,1976; Giordano, 1978; Connan, 1979; Estep eta!., 1980).As a consequence of this major