The Saturation Level of Methane Hydrate in Natural Sediments
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Saturation (graph theory)
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Carbonate minerals
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Surface chemical reactions profoundly influence the distribution of magnesium between the solid and dissolved phases in the sediment-pore water system. These reactions, in addition to bulk reactions such as mineral precipitation, replacement and alteration of basalts, illustrate the diverse geochemical behavior of magnesium. A radiotracer technique using {sup 27}Mg was developed in order to determine the empirical parameters that govern the surface chemical reactions of magnesium in seawater: notably, adsorption and ion-exchange equilibria. This new analytical technique eliminates the experimental problems commonly associated with high Mg-background of any seawater matrix. The free Mg-ion concentration in pore fluids of hemipelagic anoxic environments decreases significantly, mainly due to the formation of Mg{center dot}CO{sub 3} complexes resulting from high levels of total dissolved carbon dioxide. An important consequence of this lowering of the Mg-ion activity is the re-equilibration of the Mg-containing solid surfaces with the dissolved species, resulting in a desorption of Mg. High levels of dissolved ammonium ions also displace Mg from mineral surfaces by ion exchange. A multi-component/multi-reaction model simulates all these interrelated surface-solution reactions in a complex mixture of natural sediments and pore-fluids. This model includes the equilibrium conditions for complex formation, adsorption-desorption, and Mg-NH{sub 4} ion-exchange reactions. It predictsmore » the Mg-behavior during diagenesis by allowing the composition of the solution matrix (pore water), as well as the CEC of the solid phases (sediment) to vary.« less
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Microbial methane is generated in rapidly accumulating marine sediments (>40 m/my) where pore waters are deficient in dissolved oxygen and sulfate. Based on indirect geochemical evidence, microbial methane generation is largely confined to depths of between 10 and 1000 meters beneath the sea floor. Under shelf conditions (water depth <200 m), methane concentrations can exceed solubility in pore water and accumulate as free gas, or escape the sediment as bubbles, or be oxidized in surface sediments. Under some deeper-water conditions of continental slope and rise sediments, more of the methane can be retained and buried because of increased solubility, and because methane in excess of solubility can be stabilized as methane hydrate. Few direct measurements of methane concentration in subsurface pore waters have been made. However, methane-water phase transitions (gas-water contacts, base of gas hydrate reflector) on seismic records can be used with methane solubility relationships to estimate gas contents of sediments. Comparison of various environments shows a relatively narrow range of dissolved methane contents. In marine sediments, free gas (and methane hydrate) is stable only in contact with methane-saturated pore water. Finer-grained sediments can be supersaturated with respect to a gas (and gas hydrate ) phase because of capillarymore » pressure inhibition of bubble (or hydrate ) formation. The amount of methane dissolved in marine sediment pore water is necessarily larger than that present as gas hydrate.« less
Capillary pressure
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Clathrate hydrate
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Clathrate hydrate
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Chrysotile
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Wellbore
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The porosity of clastic sediments at deposition varies very approximately between about 45% (sands) and 85% (muds). With burial, consolidation takes place as pore water is progressively eliminated. It would be misleading, however, to attribute alterations in sediment bulk properties to physical processes alone. Very significant mineralogical changes occur and these start soon after burial, especially in mudrocks. Striking heterogeneities such as thin, laterally continuous cemented horizons or discrete concretions are commonly introduced. These shallow burial processes are predominently the result of microbial activity. Thermodynamically unstable mixtures of organic matter and various oxidants [dissolved oxygen, sulphate, nitrate, particulate Fe(III) and Mn(IV)] provide both substrate and energy source for a variety of different microbial ecosystems. Mineralogical consequences include both leaching and the precipitation of carbonate, sulphide, phosphate and silica cements. The type and extent of mineral modification depends strongly on depositional environment variables such as rate of sedimentation and water composition.At greater depths, large scale modification of detrital clay minerals (particularly the smectite‐I/S‐illite transformation) takes place. Recent work of various kinds, however, has demonstrated that these changes may not be solid state transformations: clay mineral dissolution, transport and precipitation occur much more widely than was formerly supposed. In sandstones, authigenic precipitation of clay minerals from pore solution is much more obviouis. Systematic patterns of precipitation, alteration and replacement have been documented in many sedimentary basins. Porosity and permeability are reduced by cementation and, sometimes, enhanced by mineral dissolution. Whereas the general nature of these chemical reactions is fairly well understood, it is not yet possible to predict with certainty the scale or distribution of mineralogical consequences. Much debate, for example, surrounds the mechanism of porosity enhancement in sandstones. More information is needed about amounts and rates of porewater migration at different stages of compaction and the mobility of chemical solutes in the deep subsurface. What is certain is that almost all clastic sediments encountered during deep drilling will have been modified very substantially by chemical processes during burial. Texturial characteristics such as grain size and shape, fabric and packing will have been altered in consequence.
Authigenic
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