Geology pressure solution

Elias B. P. and Hajash A., Jr. (1992) Changes in quartz solubility and porosity due to effective stress an experimental investigation of pressure solution. Geology 20, 451-454. 

Wood J. R. (1981) Fracture pressure solution experiment and theory. Geological Association of Canada, Mineralogical Association of Canada Meeting, Abstracts of papers, A63. 

Dewers, T. and Ortoleva, P. (1991) Influences of clay-minerals on sandstone cementation and pressure solution. Geology 19, 1045-1048. 

De Boer, R.B. (1974) Thermodynamical and experimental aspects of pressure solution. In Proceeding of International Symposium on Water-Rock Interactions. (Eds Cadek, J. Paces, T.), pp. 381-387. Geological Survey, Prague. 

A common approach in geology is to analyse processes as a function of temperature and pressure. However, many long-lived geological processes, in particular the dia-genetic processes in sedimentary rocks, develop under moderate temperatures and pressures. As is known, the concept of silica pressure solution in sandstones is shared by many authors (Heald and Renton 1966 Renton et al. 1969 Houseknecht 1984,1988 McBride 1989 Sibley and Blatt 1976 Williams et al. 1985a). Fundamental to supporters of this concept are the laboratory experiments by Fairbairn (1954) in which quartz sand compressively loaded and heated transforms to quartzite. 

A study of pressure solution undertaken to show which mechanism really controls the porosity has three aims (1) to evaluate quantitatively the spatial distribution of pressure solution and quartz cementation on a regional scale in the sandstones, (2) to evaluate the influence of certain geological variables like median grain size, composition of clay binders, temperature, etc. on pressure solution and (3) to evaluate its role in quartz cementation during evolution of the sandstone porosity. 

Pressure solution and quartz cement to a great extent depend on a number of geological variables and in particular on grain size of the quartz and temperature. The influence of other parameters like grain sorting, clay cement content, composition of framework grains, presence of early cement and burial history of the reservoirs also have to be considered. The volume of quartz dissolved by pressure solution denotated as OQ or overlay quartz and the volume of quartz cement (QC) are expressed as a percentage of the overall volume of the detrital quartz (DQ) (Fig. 4.11). In this context, the utilization of the same approach by Houseknecht (1984), i.e. of the ratios OQ/DQ and QC/DQ, excludes in the sandstones considered any influence of the variability of the volume of detrital quartz on the volume of dissolved quartz and quartz cement. A quantitative evaluation of quartz cement and pressure solution was carried out on the basis of a study of thin sections under the optical microscope and with the aid of cathodoluminescence. 

Quantitative petrophysical and petrographic analyses with the aid of cathodolumines-cence and SEM studies have shown that in the Saharan sandstones pressure solution has been influenced by a number of geological variables. In the complexes studied there is a linear correlation between median grain size and pressure solution by which finer-grained sandstones suffer more pressure solution. The presence of stronger clay films on the quartz grains also favours pressure solution. Sorting and variations in the compaction of the matrix of the sandstones exert little influence on the process of pressure solution. The presence of early cement dominates pressure solution and con-... 

Smirnov YB (1980) Heat flow in USSR remarks to the heat flow and deep temperatures maps in the scale 1 10000000 (in Russian), Moscow, GUGK, p 150 Sprunt ES, Nur A (1976) Reduction of porosity by pressure solution experimental verification. Geology 4 463-6... 

Sprunt ES, Nur A (1977) Destruction of porosity through pressure solution. Geophysics 42 726-741 Sprunt ES, Dengler LA, Sloan D (1978) Effects of metamorphism on quartz cathodoluminescence Geology 6 305-308... 

Trurnit P (1968) Pressure solution phenomena in detrital rocks. Sed Geol 2 89-114 Turcotte DL, Schubert G (1982) Geodynamics application of continuum physics to geological problems. Vol. iWiley, New York, 376 p... 

In the United States, Hquid HLW from the reprocessing of defense program fuels was concentrated, neutralized with NaOH, and stored in underground, mild steel tanks pending soHdification and geologic disposal (see Tanks AND PRESSURE VESSELS). These wastes are a complex and chemically active slurry. Suspended in the supernatant Hquid are dissolver soHds which never went into solution, insoluble reaction products which formed in the tank, and salts which have exceeded their solubiHty limit. The kinetics of many of the reactions taking place are slow (years) so that the results of characterization... 

The fluid pressure in the rock at the bottom of a well is commonly defined as pore pressure (also called formation pressure, or reservoir pressure). Depending on the maturity of the sedimentary basin, the pore pressure will reflect geologic column overburden that may include a portion of the rock particle weight (i.e., immature basins), or a simple hydrostatic column of fluid (i.e., mature basins). The pore pressure and therefore its gradient can be obtained from well log data as wells are drilled. These pore pressure data are fundamental for the solution of engineering problems in drilling, well completions, production, and reservoir engineering. 

America s roads more than tripled, General Motors overtook Ford as the world s largest carmaker. And as engineers improved the efficiency of internal combustion engines, the need for more efficient fuel became even more pressing. Adding to the car industry s pressure for a solution to the fuel problem, the U.S. Geological Survey announced—mistakenly—that world petroleum deposits would be depleted in a few years. 

In general, the formulation of the problem of vapor-liquid equilibria in these systems is not difficult. One has the mass balances, dissociation equilibria in the solution, the equation of electroneutrality and the expressions for the vapor-liquid equilibrium of each molecular species (equality of activities). The result is a system of non-linear equations which must be solved. The main thermodynamic problem is the relation of the activities of the species to be measurable properties, such as pressure and composition. In order to do this a model is needed and the parameters in the model are usually obtained from experimental data on the mixtures involved. Calculations of this type are well-known in geological systems O) where the vapor-liquid equilibria are usually neglected. 

If quartz is the stable phase at low temperatures, other forms of silica are nevertheless found in nature. It is apparently difficult to form quartz in the laboratory as well as in nature at low pressure and temperature conditions. The question can be asked, "at what concentration will a solution be saturated with silica " It would appear that laboratory and theory do not give a simple answer. Let us then look at forms of silica found in different geologic environments. 

It is most likely then that the effective (although metastable) SiO equilibria in most geological environments of low temperature and pressure, weathering, sedimentation and the early stages of compaction as well as surface hydrothermal alterations, are governed by the solubility and precipitation of amorphous silica in aqueous solution. As a result, the existence of quartz in an assemblage of clay minerals in these environments does not necessarily represent a compositional limit or saturation with respect to SiC and, therefore, such an assemblage cannot be considered, a priori, as a system with silica as an effective component in excess. 

Temperature and pressure effects become important in chemical systems of geological interest. Also, the chemical nature of the system is often not well characterized. Nonstoichiometric compounds and solid solutions are often present, with complex silicates frequently playing an important part. 

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