Diagenesis alteration

Carbonate reservoir rock is usually found at the place of formation ( in situ ). Carbonate rocks are susceptible to alteration by the processes of diagenesis. 

Carbonate rocks are not normally transported over long distances, and we find carbonate reservoir rocks mostly at the location of origin, in situ . They are usually the product of marine organisms. However, carbonates are often severely affected by diagenetic processes. A more detailed description of altered carbonates and their reservoir properties is given below in the description of diagenesis . 

Carbonate reservoirs are usually affeoted to varying degree by diagenesis. However the process of dissolution and replacement is not limited to carbonates. Feldspar for instance is another family of minerals prone to early alterations. 

An alternative description of iUite—smectite mixed-layer clays begins with megacrystals of smectite that incorporate smaller packets of iUite (163). These constituents are observed as mixed-layer minerals in x-ray analysis. Diagenesis increases the percentage of iUite layer and with increasing alteration the mixed-layer mineral takes on the characteristics of an iUite dominated iUite—smectite. 

Inherent in all these methodologies, which measure either absolute Sr levels or strontium isotope ratios in mineralized tissue, is the assumption that diagenesis has not altered the signal since death. This has been a matter of some considerable debate (e.g., Nelson et al. 1986), but the consensus of current opinion amongst practitioners is that the repeated acid-washing procedures used remove any diagenetic mineral, because it has a higher... 

Matthews A, Zhu X-K, O Nions K (2001) Kinetic iron stable isotope fractionation between iron (—II) and (—III) complexes in solution. Earth Planet Sci Lett 192 81-92 McManus J, Nagler TE, Siebert C, Wheat CG, Hammond DE (2002) Oceanic molybdenum isotope fractionation Diagenesis and hydrothermal ridge-flank alteration. Geochem Geophys Geosys 3 2002GC000356... 

Diagenesis and catagenesis can alter the evaporite minerals after burial. For example, high temperatures, pressures, and pore-water salinities characteristic of deep burial lead to the conversion of gypsum into anhydrite. Thus, evaporite mineralogy reflects not only the environmental conditions under which the evaporite was formed, but also those under which diagenesis and catagenesis occurred. 

The effects of wind winnowing by bottom currents could also explain why 25% of the nodules are buried in the sediments. Variations in local rates of sediment deposition and erosion could cause the periodic burial and exposure of nodules. Growth would be expected only during periods of exposure. This would also explain why buried nodules appear unaffected by diagenesis (i.e., they are not buried long enough to undergo extensive alteration). 

Fig.l. Trends on the VHMS alteration box plot showing alteration related to (a) hydrothermal activity and (b) diagenesis. 

FIoeve, J. Quirt, D.FI. 1984. Mineralization and Flost Rock Alteration in Relation to Clay Mineral Diagenesis and Evolution of the Middle- Proterozoic, Athabasca Basin, northern Saskatchewan, Canada. Saskatchewan Research Council, SRC Technical Report 187, 187 p. 

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