Diagenesis feldspars

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. 

Zeolites created in shallow sea sediments are also widespread. Other marine zeolites include analcime, merlinoite, chabazite, stilbite, and more rarely, erionite, mordenite, and laumontite. Figure 13 shows a schematic of the zoning patterns of autogenic zeolites and feldspars in tuffs of saline alkaline lakes, saline alkaline soils, deep-sea sediments, open hydrologic systems and burial diagenesis... 

Hemming S. R., McDaniel D. K., McLennan S. M., and Hanson G. N. (1996) Pb isotope constraints on the provenance and diagenesis of detrital feldspars from the Sudbury basin, Canada. Earth Planet. Sci. Lett. 142(3— 4), 501-512. 

Detrital feldspars in both sandstones and shales are subject to dissolution and replacement during late diagenesis. As previously discussed, replacement processes during diagenesis commence with... 

Detrital feldspars in modern sediments have a compositional range that reflects their composition in the source rocks (Trevena and Nash, 1981). During diagenesis, sodium-rich K-feldspars and more calcium-rich detrital plagioclase grains are more subject to dissolution (and replacement) than potassium-rich K-feldspar or sodium-rich plagioclases (Maynard, 1984 Milliken, 1988, 1992 Milliken et al, 1989). 

Small secondary pores are frequent in feldspars of modem sediments (Passaretti and Eslinger, 1987 Brantley et al., 1999). Additional dissolution of detrital feldspars proceeds through the earliest stages of diagenesis (Milliken, 1988). Sandstones flushed with large quantities of meteoric water are especially prone to loss of feldspars through dissolution (Mathisen, 1984). 

Other common, though volumetrically minor, feldspar-replacing minerals include titanite, ana-tase, sphalerite, barite, ankerite, siderite, and fluorite. With the exception of replacement driven by force of crystallization, feldspar replacements have intracrystalline distributions that are strongly localized at sites of surface-controlled dissolution. Interestingly, replacement of detrital feldspars by authigenic clays is rarely observed in late diagenesis. 

Table 1 Petrographically documented reactions of volumetric significance in late diagenesis of sandstones and shales. As written, all reactions only proceed to the right in the nonequilibrium realm of late diagenesis. Compositions are from Lynch (1996) for clay minerals and from Milliken et al. (1989) for feldspars.

The balance among the reactions in Table 1 evolves during late diagenesis as clay mineral reactions proceed, feldspars are consumed, and cements are precipitated. If local dissolution and precipitation reactions are out of balance at any point in this evolution, then transport of acid (and possibly other components) at scales larger than thin sections will be required. However, quantifying the acid balance among all the reactions in Table 1 requires information on shale composition and petrography that is not currently available. 

Land L. S. and Milliken K. L. (1981) Feldspar diagenesis in the Frio Formation, Brazoria County, Texas Gulf Coast. 

The minerals in a coal clearly reflect its geochemical environment of deposition and diagenesis rather than the composition of the source rocks. Ferromagnesian minerals such as pyroxenes, amphiboles, and ohvines are very rare in coal feldspars are also uncommon. These minerals are unstable in low-pH peat-swamp environments and are apparently destroyed in the early stages of coalihcation. In contrast, more resistant accessory minerals such as rutile, zircon, and rare-earth phosphates are relatively common in coal. 

Illite clays may result from the weathering of micas and feldspars. Their formation in soils and sediments is favored by high K" " and moderate silica concentrations. When smectites or mixed-layer smectite/illite clays are buried in deep sedimentary basins, they are gradually transformed into more stable illites by a combination of time and temperature (diagenesis) (cf. Velde and Vasseur 1992 Huang et al. 1993 Cuadros and Linares 1996). The reaction involved might be... 

Thermocatalytic sulfate reduction probably is the main source of H2S in the deep subsurface (25.81). The reactions in Table II, based on the evolution of H2S and CO2 by sulfate reduction of hydrocarbons in clastic sequences where there is available SO42- (i.e. sequences with evaporite beds or cements), illustrate the importance of this process to diagenesis. Not only does it provide a diagenetic sink for Fe via pyrite, but depending on the relative availability of reactants, can either cause precipitation or dissolution of calcite and alter feldspars to clay minerals. 

It is for these reasons that the use of clastic minerals like quartz and feldspars, with their simpler diagenesis, are attractive as tape recorders for deciphering both the type of oil, i.e. source facies and maturity, API and GOR, and also when this oil existed in the trap (cf Karlsen et al. 1993). 

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