Abstract

Compaction of the sediment and deposition of precipitated cements are the most important processes affecting the porosity of sandstones. The amount of compaction of a sandstone is evaluated by thin-section counts of (1) number of grain-to-grain contacts; (2) cumulative grain intercept length in a traverse of known length (packing density); (3) proportion of grain-to-grain contacts among all types of contacts (packing proximity). In general, closeness of packing of sand-sized particles is positively correlated with the percentage of micaceous rock fragments and depth of burial and negatively correlated with the percentage of detrital clay. Exceptions exist to these generalizations, probably largely as the result of early deposition of cements precipitated from underground waters in pore spaces.

The most effective way to obtain practical information concerning cementation of sandstones is through study of thin-sections. Such studies have revealed that (1) silica, calcite, and dolomite are the most abundant chemical elements; (2) the presence and type of chemical cement in a sandstone may vary with time so that a temporal sequence of replacements in one sand may be reversed in another; (3) the amount of chemical cement is negatively correlated with the amount of detrital matrix.

Recent field and laboratory studies of the chemistry of the cementation process have added greatly to the understanding of diagenesis. These studies have shown that natural waters are almost always under-saturated with respect to opal but are commonly super-saturated with respect to quartz and therefore some quartz cement may form without addition of silica to underground waters. However, extensive cementation by quartz is generally considered to require additional sources of silica. Dissolution of diatom and radiolarian tests and "pressure solution" of quartz grains at intergranular contacts seem to be the most common sources.

Precipitation of calcite cement in the pores of sandstones depends largely upon increasing the CO₃=/HCO₃^– ratio in underground waters. This is accomplished by increasing either temperature or pH. Replacement relationships between calcite and silica are also explained by variations in either of these parameters. Calcite solubility decreases with temperature; that of silica increases. Calcite solubility increases with decreasing pH; silica solubility is unaffected by the range of pH variation common in underground waters.

Dolomite cement probably forms only as a replacement of earlier-formed calcite.