It is now common to estimate diagenetic grades of claystones and shales using the composition of interstratified dioctahedral minerals. It is also well known that sandstones commonly have mineralogies which are different from those in associated shales. It has, therefore, been proposed that solution composition of formation waters can play an important role in determining the clay mineralogy of sandstones.

Chemiographic analysis of clay minerals present under conditions of deep burial can be used to determine the key assemblages

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which will be useful to establish maximum expandability of the mixed layered clays. This then allows one to establish the maximum pressure-temperature conditions of burial experienced by the sediments. This analysis allows one to estimate the effects of composition as well as pressure and temperature.

Several sandstone samples from the North Sea were analyzed using the electron microprobe. This study allows one to establish the compositional range of diagenetic minerals present for a given paragenesis and to estimate the relative influence of local bulk composition upon the new minerals. It appears that small variations in mineral composition can be attributed to local chemistry, but major differences in mineralogy (i.e., mixed layering) are due to differences in pressure-temperature during the recrystallization of the minerals. The extent to which the minerals have recrystallized and the apparent mobility of the constituent elements in both fine- and coarse-grained material indicate that there has been a great approach to mineral chemical equilibrium in the samples studied. This l ads one to believe that the phase equilibrium approach to clay mineralogy could be useful in rocks which have experienced burial diagenesis.

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