Illite and illite/smectite, which occur as authigenic pore-fill in some sandstones, are excellent argon-retentive clocks. Dating their mean time of growth in conjunction with other basin information may add to predictive understanding of illitic cement distribution. Paragenesis and distribution of authigenic clay in the sandstone body with respect to hydrocarbon occurrence may allow the measured age to be related to time of hydrocarbon emplacement. For certain petroleum-source shales, the mean age of illite/smectite burial metamorphism, and thus petroleum maturation, can be measured directly and then compared to the age of clay authigenesis in reservoir sandstones to see if the two phenomena are related.

Three sandstones have been examined: the Cretaceous Muddy on the eastern flank of the Powder River basin; the Jurassic Nugget of the Wyoming Overthrust belt; and the Permian Rotliegendes of northern Europe. For the regressive poorly sorted Muddy, logistic problems arise because of the low abundance of authigenic illite/smectite compared to kaolinite and the common occurrence of old detrital illite. The age of illite/smectite authigenesis thus is asymptotically derived from analyses of a series of progressively finer (and authigenically purer) clay size fractions down to < 0.05µ. Over a broad region, this derived age is about 40 m.y., excluding Bell Creek which is anomalous. This age may associate illite/smectite growth in the reservoir with the early Tertiary deep burial of th basin interior. Because similar ages occur in both oil and water zones, illite/smectite growth may either have preceded or accompanied oil emplacement.

Logistic problems are minimized for the abundant, high-K₂O, discrete illite cements of the well-sorted aeolian Nugget and Rotliegendes. For both of these, precise sets of ages have been obtained for individual fields in both water and hydrocarbon zones, implying cementation may have been a relatively short-lived "event." The Nugget samples, all from the Absaroka sheet, give late Mesozoic ages. In the Rotliegendes, complex block faulting has led to unique post-late Jurassic burial and gas generation histories for adjacent fault blocks. However, thus far the three areas of the Rotliegendes examined indicate illite growth largely preceded this history.

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