| Summary: | Illitization in the Beaufort-Mackenzie Basin is characterized by random, a mixture of random and ordered, R1-ordered, and R $>$ 1-ordered illite/smectite (I/S). Proportions of I-layers increase with depth, while the dominant I/S type changes in that order. Illitization in offshore wells is generally more advanced than onshore because of higher geothermal gradients. Illitization in geopressure zones and sandy intervals appears to be faster than under normally pressured conditions and in shales, respectively, due to higher water/particle ratios. Hydrocarbons generally occur above and in the interval of random I/S, but rarely below the depth where ordered I/S is found. === Layer formulae of S- and I-layers are estimated to be $ rm lbrack Al sb{1.57}Fe sb{.19}Mg sb{.31}Ti sb{.07} rbrack lbrack Si sb{3.84}Al sb{.16} rbrack O sb{10}(OH) sb2$ and $ rm lbrack Al sb{1.84}Mg sb{.16} rbrack lbrack Si sb{3.33}Al sb{.67} rbrack O sb{10}(OH) sb2,$ respectively. The increase in concentrations of $ rm K sb2O,$ Rb and rare earth elements (REE), the decrease in octahedral elements (Mg, Ti, Sc, Zn, Zr), and the increase in tetrahedral elements (Al, Be, V) can be attributed to the conversion of S-layers to I-layers with increasing depth. The REE appear to be mobilized during illitization. $ rm delta sp{18}O$ values range from 2.91 to 15.72$ perthous$ (SMOW), and increase with depth, contrasting to the trends observed in the Gulf Coast and elsewhere. This probably is related to the rapid increase in $ rm delta sp{18}O$ of pore waters with depth.
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