ILLITE SMECTITE DIAGENESIS IN THE BEAUFORT-MACKENZIE BASIN, ARCTIC CANADA - RELATION TO HYDROCARBON OCCURRENCE/

The burial-diagenetic trends of illite/smectite (US) mixed-layer clays in the Beaufort-Mackenzie Basin have been established from detailed s tudies of 215 core and drill-cutting samples from 22 exploratory wells onshore and offshore. Illitization in the Beaufort-Mackenzie Basin is characterized by four types of VS: random, a mixture of random and or dered, R1-ordered, and R>1-ordered US. In general, the proportion of I -layers increases with depth, while the dominant type of I/S changes f rom random to R>1-ordered I/S. The increase of I-content is fast and s hows a parabolic trend with respect to depth in the interval of random US, whereas it is slow and linear in the range of ordered I/S. Temper ature appears to be the important control for the illitization reactio n, while geopressure, lithology, and other geological variables contri bute substantially to local variations in I/S composition. A temperatu re of more than 50 degrees C is the threshold temperature for activati on of the illitization reaction. The transitions from random to R1, an d from RI to R>1-ordered VS occur at 70 degrees-110 degrees C and 110 degrees-140 degrees C, respectively. Illitization in geopressure zones and sandy intervals appears to be more advanced than in normally pres sured sections and shales, respectively. Accelerated VS diagenesis in geopressured environments is probably related to higher water/rock rat ios compared to normally pressured conditions. The same reasoning may apply to sandy sections because of their greater porosity and permeabi lity. Hydrocarbon occurrence in the Beaufort-Mackenzie Basin appears t o be related to I/S diagenesis. Hydrocarbon reservoirs are located abo ve or in the top portion of the active illitization zone within the in terval of random I/S interval, but rarely below the depth where ordere d I/S appears. Smectite-water derived from I/S diagenesis may have pla yed a role in the emplacement of hydrocarbons.