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Abstract

A review of available stable isotope geochemistry for diagenetic minerals from several Mesozoic sandstones and conglomerates in the Western Canada sedimentary basin has permitted the general nature of pore water evolution to be traced from deposition to the present time. The importance of meteoric water throughout the evolution of these rocks has been demonstrated by this analysis. Meteoric water was present during early diagenesis of some sandstones simply because they were deposited in fluvial environments. In other cases, deposition in transitional fresh-water to marine settings resulted in a variable but nevertheless significant meteoric contribution to early pore waters. Diagenesis of the Lower Cretaceous Clearwater Formation oil sands, for example, has been substantially influenced by the prevalence of meteoric water early in their history. Meteoric water was also introduced early into sandstones and conglomerates deposited in marine environments. In many cases, this situation reflected post- depositional meteoric infiltration related to sea-level fluctuations, but the Inland Seaway itself may also have been brackish at some times.

The diagenetic mineral record also indicated that δ18O values of pore waters rose during burial. This change resulted mostly from mixing between connate waters in the sandstones and more 18O-enriched waters from underlying Paleozoic carbonates. Significant contribution of 18O-rich shale pore waters to the sandstones (via the smectite to illite reaction) could not be demonstrated.

The compositions of late diagenetic minerals confirmed that 18O-enrichment of sandstone pore waters was reversed by large-scale, gravity-driven introduction of meteoric water—a response to uplift of the Western Canada sedimentary basin in early Eocene (post-Laramide) time. The influx of low-18O water dominated late diagenesis of most sandstones, resulting in formation waters largely controlled by mixing between the post-Laramide, meteoric flux and earlier, more evolved pore waters.

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