Petrographic study of the Frewens sandstone, Upper Cretaceous Frontier Formation, documents reservoir-scale diagenetic heterogeneity. Iron-bearing calcite cement occurs as large concretions that generally follow bedding and are most common near the top of the sandstone. Median thickness of the concretions is 0.6 m, length 4.5 m, and width 5.7 m; median volume is 5.2 m3. Concretions comprise 12% of the sandstone.

The minus-cement porosity of concretion samples is low, indicating that the calcite precipitated near maximum burial depth. Isotopic and burial history data suggest that the calcite precipitated at ~54°C from evolved meteoric water enriched in 18O or from a mixed meteoric–marine pore-water. Shell-bearing transgressive shales above the Frewens sandstone are interpreted to be the source of calcium carbonate. Concretions of this size and distribution would influence fluid flow in a reservoir and would reduce the amount of hydrocarbons in place.

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