Correlation of calcite-cemented layers in shallow-marine sandstones of the Fensfjord Formation in the Brage Field

Abstract

Shallow-marine sandstones of the Fensfjord Formation in the Brage Field contain numerous calcite-cemented layers with thicknesses up to 1.6 m. Most of the calcite-cemented layers are located within bioturbated coarse silt to fine-grained sandstone, and are similar to calcite-cemented layers with lateral extents greater than 3 km in the Bridport Sands in Dorset. Less common horizontally or hummocky laminated calcite-cemented intervals in the Fensfjord Formation resemble calcite-cemented concretions in the Bencliff Grit. The dominant source of Ca²⁺ in the calcite cement in the three formations is probably biogenic carbonate. Isotopic analysis indicates that calcite cement in the Fensfjord Formation contains a mixture of carbon from decomposition of organic matter and from biogenic carbonate. In the Bridport Sands carbon is probably predominantly derived from biogenic carbonate, whereas calcite cement in the Bencliff Grit contains a large proportion of carbon from fermentation reactions. Wireline log correlation, supported by core description and comparison with the studied outcrops, indicates that the most extensive calcite-cemented layers in the Fensfjord Formation have lateral extents of at least 6 km. Geochemical analysis of closely spaced samples from the correlated calcite-cemented intervals in four wells show that isotopic signatures and trace element contents are widely different within calcite-cemented intervals thought to belong to the same layer. Samples located along two vertical sections through a calcite-cemented layer in the Bridport Sands also have different isotopic signatures and trace element contents, even though the lateral separation between the two sample sets was only 30 cm. This suggests that geochemical analysis may be of limited value for correlation of calcite-cemented layers.