Geochemistry and diagenesis of stratabound calcite cement layers within the Rannoch Formation of the Brent Group, Murchison Field, North Viking Graben (northern North Sea)

Abstract

Up to four calcite-cemented horizons (doggers) form impermeable barriers to fluid flow within the Middle Jurassic Rannoch Formation and are correlatable across the Murchison Field. Calcite precipitated during early diagenesis, within high porosity/permeability sandstones at the top of coarsening (shoaling) upward shoreface cycles. Calcite δ 13C and δ 18O compositions range from -4.1 to -13.4‰ PDB, and -6.6 to -16.7‰ PDB, respectively. Sr concentrations of up to 1334 ppm are consistent with marine carbonate sources (probably shell fragments), but no viable intraformational carbonate source has been identified in the Murchison Field area. Initial 87Sr/ 86Sr compositions (0.71109–0.71266) are higher than Middle Jurassic seawater (0.7073), and consistent with precipitation from modified porewaters containing significant proportions of continentally derived “meteoric” fluids enriched in 87Sr as a result of basement weathering, or percolation through hinterland soils/unconsolidated detritus. An internal source of 87Sr is not considered viable in view of the high proportion (up to 25‰ clastic constituents) of unaltered detrital alkali feldspar and mica within the Rannoch Formation. Geochemical and isotope data indicate correlations between increasing δ 18O composition and increasing iron and magnesium content within calcite. Calcium concentrations decrease with increasing δ 18O for calcites. Geochemical data trends can be interpreted differently in terms of either “static” or “evolving” δ 18O porewater models. Static δ 18O porewater models using Jurassic/Early Cretaceous “meteoric” water ( δ 18O = −6‰; SMOW ) predict cement precipitation temperatures of 17–77°C. However, δ 13C compositions are more depleted than those typical of carbon derived from shell debris, and correlation between decreasing δ 18O and decreasing δ 13C suggests modification of a meteoric-derived porewater system during burial via contribution of low δ 13C carbon derived from a deep basinal source. However, it is difficult to constrain accurately the degree to which porewater δ 18O may have evolved during burial. The degree to which porewater δ 18O could have evolved from that of a Jurassic/Early Cretaceous meteoric water can be loosely constrained as from δ 18O ∼ −6‰ SMOW to δ 18O ∼ −2‰ SMOW if the temperature at which the most 18O-depleted cements precipitated is limited by present-day bottom-hole temperatures (∼110°C) for wells currently at maximum burial depth.