Carbonate cement genesis and links to siliciclastic environments of deposition: Hibernia Formation, Jeanne d’Arc basin, offshore Newfoundland

Abstract

Carbonate cements filling intergranular pores in siliciclastic reservoirs dramatically reduce porosity and permeability, creating baffles and flow barriers that impact reservoir performance and drilling. Understanding the origin and controls of carbonate cementation aids in identifying and predicting carbonate-cemented intervals in reservoir stratigraphy. Sedimentologic and diagenetic studies of the Lower Cretaceous Hibernia Formation in the Jeanne d’Arc basin, Newfoundland, Canada, were conducted to explore links between carbonate cements, facies associations and sea-level fluctuations. The Lower Hibernia zone records the transition from a deltaic to an estuarine environment, characterized by two regressive sequence boundaries and comprising fluvial, tidal-fluvial, and tidal facies associations. Petrographic analyses revealed three main pervasive carbonate cements: early calcite, early dolomite and late ferroan calcite. The δ¹⁸O (δ¹⁸O_calcite = −7.6 to −4.2 ‰ VPDB; δ¹⁸O_dolomite = −4.1 to −1.6 ‰ VPDB) and trace element geochemistry of calcite and dolomite suggest that parent fluids were influenced by mixing of marine-meteoric water under relatively reducing conditions at shallow depths. Dolomite cementation is linked with seawater retention within sediments during sea level regressions and fluid expulsion due to compaction of the underlying shales. Conversely, calcite cements are associated with transgressions and an increased marine water flux into the recently deposited sediments. Acidic pore water dissolved early carbonate cements, developing secondary porosity predating ferroan calcite cementation. The wide distribution of ferroan calcite in the overall stratigraphy, its depleted δ¹⁸O composition (δ¹⁸O_{ferroancalcite} = −10.2 to −9.1 ‰ VPDB) and high Fe and Mn concentrations suggest that ferroan calcite cementation occurred during early burial.