Sedimentary architecture and links to carbonate cementation in the Hibernia Formation, Hibernia Field, offshore Newfoundland

Carbonate cements filling intergranular pores in siliciclastic reservoirs dramatically reduce porosity and permeability, representing baffles and flow barriers that impact reservoir performance and drilling operations. Understanding the origin and controls of carbonate cementation allows deciphering and predicting carbonate-cemented intervals in the reservoir stratigraphy. Sedimentologic and diagenetic studies of the Lower Cretaceous Hibernia Formation in the Jeanne d’Arc basin, Newfoundland, Canada, were performed to test 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 (δ¹⁸Ocalcite= -7.6 to -4.2 ‰ VPDB; δ¹⁸Odolomite= -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 in 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. The percolation of acidic pore waters promoted the dissolution of early carbonate cements and the development of secondary porosity, predating ferroan calcite cementation. Finally, the common distribution of ferroan calcite in the overall stratigraphy, and depleted δ¹⁸O composition (δ¹⁸Oferroancalcite= -10.2 to -9.1 ‰ VPDB) and high Fe and Mn concentrations, suggests that ferroan calcite cementation occurred during early burial.