Diagenesis and porosity reduction in the Late Cretaceous Wyandot Formation, Offshore Nova Scotia: a comparison with Norwegian North Sea chalks

Abstract

Abstract Porous chalk units have produced significant oil and gas discoveries around the world, including the world-class Tor Field of the Norwegian North Sea. On the Scotian Shelf of eastern Canada, the Upper Cretaceous Wyandot Formation is widespread, extending approximately 500 km along the length of the margin, and consists primarily of limestone with major chalk intervals. These chalks have porosities of up to 30% and are the reservoir for a gas and oil discovery at the Primrose N-50 well (1972) and a gas show at the Eagle D-21 well (1972). However, despite its potential as a hydrocarbon reservoir and/or seal, the Wyandot Formation is under-studied. In this paper, the Wyandot chalk is studied using conventional core samples, petrophysical logs, isotope geochemistry and SEM images to enhance the understanding of the depositional history, diagenesis and porosity-reducing mechanisms within the Wyandot Formation. Results indicate that the Wyandot chalks are in situ pelagic deposits, as opposed to the allochthonous North Sea chalks, and that mechanical compaction and dissolution/re-precipitation are the dominant mechanisms of porosity reduction. Given that significant volumes of Wyandot chalk have been eroded on parts of the Scotian Shelf, it is possible that North Sea type allochthonous reservoirs exist is distal locations on the Scotian Slope, and therefore an increased understanding of the sedimentology and porosity distribution of the in situ Wyandot Formation is important for further exploration in this frontier area.