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Heavy Minerals and U-Pb Zircon Ages of Upper Jurassic Reservoirs in the Flemish Pass and Orphan Frontier Basins, Offshore Newfoundland


Offshore Newfoundland and Labrador is considered one of the best frontier regions for oil and gas exploration today with over 20 deep-water basins, many drilled only sparsely. The most advanced exploration of the frontier basins has been carried out in the Flemish Pass basin where five discoveries have been announced since 2008. Jurassic reservoirs of high porosity and permeability with mature source rocks are of particular interest. Recent assessments suggest that there are potentially 12.5 billion barrels of oil in the Flemish Pass basin and twice that in the Orphan basin to the west. The objective of this study is to compare heavy-mineral proportions and uranium-lead detrital zircon ages of late Jurassic sandstones from the Flemish Pass and Orphan basins to determine whether their detritus had the same provenance and thus may have been deposited from the same large-scale fluvial system. Cuttings from 5-m intervals of Upper Jurassic sandstones were selected from the Mizzen F-09 well of the Flemish Pass Basin and, 160 km to the northwest, the Great Barasway F-66 well of the Orphan Basin. The samples were washed and sieved and the 63-180 micron fraction was separated into a heavy (>2.89 g/cc) fraction using bromoform. At Memorial University of Newfoundland, heavy minerals were identified and counted using a Quanta 650F (field emission) scanning electron microscope equipped with mineral liberation software. Detrital zircon U-Pb ages were determined using a Lambda Physik ComPex Pro 110 ArF GeoLas laser ablation system coupled to an ELEMENT-XR single collector magnetic sector inductively coupled plasma mass spectrometer. The results indicate that the detrital heavy mineralogy of the Upper Jurassic intervals in the Flemish Pass and Orphan wells is rather similar. Both are dominated by magnetite and garnet, with variable proportions of ilmenite and aluminosilicate, and minor detrital apatite. Detrital zircon U-Pb ages show subtle differences, however. Avalonian ages are dominant in the Upper Jurassic Mizzen intervals, whereas Avalonian ages are sub equal in abundance with Grenvillian, Makkovik-Ketilidian and Archean ages in the Upper Jurassic Great Barasway sections. Ordovician ages are present and Variscan ages absent from the Upper Jurassic Mizzen intervals but the reverse is true for the Upper Jurassic Great Barasway sections. Lithologic differences are therefore indicated between the Upper Jurassic reservoirs in these two basins.