Salt Tectonics and Depositional Systems Offshore Nova Scotia

Matt Luheshi¹, David Roberts³, Bernard Colletta², Hamish Wilson¹, Janice Weston¹, Mark Deptuck⁴, Kris Kendal⁴, and Andrew McCrae^5
1RPS Energy, Henley-on-Thames, United Kingdom.
²BEICIP FRANLAB, Rueil-Malmaison, France.
³Roberts Geosciences, Henley-on-Thames, United Kingdom.
⁴Canadian Nova Scotia Offshore Petroleum Board, Halifax, NS, Canada.
⁵St Mary’s University, Halifax, NS, Canada.

A new rigorous evaluation of the tectonic evolution of the Nova Scotia margin provides the framework for a detailed evaluation of the sediment basin fill, from the earliest syn rift to the present day. The Play Fairway Analysis project involved a comprehensive review of a complete data set for the Scotia Margin - including key well data along with 70,000km of 2D data and 30,000km2 of 3D data. This has enabled the project to develop a robust sequence stratigraphic and sedimentological interpretation of the margin.

The seismic interpretation has concentrated on understanding the stratigraphic architecture of the Late Jurassic and early Cretaceous deltas that form the main hydrocarbon reservoirs on the margin.

Late Triassic rifting formed basins that were the locus of salt deposition. Backstripping suggests that these basins contain over 4km of salt. These rifted basins abut the continent ocean transition where there is a marked ‘basement step’ interpreted to be subaerial volcanics associated with break up. The presence of salt and subaerial volcanism invokes a model of a large restricted marine basin immediately post rift that gives way to deeper marine environments as the basin subsides. The marine basins were rimmed by carbonates on both sides of the conjugate margin.

The late Jurassic Avalon uplift to the Northeast of Nova Scotia, associated with Northern Atlantic rifting between Iberia and the Grand Banks, caused large-scale deltaic input in the Northern part of the Scotian margin. This overran the carbonate-rimmed basin and filled depocenters controlled by salt. These sediment loads caused enhanced diapirism and salt nappes partially expelling salt from the basin. From the Early Jurassic, a carbonate rimmed platform was established along the Southwest of the margin, with carbonate reefs in the North east on basement highs in localized starved conditions. A Tithonian maximum flooding event marked the end of this phase of deposition.

Continued uplift throughout the early Cretaceous lead to three pulses of deltaic input with the locus of deposition migrating to the west through time. The first of these, of Berriasian-Valanginian age, formed the well known Banquereau slide as deltaic sediments detached on early salt glaciers. The second delta formed the Mississauga formation which contains the majority of Nova Scotia’s offshore production in the Sable sub-basin. Transgression of this delta over the shelf and onto the slope was controlled by the interaction between sediment load and salt tectonics. To the east, salt nappes formed, while further west sedimentation was related to growth faults that detach on older salt features. The third delta, the Logan Canyon formation, of Albian- Aptian age evolved in similar conditions but further to the west. These Cretaceous deltas have a well defined westernmost edge on the carbonate bank with starved conditions existing throughout the early Cretaceous in the west. In deeper water, salt features produce a paleobathymetry that controlled turbidite flows along the strike of the margin and out into the deeper water.

The structural maps, and well data were used as input to the DIONISIS sediment facies forward model. This model has allowed a calibration of the position of the shelf slope break through time, for each of the deltaic sequences and developed a predictive approach for reservoir distribution in the basin, slope and onto the shelf.

 

AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.

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