Growth-faulted shelf-margin deltas: a new (but old) play type, offshore Nova Scotia

Abstract

Abstract Shelf-margin deltas have not been explicitly recognized as a play type in the passive margin basin offshore Nova Scotia, perhaps because Mesozoic shelf-margin positions are rarely well resolved in seismic data. It is argued, however, that several of the largest gas accumulations offshore Nova Scotia (Venture, Alma, Glenelg) occur in growth-faulted shelf-margin delta complexes that formed at or near the paleoshelf edge during periods of low relative sea-level. Shelf-margin deltas are excellent exploration targets for several reasons: they commonly have laterally-extensive shale seals and expanded sandstone reservoirs, are associated with early movement on growth fault structures, and are commonly overpressured. Also, because they commonly link downdip to sand accumulations on the slope and basin floor, identification of shelf margin depocentres can help identify the location of deep-water exploration targets. Criteria that have proved useful in identifying shelf-margin deltas offshore Nova Scotia include: (1) proximity to large-scale (i.e. at least several hundred metres relief) basinward-dipping slope reflections; (2) association with large-scale listric growth faults; (3) evidence of delta-front/prodelta gravity-flow deposition in core; (4) relatively large scale upward-coarsening deltaic/shoreface successions (several tens-of-metres to over two hundred metres thick); and (5) paleoecological evidence for rapid shallowing. Recognition of the growth-faulted shelf-margin play type will not only improve exploitation strategies in offshore Nova Scotia, but provide an important framework to guide the identification of new exploration opportunities in genetically-related parts of the stratigraphic section.