The stratigraphy and structrural history of the Mesozoic and Cenozoic of the Central Nova Scotian Slope, Eastern Canada

摘要

The continental shelf and slope of Nova Scotia is underlain by a number of interconnected rift sub-basins that collectively form the Scotian Basin. Petroleum exploration companies have been moderately successful on the shelf region of the basin, close to Sable Island, where several significant hydrocarbon discoveries have lead to the development of the Sable Project. This success has sparked interest in exploration of the adjacent frontier slope region within water depths between 200 and 2500m. However, the Scotian Slope Basin has been the focus of only limited regional geologic studies. Present accounts of the slope are largely extrapolated from shelf descriptions and/or modeled after play types and depositional systems typically associated with deep water exploration in other Atlantic margin areas. A discrete study area was defined for this project within the central slope region approximately 125 kilometres southwest of Sable Island. The area is approximately 120 square kilometres and contains five of the ten Scotian Slope exploration wells, three shelf wells and 4 500 kilometres of 2D seismic data. -- Scotian Basin development commenced in the Late Triassic - Early Jurassic with rifting of the Pangean Supercontinent and opening of the Atlantic Ocean. Red bed and evaporate deposition characterized the rift phase, while the drift phase was characterized by clastic progradational with periods of carbonate deposition. A prominent carbonate bank developed in the western part of the basin during the Late Jurassic, the eastern extent of which was limited by a Late Jurassic - Early Cretaceous Sable Delta. As relative sea level rose throughout the Late Cretaceous and Tertiary major transgressive sequences were deposited. This overall transgression was punctuated by major sea level drops resulting in the deposition of regressive lowstand sequences partially comprising turbidite deposits. -- Seismic stratigraphic analysis of the study area identified ten major sequence boundaries on the basis of reflection character and termination patterns. The sequence boundaries divide the Mesozoic through Cenozoic Scotian Slope Basin fill into nine depositional sequences. There are major changes in depositional style and thickness distribution patterns of the depositional sequence through time. Depositional patterns are closely linked to the tectonic, structural and halotectonic evolution of the basin. -- Five fault families were defined within the study on the basis of their regionality, duration of movement and depths of detachment; the Slope Basin-Bounding Fault Family, the Basement-Involved Fault Family, the Listric Growth Fault Family, the Major and Minor Sedimentary Fault Family and the Halokinetically Induced Fault Family. The existence of a sixth fault family, the Transfer Fault Family, is implied by local structural and stratigraphic architecture, however, the signature of this potential transfer fault is not clear enough on the available seismic data to allow for confident mapping. -- The complete spectrum of salt structures typical of passive margins has been identified and mapped within the study area. Five halotectonic structural associations with variable areal distributions have been identified. These associations are: the Trough and Swell Association, the Intra-Salt Detachment Association, the Diapiric Association, the Secondary Weld Association, and the Allochthonous Salt Association. -- The integration of seismic stratigraphic, structural and halotectonic analysis of the study area allowed for several conclusions regarding implications of a potential Scotian Slope petroleum system to be proposed. Considering all the elements and processes necessary for working hydrocarbon system, the most likely plays within the mapped study area consist of: 1) a reservoir of Cretaceous to Tertiary turbidite channel, levee or lobe sands, 2) a source rock most likely within the Kimmeridgian Verrill Canyon Formation (possible contribution from Jurassic Mohican Formation or Late Triassic to Early Jurassic early Syn-rift and/or Post-rift Lacustrine deposits, 3) a seal of Verrill Canyon Formation, Dawson Canyon or Banquereau shale or allochthonous salt, and 4) structural, tectonic and/or halokinetic traps ranging from Triassic to Cretaceous in age. -- Failure to discover hydrocarbons in recent slope explorations wells may point to the limitations of seismic resolution to predict reservoirs within the late Jurassic-Early Cretaceous successions. It also shows that drilling on the Scotian Slope is high risk exploration; more regional and better correlation of seismic with lithologies encountered in the wells are needed.udud