Overpressures and hydrocarbon generation in the Sable sub-basin, offshore Nova Scotia

One of several interconnected depocentres lying offshore eastern Canada, the Sable sub-basin preserves a thick sequence of Mesozoic-Cenozoic clastic sediments, significant gas accumulations and an extensive development of abnormal pressures. In order to understand the basin's hydrocarbon generation, migration and accumulation history it is necessary to quantify the interplay between its burial, thermal, and maturation history, and to determine the influence on these of the basin's excess pressure history. Simple, one-dimensional reconstructions of maturity and pore pressure histories are derived for exploration well and pseudo-well locations on a seismic line running from the basin's structural high to its depocentre. Calibrated, where possible, by reference to measured maturity, temperature and pressures, these models provide a basic dynamic framework within which it is possible to consider the generation history of the basin's source rocks. Late Jurassic to Early Cretaceous sediments underwent an initial rapid, rift-related subsidence. The thermal/maturation models suggest that source rocks lying within these intervals quickly matured and began generating gas and condensates. Similarly, this rapid burial was translated, through sediment compaction disequilibrium processes, into an early expression of abnormal pressures. The pore pressure/time reconstructions in the modelling assume that sediment compaction disequilibrium and gas generation are the principal causal mechanisms. Matching pore pressure reconstructions with present-day pressure-depth profiles is particularly sensitive to assumed seal permeability profiles. Although the seal permeabilities used as model input are based on actual measured permeabilities at the present day, this does not mean that the permeability-time curves derived through the model's decompaction assumptions accurately reflect seal permeability evolution.