Abstract The Lower Cretaceous Missisauga sandstones, which form part of a major southward-prograding delta system, are the principal reservoir units in the South Sable Basin (Figure 1). In Missisauga sandstones of similar lithology and burial depth, porosity values are highly variable ranging from 12% in the Glenelg wells (3500 m) to over 20% in North Triumph (3850 m). Variations in porosity and the occurrence of porous and permeable rocks at depth can result from geopressuring, early hydrocarbon migration into the reservoir, and/or-the dissolution of a specific component or components in the rock. To investigate porosity occurrence as a function of geopressuring, pressure-depth plots, as well as plots of shale interval transit time (ITT) versus depth were constructed. From these plots it is evident that hydropressuring exists from surface to the base of the Missisauga Formation. This study excludes deeper geopressured regions such as the Venture gas field. Petrographic examination of the Missisauga sandstones revealed that in the poorer quality reservoirs much of the primary porosity was destroyed by the combined effects of compaction and silica cementation, while some secondary porosity resulted from the dissolution of-feldspars and rock fragments. In the more porous and permeable sandstones the pore system resulted from the dissolution of a pervasive, texturally early ferroan calcite cement. The creation of porous reservoir units through a similar dissolution process has also been observed in sandstones from other areas such as the North Sea, offshore Newfoundland and Mesozoic reservoirs in Alberta. The porosity in these reservoirs depends not only on

Abstract The tectonic evolution of the Nova Scotian pas-sive margin includes a Late Triassic-Early Jurassic rift phase and two major episodes of postrift subsi-dence spanning Jurassic to early Tertiary time. Rifting resulted in two classes of extensional basins characterized by opposite polarities of basin- forming faults, and different levels of crustal detachment. Episodic passive-margin development is recorded by unconformity-bounded sequences that reflect differences in depositional systems and distribution of sedimentary facies. Seismic- stratigraphic sequences, separated by unconformi-ties, match these cycles of rift and postrift subsidence, and reflect major reorganizations of oceanic plates. Transfer fault zones accommodate four extensional tracts that are characterized by different extensional and postrift subsidence. The Sable extensional tract became the major postrift depocen- ter, as localized drainage patterns along its eastern edge fed elastics into Sable basin. Postrift structural modifications include seaward-dipping detachments within the sedimentary cover, as well as salt mobility. The combined effect of increased subsidence, structurally controlled dispersal systems, and postrift faulting resulted in the accumulation of hydrocarbons in Sable basin.