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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

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