ABSTRACT

Overpressured formations with pressure gradients of nearly twice the normal hydrostatic gradient occur over an area of approximately 10,000 km² in the Scotian basin, offshore Nova Scotia. In the Venture Field, the overpressured zone is confined below a depth of 4,500 m, and contains prolific gas/condensate sandstone reservoirs and hydrocarbon source shales of Late Jurassic-Early Cretaceous age. The Venture Field exemplifies a hard-rock overpressured system developed within normally compacted strata. Late hydrocarbon generation and migration at depth are chiefly responsible for the overpressuring.

Venture overpressure differs from U.S. Gulf Coast-type overpressure in that the former occurs within normally compacted shales interbedded with thick, vertically-stacked sandstone reservoir beds.

The diagenetic history of the shales and sandstones in the Venture Field has been investigated through petrographic and SEM work. Clay studies show the overpressured shales are well indurated and display similar composition and texture above and below the onset of overpressure. Progressive shale diagenesis over time with compaction has led to near total loss of permeability. The overpressured sandstones exhibit textures and fabrics diagnostic of normal compaction with secondary reservoir porosity developed at depth via leaching of aluminosilicates and porefilling calcite cement. Three depth levels of secondary porosity generation are recognized across both normally and abnormally pressured strata.

Hydrocarbon generation and migration lead to pore pressure buildup at depth and are postulated to be the main driving force behind Venture-type overpressure. Peak gas generation was initiated late in the compaction history of the basin. Diagenetic seals, such as impervious shales, sandstones and limestones, within and above the zone of peak gas generation ("hydrocarbon cooking machine"), are believed to behave as aquitards ("pressure cooker"). This results in net increments in rock pore pressure and the establishment of an overpressured zone.

The recognition of Venture-type overpressure is important in that it provides clues as to the mechanisms of hydrocarbon generation and migration in the basin. It also defines a model that may assist in understanding overpressured regimes in other frontier basins.

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