Venture gas field, offshore Nova Scotia; case study of overpressuring in region of low sedimentation rate

The Venture gas field is a deep, overpressured gas reservoir on the Scotian Shelf off eastern Canada. Overpressuring occurs at depths below 4.5 km and is associated with significant quantities of gaseous hydrocarbons. The overpressured zone is composed of high-porosity sandstones interbedded with low-porosity, low-permeability shales, which have nearly normal levels of compaction. Pore pressures in this zone show a stepwise increase with depth where pressure jumps correlate with low-permeability (<10-20m2, where 10-12m2 = 1.01 d) shale beds. Petrophysical and lithological data for sandstones and shales in the overpressured zone are used to constrain a one-dimensional, single-phase model of pore-pressure development. The conversion of kerogen to hydrocarbons is simulated by simple kinetic-reaction theory. Estimates of the effects on overpressuring of sediment compaction, temperature changes, and hydrocarbon generation were obtained from this model using the above data. Disequilibrium compaction apparently is responsible for much of the overpressuring in the Venture gas field. In our simple single-phase model, the contribution of hydrocarbon generation to overpressuring was slight.