Geology and Genesis of Overpressured Sandstone Reservoirs in Venture Gas Field, Offshore Nova Scotia, Canada

Lubomir F. Jansa, Victor H. Noguera

Overpressured formations with pressure gradients up to 1.9 times normal hydrostatic occur over an area approximately 10,000 km² offshore Nova Scotia, Canada. In the Venture field, the abnormal pressures are confined below 4,500 m and are associated with gas- and condensate-bearing Upper Jurassic-Lower Cretaceous sandstone reservoirs. Venture overpressures differ from Gulf Coast-type overpressures in that they occur within normally compacted shales containing numerous overpressured sandstone reservoir beds. Normal compaction is indicated indirectly by gradual increase in bulk density, sonic velocity, and shale resistivity with depth. Plots of temperature gradients, organic maturation gradients, and chemical composition of formation waters vs. depth across the ov rpressured zone also indicate normal sediment compaction. Clay mineral studies show the overpressured shales are well indurated, with the transformation of smectite into mixed-layer clay minerals occurring 2,500 m above the top of the overpressured zone. Furthermore, the overpressured sandstones exhibit textures indicative of normal compaction, with secondary porosity developed in both normally compacted and overpressured strata. The Venture represents hard-rock overpressures within normally compacted strata. As a result of a low geothermal gradient in the basin, the peak gas generation was reached late after most of the lithologies had lost their effective permeability due to progressing sediment diagenesis. Formation of diagenetic seals above the zone of peak gas generation in addition to continuing release of fluids as a result of shale diagenesis contribute to formation of overpressures, but organic matter maturation and hydrocarbon expulsion are the main driving forces behind the Venture overpressures.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.