Geology and genesis of overpressured sandstone reservoirs in Venture gas field, offshore Nova Scotia, Canada

Conference · Wed Mar 01 04:00:00 EST 1989 · AAPG Bull.; (United States)

OSTI ID:5787725

Jansa, L F; Noguera, V H

Overpressured formations with pressure gradients up to 1.9 times normal hydrostatic occur over an area approximately 10,000 km/sup 2/ offshore Nova Scotia, Canada. In the Venture field, the abnormal pressures are confined below 4500 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 overpressured 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 2500 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.

• Research Organization: Atlantic Geoscience Centre, Dartmouth, Nova Scotia (Canada)
• OSTI ID: 5787725
• Report Number(s): CONF-890404-
• Conference Information: Journal Name: AAPG Bull.; (United States) Journal Volume: 73:3
• Country of Publication: United States
• Language: English

Related Subjects

03 NATURAL GAS
030200* -- Natural Gas-- Reserves
Geology
& Exploration
CANADA
CLAYS
DIAGENESIS
GEOLOGIC DEPOSITS
GEOLOGIC TRAPS
GEOLOGY
MATURATION
MINERAL RESOURCES
NATURAL GAS DEPOSITS
NATURAL GAS FIELDS
NORTH AMERICA
NOVA SCOTIA
ORIGIN
RESERVOIR ROCK
RESOURCES
ROCKS
SANDSTONES
SEDIMENTARY ROCKS
SOURCE ROCKS