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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 74 (1990)

Issue: 10. (October)

First Page: 1640

Last Page: 1658

Title: Geology and Diagenetic History of Overpressured Sandstone Reservoirs, Venture Gas Field, Offshore Nova Scotia, Canada (1)

Author(s): LUBOMIR F. JANSA (2) and VICTOR HUGO NOGUERA URREA (3)

Abstract:

Deep exploratory wells in the Scotian Basin, offshore Nova Scotia, Canada, have encountered overpressured formations with pressures 1.9 X the normal hydrostatic gradient. The overpressures occur over an area of approximately 10,000 sq km. In the Venture field, the abnormal pressures are confined below a depth of 4500 m and are associated with Upper Jurassic-Lower Cretaceous gas- and condensate-bearing sandstone reservoirs. The overpressures occur within normally compacted shales containing numerous overpressured sandstone reservoir beds. The development of overpressures, seals, and secondary reservoirs are all diagenetically driven. The volumetric increase achieved by kerogen to petroleum-gas conversion and hydrocarbon expulsion are believed to be the main driving forces or Venture overpressures. Three secondary porosity depth levels, which top at 2500 m (65 degrees C), 3700 m (95 degrees C), and 4600 m (130 degrees C), correlate with major steps in the organic matter maturation in the basin. Secondary porosity is initially achieved by alumosilicate dissolution, with ferroan sparry calcite cement dissolution dominating below 4000 m. Porosity enhancement and preservation is not the result of a single diagenetic event but instead the result of a series of diagenetic events that overlapped in time. Formation of dynamic diagenetic barriers within the zone of peak gas generation helps retard the diffusive migration of hydrocarbons and other fluids expelled during shale diagenesis resulting in pressure build up.

The preservation of up to 32 percent porosity under 500-1000 atm of pressure could not be achieved without simultaneous pressuring of developing voids. Thus, the expansion of secondary reservoirs in the overpressured zone also had to be late (50 Ma or younger) and about synchronous with pressurizing of the reservoirs. Significant for hydrocarbon exploration is that Venture-type diagenetic overpressures are not associated with undercompacted sediments and, hence, they cannot be predicted from compaction trends during drilling. Petrographic, diagenetic, and lithofacies studies can be instrumental in predicting potential areas of deep subsurface secondary reservoirs development.

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