Geology and diagenetic history of overpressured sandstone reservoirs, Venture Gas field, offshore Nova Scotia, Canada

Journal Article · Mon Oct 01 00:00:00 EDT 1990 · AAPG Bulletin (American Association of Petroleum Geologists); (United States)

OSTI ID:5890269

Jansa, L F ^[1]; H N, Urrea V ^[2]

---

1. Bedford Institute of Oceanography, Dartmouth, Nova Scotia (Canada) Dalhousie Univ., Halifax, Nova Scotia (Canada)
2. Chevron Canada Resources, Calgary, Alberta (Canada)

Deep exploratory wells in the Scotian Basin, offshore Nova Scotia, Canada, have encountered overpressured formations with pressures 1.9 {times} the normal hydrostatic gradient. The overpressures occur over an area of approximately 10,000 km{sup 2}. In the Venture field, the abnormal pressures are confined below a depth of 4,500 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. Three secondary porosity depth levels, which top at 2,500 m (65C), 3,700 m (95C), and 4,600 m (130C), correlate with major steps in the organic matter maturation in the basin. Secondary porosity is initially achieved by aluminosilicate dissolution, with ferroan sparry calcite cement dissolution dominating below 4,000 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% porosity under 500-1,000 atm of pressure could not be achieved without simultaneous pressuring of developing voids. 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 dependent.

• OSTI ID: 5890269
• Journal Information: AAPG Bulletin (American Association of Petroleum Geologists); (United States), Vol. 74:10; ISSN 0149-1423
• Country of Publication: United States
• Language: English

Related Subjects

03 NATURAL GAS
NATURAL GAS DEPOSITS
DIAGENESIS
NOVA SCOTIA
RESERVOIR ROCK
POROSITY
CRETACEOUS PERIOD
EXPLORATORY WELLS
GEOLOGY
GEOPRESSURED SYSTEMS
JURASSIC PERIOD
OFFSHORE SITES
SANDSTONES
SEDIMENTARY BASINS
CANADA
DEVELOPED COUNTRIES
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GEOLOGIC STRUCTURES
MESOZOIC ERA
MINERAL RESOURCES
NORTH AMERICA
RESOURCES
ROCKS
SEDIMENTARY ROCKS
WELLS
030200* - Natural Gas- Reserves
Geology
& Exploration