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The AAPG/Datapages Combined Publications Database

CSPG Bulletin


Bulletin of Canadian Petroleum Geology
Vol. 40 (1992), No. 2. (June), Pages 151-169

Timing of Gas and Overpressure Generation in the Sable Basin Offshore Nova Scotia: Implications for Gas Migration Dynamics

Mark A. Williamson, Carolyn Smyth


Gas accumulations at the Glenelg and Venture locations offshore Nova Scotia, eastern Canada, occur in hydropressured and overpressured reservoirs respectively. The lithological, structural and paleohydrological characteristics of these two locations differ significantly. Burial history, thermal and maturation models suggest that the source datum (152 Ma) began significant gas generation at 115 Ma (Glenelg) and 85 Ma (Venture). Two-phase one-dimensional numerical models of formation pore pressures within the source datums suggest two periods of excess pore pressures (above hydrostatic). The first is at 90 Ma and 110 Ma for Glenelg and Venture respectively. A second builds from 20 Ma to the present day for both areas. Both events can be accounted for through sediment disequilibrium compaction processes with minor pressuring from hydrocarbon generation. The coincidence of excess pore pressure and gas generation events at Glenelg together with the occurrence at this location of major vertical growth fault conduits was probably responsible for vertical gas migration from a mature, overpressured source into hydropressured reservoirs. The lack of coincidence of these events and the absence of major vertical growth faults at Venture probably combined to restrict most of the gas at Venture to overpressured accumulations. The models, which attempt to define the geometrical relationships between the top predicted overpressures, top predicted gas window and top source datum through time, have implications for diagenetic and fluid histories within the rock column.


Des accumulations de gaz aux localites de Glenelg et Venture au large de Terre Neuve, Canada de l'est, sont presentes dans des roches reservoir sous pression hydrostatique et supra-hydrostatique, respectivement. Les caracteristiques lithologiques, structurales et paleo-hydrologiques different significativement entre ces deux localites. Des modeles d'historie d'enfouissement, d'evolution thermique et de maturation suggerent que la roche mere (152 Ma) commenca a generer des quantites significatives de gaz a 115 Ma (Glenelg) et 85 Ma (Venture). Les modeles numeriques de pression interstitielle a une dimension et deux phases suggerent que deux periodes de pression excessive (supra-hydrostatique) eurent lieu dans la roche mere. La premiere periode eut lieu a 90 Ma et 110 Ma a Glenelg et Venture, respectivement. La seconde periode commenca a 20 Ma et est toujours d'actualite dans les deux regions. Chacune de ces deux periodes peut etre expliquee par le procede de desequilibre de compaction, avec une contribution mineure due a la genese d'hydrocarbures. La coincidence de pression interstitielle excessive et de genese de gaz a Glenelg, en plus de la presence a cette localite de conduits sous forme de failles syngenetiques verticales majeures, a probablement ete responsable de la migration verticale de gaz depuis une roche mere mature et surcomprimee vers des reservoirs sous pression hydrostatique. L'absence de coincidence entre ces evenements et l'absence de failles syngenetiques verticales majeures a Venture se combinerent probablement pour limiter la plupart du gaz a Venture aux accumulations surcomprimees. Les modeles, qui tentent de definir les relations geometriques entre le dessus des zones surcomprimees predites, le dessus de la fenetre a gaz predite et le dessus du niveau de la roche mere a travers le temps, ont des implications pour l'histoire diagenetique et des fluides dans cette sequence sedimentaire.

Traduit par Patrice de Caritat

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