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

WILLIAMSON, MARK A., and CAROLYN SMYTH, Atlantic Geoscience Centre, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada

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 and thermal and maturation models suggest that the source datum (152 Ma) began gas generation at 115 Ma (Glenelg) and 85 Ma (Venture). 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. 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 hy rocarbon generation. The coincidence of excess pore pressure and gas generation events at Glenelg together with the occurrence at this location of major vertical 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 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 with the rock column.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)