ABSTRACT

A comparative study of the Venture and Glenelg gas fields yields some insight into their respective gas migration history. Gas accumulations at Venture are geopressured and are associated with rollover into major east-west faults. These faults do not extend upward to breach the hydropressure-geopressure transition, which occurs (at 4500 m) in sediments that have a relatively high sand/shale ratio (1:1). Entrapment is provided by simple closure associated with the fault rollover. Fault closure, if present, is a minor element. Gas pay at Venture occurs in reservoirs that currently reside in the mature for gas zone (R_o = 1.2+). Conversely, Glenelg accumulations are hydropressured, and associated with major growth faults that not only penetrate the top geopressure zone but also extend upward to very shallow depths. Entrapment seems to be entirely fault seal dependent. The hydro-geopressure transition occurs at 3900 m in sediments that have a relatively low sand/shale ratio (0.45:1). Gas pay occurs in reservoirs that are not currently mature for gas (R_o = 0.9).

Numerical modelling of the maturity and pore pressure evolution of the two fields provides clues to their gas charge dynamics. In situ gas generation at Venture is an insignificant contributor to observed geopressures. A lateral gas charge has to be invoked to account for in-place reserves. At Glenelg, a significant vertical component to gas charge is required. It is postulated that the geopressure-penetrating growth faults in the Glenelg area have acted as conduits to gas migration, requiring that fault transmissivity for gas has changed through time.

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