--> Abstract: Geopressured Devonian Limestone Reef Reservoirs In The Wild River Area, West-Central Alberta, by J. Wendte and G. Ward; #90928 (1999).

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WENDTE, JACK1, VERN STASIUK1, LLOYD SNOWDON1, and GRANT WARD2
1Geological Survey of Canada, Calgary
2Ward Hydrodynamics, Ltd.

Abstract: Geopressured Devonian Limestone Reef Reservoirs in the Wild River Area, West-Central Alberta

Most exploration in deeply buried carbonates is targeted for hydrocarbon accumulations in dolostones, because most limestones in these settings have very low porosities and are impermeable.

A major exception to this are overpressured, deeply buried (>4 km) gas-bearing Leduc reefs of Late Devonian age in the Wild River area of West-Central Alberta. Gas accumulations in these reefs differ from those in associated dolostone reservoirs: extremely high versus normal fluid pressure gradients, sweet methane versus sour wet gases, and no associated water leg versus a thick water leg.

These geopressured reefs have recoverable gas reserves of approximately 10 BCF, with producing intervals having an average porosity and permeability of approximately 5% and about 1 md, respectively. Most voids are micron-scale intercrystalline pores in peloids and stromatoporoid bioclasts. Most larger primary pores were infilled by calcite and saddle dolomite cements prior to burial of 3 km. Permeability is due to the presence of microfractures confined mainly to compact stromatoporoid particles.

These microfractures commonly have subhorizontal to horizontal orientations and are lined by pyrobitumen with high bireflectance values. This implies that the microfractures formed from excessively high geopressures created from the cracking of oil to gas. These overpressured conditions developed during deep burial (>5.5 km) and reflect separation of the limestone reef reservoirs from the regional flow system.

The steady decline in production rate for one of these reefs from an initial rate of 9MMcf/d in 1992 to the present rate of approximately 2 MMcf/d is typical of gas depletion reservoirs.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas