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The AAPG/Datapages Combined Publications Database

CSPG Special Publications

Abstract


The Geology of Selected Carbonate Oil, Gas and Lead-zinc Reservoirs in Western Canada, 1977
Pages 107-108

Meekwap Field – A Nisku (Upper Devonian) Shelf Edge Reservoir

S. G. Cheshire, J. W. Keith

Abstract

The Meekwap Field is located in Ranges 15 and 16, Township 66W5M, approximately 230 kilometres (140 miles) northwest of Edmonton. The Field lies in the Nisku Formation, immediately above the shelf break in the underlying Ireton Formation. During Nisku times an extensive basin lay to the south, and northwards a carbonate shelf formed. On this shelf the Nisku may be divided into three units.

The lower unit, a maximum of 20 metres thickness, consists of interbedded clean and argillaceous limewacke and mudstones, with a sparse fauna of branching and laminar stromatoporoids, corals and brachiopods. Very little intergranular porosity is developed. This unit is of regional extent and illustrates an environment influenced by argillaceous sediment input from the north.

The middle unit shows local development of small, upraised lobes and several channels cutting back into the shelf. The Meekwap Field consists of at least two small (5 by 2 kilometre), poor ecological reefs located on these lobes. These reefs are oriented perpendicular to the shelf trend, and extend into slightly deeper water. They are separated partially by a narrow channel, and partially by an Amphipora shelf facies. The reef fauna is restricted to thin tabular and encrusting stromatoporoids intimately associated with Renalcis, with some development of Stachyodes and tabulate corals. These reefs grew at a greater depth than did the typical well-developed Devonian stromatorporoid reefs. A groundmass of limewacke and grainstones, with fair intergranular porosity, is preserved.

Shelfwards, this unit grades into an Amphipora dominated facies, with intergranular porosity in packstone and grainstone beds. There is some development of a non-porous wackestone. The channels separating reefs are filled by carbonate mud washed from the reef and shelf, and some coarser carbonate debris close to the reefs. A cyclical input of argillaceous material is superimposed to give interbedded clean and argillaceous limewackestones of practically no original porosity. The middle unit shows an approximate thickness of 15 metres, and includes a 1 metre thick, shallow water sequence at its top. This may be tentatively traced as a gently seaward dipping sheet over the southern part of the Meekwap Field. It grades from subtidal muds through the intertidal zone into possibly supratidal algal laminates.

The top unit of 5 to 10 metres thickness consists of a tight argillaceous limewackestone with a sparse fauna similar to the lower unit. It contains thin green shales, and is immediately overlain by the Calmar shales.

Dolomitization, localized on the reef bodies, is important in enhancing porosity and has formed much vuggy and channel porosity. A later anhydrite event had a minor affect in reducing porosity, but sealed many fractures, and in the lower unit there is evidence of dolomite being replaced by anhydrite.

The Meekwap Field is a stratigraphic reef trap dipping gently to the southeast with updip impermeable channnel sediments and a shale caprock.


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