The Limestone reef reservoir at Swan Hills consists essentially of a "buildup" of successively smaller atoll-like layers. Precipitation of calcium carbonate within the lagoonal area of each layer essentially kept pace with growth of the outer organic lattice. Within the reef mass, six depositional environments characterized by specific suites of fossils (or fossil fragments) and associated limestone textures are recognized as follows.

1. Aerated moderately agitated water of normal marine salinity (widespread shelf or submerged reef-built platform). Thamnopora-type corals, Amphipora, "pancake" stromatoporoids, crinoids, articulate brachiopods, rare rugose cup corals, and ostracods occur in light gray-buff skeletal microgranular limestone of low permeability.

2. Semi-stagnant quiet water of normal marine salinity (leeward side of reef). Crinoids, articulate brachiopods, rare thamnopora-type corals, Amphipora, pancake and bulbous stromatoporoids, rugose cup corals, ostracods, and gastropods occur in impermeable dark brown argillaceous skeletal calcilutite with thin black bituminous shale beds and rare dark chert nodules.

3. Aerated highly agitated water of normal marine salinity (organic lattice). Light buff stromatoporoid bulbs and Amphipora, rare thamnopora-type corals, rugose cup corals, crinoids, and articulate brachiopods form permeable patch reefs and interbedded skeletal calcirudites.

4. Semi-stagnant quiet marine water of slightly increased salinity (partly restricted shelf). Dark gray biostromal "buildups" of stromatoporoid bulbs and minor Amphipora occur in impermeable black shaly matrix.

5. Aerated, quiet to highly agitated marine water of increased salinity (mud flats, tidal channels, and shoals within atoll-lagoon). Amphipora, rare ostracods and gastropods occur in light buff impermeable to highly permeable precipitated limestone (lithographic, microgranular, pseudo-oolitic, and intra-formational conglomerate).

6. Semi-stagnant quiet marine water of increased salinity (deeper protected pools in atoll-lagoon). Amphipora, rare stromatoporoid bulbs and ostracods occur in impermeable dark brown slightly argillaceous calcilutite of precipitated origin.

Mapping of these environments is of great value in outlining field extensions and evaluation of acreage during development drilling. Use of these techniques, in addition to contruction of isopach maps of critical intervals, should also be highly effective in exploration for undiscovered reef oil and gas fields.

End_of_Article - Last_Page 265------------