ABSTRACT

A reef-fringed carbonate bank, the Swan Hills Member, of probable early Late Devonian age, is present in the subsurface of west-central Alberta, at Judy Creek. Five million barrels of 41° API gravity oil were produced from this bank in 1963. In 1964, there were 169 producing oil wells and reserves were estimated at 770 million barrels of oil in place.

Thirty-two diamond cores from the Swan Hills Member were studied. Three hundred and thirty thin sections and one hundred and fifty acetate peels were examined petrographically in order to determine grain origin, grain/micrite ratios, types of fossils present and diagenetic history.

The Swan Hills Member limestones are composed of: (1) Organic framebuilders--dominantly massive and tabular stromatoporoids with locally significant quantities of solenoporid algae and tabulate corals. The dendroid stromatoporoids, mainly Amphipora and Stachyodes, are not considered to be framebuilders although they were exceedingly important rock builders. (2) Grains--mostly skeletal, but pellets dominate locally. Rare intraclasts and coated grains are also present. (3) Micrite. (4) Sparry calcite. (5) Void space.

The Swan Hills Member is divisible into nine informal stratigraphic units. The major rock types and environments of deposition of these informal units are given below in ascending order.

1. The basal biomicrite beds consist mainly of micritic-skeletal limestone which contains a brachiopod-gastropod-echinoderm fauna. The environment of deposition is essentially similar to that of the Waterways Formation but is believed to represent shallower water.
2. The coral beds are composed mainly of stromatoporoidal limestone with large quantities of Amphipora, Stachyodes and lesser quantities of Thamnopora; these beds form a widespread biostromal deposit.
3. During deposition of the next unit, the Amphipora-stromatoporoid beds, a broad shelf lagoon developed over a wide area; stromatoporoid reefs grew on the eastern margin of the shelf. The lagoon was densely populated with Amphipora and bulbous stromatoporoids which grew, in part, on a pelleted lime-mud bottom. Local areas of supratidal conditions prevailed.
4. The overlying Amphipora-spar beds, characterized by Amphipora limestone with abundant sparry calcite and interbedded with micritic-Amphipora limestone, represent a transition between the underlying lower energy and the overlying higher energy environments. Emergent conditions appear to have prevailed in some areas after deposition of this unit.
5. The next depositional unit, the dendroid stromatoporoid beds, consists mainly of stromatoporoidal limestone, which was deposited on a relatively flat surface interpreted as a table reef.
6. The succeeding Amphipora beds, chiefly micritic-Amphipora limestone, were deposited in a shallow central lagoon surrounded by reefs (Unit 8) on the bank margins.
7. This differentiation into a shallow central lagoon and marginal reefs ceased during sedimentation of the grey limestone-green shale beds; deposition occurred in brackish ponds, supratidal flats and subtidal environments.

1 Manuscript received January 5, 1966; revised manuscript received February 22, 1966.

2 Department of Geology and Institute of Oceanography, University of British Columbia.

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8. Reef growth on the bank margins was contemporaneous with the deposition of Unit 5, and most of Unit 6. These reefs were built primarily of massive and tabular stromatoporoids with lesser amounts of solenporid algae and tabulate corals; they form an important petroleum reservoir which encircles Judy Creek field.
9. The culmination of the Swan Hills depositional history was the formation of a widespread skeletal and pelleted limestone over the entire Judy Creek bank. This unit forms an important petroleum reservoir through Judy Creek field.